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Article

Potentials and Limitations of Fluviomarine Pollen Records to Reconstruct Spatiotemporal Changes in Coastal Ecosystems During the Holocene: A Case of Study from Ría de Vigo (NW Iberia)

by
Alberto Castro-Parada
1,2,
Nerea Cazás
1,2,
Víctor Cartelle
3,
Javier Ferreiro da Costa
4,
Natalia Martínez-Carreño
5,
Soledad García-Gil
1,6 and
Castor Muñoz Sobrino
1,2,*
1
Centro de Investigación Mariña, Universidad de Vigo, E-36310 Vigo, Spain
2
Departamento de Bioloxía Vexetal e Ciencias do Solo, Facultade de Ciencias s/n, Universidade de Vigo, E-36310 Vigo, Spain
3
Flanders Marine Institute (VLIZ), InnovOcean Campus, Jacobsenstraat 1, 8400 Oostende, Belgium
4
GI 1934-TB (Territorio, Biodiversidade), Instituto de Biodiversidade Agraria e Desenvolvemento Rural (IBADER), Universidade de Santiago, Campus de Lugo s/n, E-27002 Lugo, Spain
5
Instituto Español de Oceanografía, IEO-CSIC, C. O. Vigo, E-36390 Vigo, Spain
6
Departamento de Xeociencias Mariñas, Facultade de Ciencias s/n, Universidade de Vigo, E-36210 Vigo, Spain
*
Author to whom correspondence should be addressed.
Land 2025, 14(3), 540; https://doi.org/10.3390/land14030540
Submission received: 29 January 2025 / Revised: 24 February 2025 / Accepted: 27 February 2025 / Published: 5 March 2025
(This article belongs to the Special Issue Pollen-Based Reconstruction of Holocene Land-Cover)
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Abstract

:
The study of marine and terrestrial palynomorphs in fluviomarine environments has been successfully used in combination with different geophysical approaches to understand high-resolution relative sea-level oscillations and to reconstruct the environmental changes affecting estuaries and adjacent inland ecosystems. However, erosion during the postglacial marine transgression frequently causes sedimentary discontinuities or may lead to the redeposition of ancient upland sediments, including secondary, recycled and rebedded pollen. Therefore, a robust seismic and chronological control of the sedimentary facies is essential. In addition, studies of modern pollen sedimentation and its relationship to contemporaneous vegetation are valuable for obtaining a more realistic interpretation of the sedimentary evidence. To explore the significance of the experimental evidence obtained and to support the interpretation of sedimentary records from the same basin, we analysed a large set of modern pollen data from the Ría de Vigo (NW Iberia). The pollen samples derived from different sedimentary environments were compared with the local and regional vegetation cover. Pollen evidence from the various limnetic systems studied allows the identification of major vegetation types in the basin. However, in all the cases, the reconstructed relative pollen contributions of each vegetation unit are often distorted by the overrepresentation of certain anemophilous pollen types, the underrepresentation of some entomophilous species, and the specific taphonomy of each site of sedimentation. The ability of the seabed pollen evidence to represent the modern deciduous and alluvial forests, as well as the saltmarsh vegetation onshore, increases in the shallowest points of the ria (shallower than −10 m). Conversely, pastures and crops are better represented at intermediate depths (shallower than −30 m), while scrubland vegetation is better represented in samples at more than 20 m below modern sea level. It is concluded that shallow seabed pollen can provide information on the main elements of the modern vegetation cover of the emerged basin, including the main elements of the vegetation cover. However, the selection of the most suitable subtidal sites for coring, combined with pollen data from several environmental contexts, is critical for achieving an accurate reconstruction of the changing conditions of the emerged basin over time.

1. Introduction

Northwestern Iberia, located at a mid-latitude in the eastern North Atlantic margin (Figure 1), has been identified as a sensitive area to the major climatic oscillations affecting both the subtropical and the boreal North Atlantic [1,2,3]. Additionally, the changing marine conditions at this latitude likely contributed to governing the postglacial vegetation dynamics inland [3,4]. Over the past decades, several high-resolution pollen records were produced from aquatic systems in mountain areas and inner depressions [5,6,7,8]. Nevertheless, reconstructing the postglacial vegetation dynamics in coastal areas is challenging due to the scarcity of suitable pollen records, which are affected by both marine transgression and human modifications to the coastline. Pollen studies from the coastline are often restricted to discontinuous sequences from inactive organic deposits [9,10,11] or intermittently active supralittoral sedimentary systems [12,13,14] that can overestimate local pollen evidence [15]. To overcome these limitations, the pollen samples from shallow marine environments offer a promising alternative as they can facilitate the reconstruction of major changes affecting the coastal areas. This is particularly evident in the deepest parts of the unglaciated, partially submerged valleys (rias) [16], where deposition is almost continuous, and sedimentation rates are higher than in other parts of the basin. These environments also integrate data from both continental and marine realms [17,18,19,20]. Therefore, these complex aquatic systems present clear advantages for paleoenvironmental reconstruction in coastal areas but also pose some challenges, which can often be addressed through a multidisciplinary approach.
Fluviomarine systems are key ecological zones, acting as broad ecotones between marine and upland ecosystems [21,22]. As such, they are crucial for understanding how environmental changes, linked to both climate and human activities, impact coastal and inland ecosystems [23,24]. Marine palynomorphs from fluviomarine facies can serve as independent proxies for regional climatic inferences [25]. Furthermore, fluviomarine systems are complex sedimentary environments strongly influenced by global and relative sea-level changes [26,27]. However, spatial and temporal changes in sedimentation rates during the postglacial period result from sediment discontinuities related to marine transgression (Figure 2A), sediment reworking, biogas formation, or aquaculture practices. Other factors affecting sediment deposition in the emerged or submerged basin are shifts in weather or runoff regimes, deforestation, agricultural activity, or engineering works [20,25,26,27,28,29].
Another key aspect of obtaining high-quality paleoenvironmental reconstructions is the availability of a properly defined chronological context. Biogenic carbonates are commonly used to date marine sediments via radiocarbon dating. However, radiocarbon dating in upwelling areas presents inherent calibration challenges. A correction is required to compare marine and terrestrial samples, but due to complexities in ocean circulation, the appropriate correction varies by location. Northwest Iberia, as an active upwelling area, experiences variability in upwelling intensity depending on wind strength, marine currents, and latitude [29,30], which may have fluctuated throughout the Holocene [25,31].
The regional difference from the average global marine reservoir correction is designated ΔR [32]. In the absence of a precise ΔR value for the specific region and period of interest, a suitable regional value (i.e., an appropriate but imperfect approximation) should be used. This is typically based on a series of sample pairs (marine and terrestrial) from different sites and comparable periods [33,34]. In the case of the NW Iberian Atlantic coast, the commonly used data and calculations [25,26] are provided by http://calib.org/marine/ (accessed on 24 February 2025), which allows the selection of the 10 nearest data points available along the Atlantic coast of Iberia and employs a standardised operating method. Another limitation is that the macrofossils required for obtaining accurate dating may be sparse or irregularly present in fluviomarine sediments. This is in part due to the shallow diagenetic conditions in the Galician rias, which have resulted in widespread carbonate dissolution in recent horizons [35,36]. Additional uncertainty arises due to the marine reservoir effect, the possibility of bioturbation, and the intrusion of freshwater, which introduces older carbon (Figure 2B). Moreover, dating bulk sediment, frequently a mixture of terrestrial and marine materials in variable proportions, requires the use of mixed calibration curves. However, the reliability of some of the radiocarbon results can be compromised by the presence of gas within the sediments or the risk of carbon reservoir effect from terrestrial materials [34,36,37,38].
Pollen is present in many different types of sediments and ideally throughout an entire sequence. If sufficiently abundant, it can be used in radiocarbon dating when effectively isolated from other carbon-bearing materials [25,39,40]. Pollen grains are frequently abundant in estuarine and fluviomarine sediments [25,26], but maximum deposition often occurs in sedimentary environments located at some distance from the shore [41]. This is likely because the diameter (4–16 μm) and sinking rate of mineral grains closely match those of the pollen grains [42]. Furthermore, pollen is of terrestrial origin and, therefore, is not subject to the marine reservoir effect. However, dating pollen presents challenges due to differential preservation over time, which is influenced by changing taphonomic conditions [43], as well as by the intrinsic characteristics of each palynomorph. The preservation of pollen grains is directly dependent on the sporopollenin content of the grain wall [25,44,45]. Additionally, an important consideration when dating pollen is the deposition of secondary, recycled and rebedded pollen [25,46,47]. To address the challenges, combining multiple techniques, such as high-resolution seismic stratigraphy, differential radiocarbon dating, multiproxy analyses of the microscopic sediment content, or molecular studies [48], can significantly improve the interpretation of the available evidence (Figure 2B).
Nevertheless, the most important topic related to pollen evidence from fluviomarine sedimentary contexts is assessing their potential in the reconstruction of local and regional onshore changes, particularly those impacting the emerging sectors of the basin. To achieve this, pollen evidence can support reliable qualitative reconstructions and, ideally, quantitative assessments of upland vegetation [49]. Initial studies suggest that modern seabed sediments are unable to accurately capture the vegetation composition of the emerged basin [41]. For instance, Pinus, a bisaccate conifer pollen that is abundantly produced, relatively buoyant [50] and rich in sporopollenin [44], is frequently overrepresented in both past [23] and modern [51,52,53] fluviomarine sediments. In contrast, entomophilous taxa are consistently underrepresented [20,41], although this is a common limitation in most sedimentary contexts.
In this study, we present and discuss extensive modern pollen evidence collected from different sedimentary environments within the same fluviomarine basin, the Ría de Vigo (NW Iberia). These environments include coastal lagoons, upland ponds, and subtidal seabed sediments. The pollen spectra from each sedimentary context are compared with the regional upland vegetation composition to assess the utility, advantages, and limitations of subtidal pollen evidence for palaeoenvironmental and palaeoecological studies.

2. Regional Setting

The Galician Rías Baixas, located in NW Iberia (Figure 1), consists of a coastal area characterised by an undulating relief, with medium-altitude elevations (frequently N-S oriented), alternating with small depressions. The main watersheds end in a few major, non-glaciated, flooded fluviomarine systems (rias) [16]. The bedrock is predominantly granitic, with some acid metamorphic rocks. The climate is temperate with a slight Mediterranean tendency [54], featuring an average annual temperature of 13.4 °C and total annual rainfall of 1864.3 mm [55]. This Eurosiberian territory belongs to the European Atlantic Province of the Atlantic-European Subregion [56], including the thermotemperate and mesotemperate belts. It hosts a variety of characteristic species such as Acer pseudoplatanus L., Betula pubescens Ehrh., Daboecia cantabrica (Huds.) K. Koch, Frangula alnus Mill., Fraxinus excelsior L., Hypericum androsaemum L., Ilex aquifolium L., Arbutus unedo L., Laurus nobilis L., Glandora prostrata (Loisel.) DC Thomas, Quercus suber L., Quercus robur L., Ulex europaeus L., Ulex minor Roth, and Castanea sativa Mill [54].
Particularly, the Ría de Vigo (Figure 1), the southernmost ria, is a drowned fluvial valley where the sea extends approximately 30 km inland from the ria’s mouth to the San Simón Bay (Figure 1). Water exchange between the outer and middle parts of the ria and the San Simón Bay is limited by a narrow strait known as the Rande Strait. This shallow ria (<53 m depth) may be a key location for understanding the final stages of the marine transgression. However, the study of the shallow subsurface using seismic methods can only be conducted in some marginal areas due to the presence of shallow gas in the sediments [27]. The connection between the ria and the shelf is partially blocked by the Cíes Islands, turning the estuary into a sedimentation trap, with notable differences in grain size and sedimentation rates along its main axis [41]. Nevertheless, the available calculations for sedimentation rates are often imprecise and difficult to compare. These rates are typically derived from seismic interpretation techniques that average the physical properties, such as the estimated sound speed in different types of sediment, or from sedimentary records of varying temporal resolution obtained using different extraction methods (e.g., gravity cores, vibracores) and dating techniques (e.g., Pb isotopes, radiocarbon dating, pollen, markers) [17,18,19,20,27,35,57].

3. Materials and Methods

3.1. Pollen Samples

Lacustrine and marine deposits are important pollen reservoirs, but their pollen records can vary depending on the intrinsic characteristics of the different sedimentary environments. To determine whether the modern pollen content from sediments of coastal lagoons, upland lakes, and marine seabed deposits reflects the actual regional and extra-local vegetation composition, we analysed sixty-one modern pollen samples (Table 1) collected from the same fluviomarine basin (Ría de Vigo, NW Iberia).
In this study, we analysed the pollen content of sediments from Lagoa dos Nenos (LN), a barrier–lagoon complex in the Cíes Islands (Figure 3). It consists of a natural 1 km-long sand barrier that confines a shallow saline lagoon, whose western margin is permanently connected to the ocean, allowing water exchange during the tidal cycle (Figure 3). On the eastern side, the lagoon has another ephemeral connection to the Ría de Vigo, which is only open during winter when storm waves coincide with spring tides, causing the formation of an ephemeral inlet [14,19]. A pine plantation is located along the southern margin of the lagoon (Figure 1 and Figure 3); the extra-local and regional vegetation include Eucalyptus stands, dry heaths and other plants characteristic of intertidal plains, salt marshes and meadows, salt dunes, and cliffs [19]. Pollen analysis of sediments from this open coastal lagoon (Table 1; Figure 3) aims to evaluate the influence of insularity and tidal regimes on the extra-local and regional pollen records.
We included previously published modern pollen samples from the region [41,50] in our study. Pollen data from three upland ponds and the drainage channel of one of them were considered (Figure 4; Table 1). These sites are located on the main watershed at the southern margin of the Ría de Vigo. These three ponds of semi-natural origin are situated at similar altitudes (the maximum difference between them is 71 m) and within a radius of less than 1 km. However, they differ in size, the area covered by tree canopy, relative position along the main axis of the sub-basin, local vegetation and seasonality of the water table (Table 2).
We retrieved a total of 26 seabed sediment samples (Table 1) distributed across the Ría de Vigo (Figure 1). Six samples from the water-sediment interface were collected in 2012 from the shallower areas of San Simón Bay using a van Veen grab sampler. We collected additional 20 samples of surface sediment from gravity and vibracores, which were retrieved during various surveys conducted by the University of Vigo in the Ría de Vigo between 2006 and 2014. Previous studies on these samples have re-evaluated the presence of certain key palynomorphs and examined the relationships between their absolute and relative abundances and different environmental variables, including the sediment grain size, annual/seasonal precipitation, the size of the source areas, the distance to the most probable source areas, the total estimated sub-basin flow and the flowering periods [41]. The surface marine samples were divided into two main groups (Figure 1) for further analyses: 8 samples from San Simón Bay and 18 samples outside the Rande Strait. The latter group was further subdivided into two subgroups (8 from the middle ria sector and 10 from its outer part).

3.2. Chemical Treatments and Pollen Identification

All samples were dried at 80 °C for 24 h, and the dry weight and volume of each sample were measured. Lycopodium spores (batch 1031, Lund University, Sweden; average concentration 20,848 spores/tablet) were added to all samples to calculate pollen concentrations. The upland samples were treated with KOH [58], and the coarse fraction (>250 μm) was removed by sieving. The lagoon and seabed samples were treated with HCl and HF at room temperature to preserve the dinoflagellate cysts and foraminiferal linings content. This was followed by sieving to remove coarse (>250 μm) and fine (<10 μm) material. The 250–10 μm fraction was repeatedly washed, centrifuged (3500 rpm), and kept in a 50% glycerine–water mixture until analysis. Finally, the slides were mounted in glycerol and examined at 400× and 600× magnifications using a Nikon ECLIPSE 50i microscope with Nikon Ds-Fi1 photographic equipment (Nikon Instruments Inc., Melville, NY, USA). Various keys and pollen atlases were used to identify the pollen types [58,59,60,61,62].

3.3. Pollen Representation

The TILIA 2.6.1 software [63] was used to process the data and generate the pollen diagrams (Figure 5, Figure 6, Figure 7 and Figure 8). The percentages of pollen and fern spores were based on a total terrestrial pollen sum, which included trees, shrubs, upland herbs and ferns. The percentages of aquatic taxa were based on the total pollen sum, encompassing all the pollen and fern spores. Certain freshwater-related vascular taxa (e.g., Ranunculus-type, Cyperaceae, Isoetes, cf. Juncus) were grouped as aquatics, but we assume that some species within these groups may be indistinctly hydrophytes, helophytes or simply developing on wet soils. The modern samples from each one and all the analysed sedimentary systems were independently grouped using the unconstrained clustering analysis of the incremental sum of squares using the Tilia software CONISS tool (Figure 5, Figure 6 and Figure 8). Local aquatic pollen and ferns were excluded from this grouping, focusing solely on data from trees, shrubs, and upland herbs. Palynomorph concentrations are expressed as grains cm−3 (Figure 8B).

3.4. Hydrological Model of Lagoa dos Nenos

Tide and water circulation dynamics are key factors in understanding sedimentation inside the Lagoa dos Nenos (LN) coastal lagoon. To analyse these dynamics, available information relative to the local bathymetry and tidal residence times (i.e., water permanence) [64] was refined using a digital terrain model (DTM) with a 5 m resolution derived from PNOA-LIDAR data (2015–2021) [65]. These analyses were performed using the QGIS 3.40 software [66]. Georeferenced samples were processed and visualised using the same software (Figure 3).

3.5. Modern Vegetation Cover

A comprehensive study of the entire Ría de Vigo basin was also conducted using QGIS [66]. The analysis utilised a raster layer of a digital elevation model of Galicia with a resolution of 25 m, incorporating contour lines and drainage basins across the whole basin area (Figure 1). The vegetation cover maps (Figure 1) were generated through photointerpretation of satellite imagery and a land-use layer [67]. The percentage of the area occupied by each vegetation unit in the whole basin was calculated using this vegetation cover and land-use map. For comparison with pollen data, vegetation units that do not produce a characteristic pollen signal (e.g., urban areas, sandy beaches, water bodies) were excluded from the analysis (Table 3). Therefore, seven main vegetation units were chosen for comparison with the modern pollen data (Figure 1): pastures and crops lands; conifers/mixed stands; Eucalyptus stands; coastal wetlands; scrublands; native hardwood stands; and exotic hardwood stands.

3.6. Numerical Analyses

Statistical analyses, including normality tests (Shapiro–Wilk test), Student’s t-test, Wilcoxon test and principal component analyses (PCA) of LN samples, were performed using PAST v.4.16c [68] and RStudio v.2022.12.10 [69]. For the PCA based on pollen percentages of modern LN samples (Figure 5), only taxa with values greater than 2% in at least two of the studied samples were considered (Figure 9). The PCA was used to identify and group the samples with similar characteristics (e.g., the role of the permanent connection to the sea, the potential sources, the transport pathways, the tidal residence times, etc.).
To evaluate the ability of pollen data to capture the vegetation diversity and composition of the whole Ría de Vigo basin (Figure 10), the different pollen types were associated with each vegetation type (Table 3), and the sums of all the percentages of the pollen types considered in each category were calculated (Figure 11, Figure 12 and Figure 13; Table 4). For the lagoon and upland ponds, additional vegetation maps were created using a 500 m radius (extra-local) and a 2 km radius (regional) from the centre of the sedimentary systems. These maps were used to compare vegetation cover and pollen assemblages across different spatial scales (extra-local, regional and the entire basin). Marine samples were compared only with the vegetation cover of the complete basin. However, given the uneven distribution of palynomorphs along the main axis of the Ría de Vigo, as revealed by previous studies [41], several assessments were made to compare vegetation cover with different groups/subgroups of seabed pollen samples. For this, the entire Ría de Vigo dataset, the subset of samples from the middle and outer parts of the ria (outside the Rande Strait), the group of samples from the inner ria (San Simón Bay), the subgroup of samples from the middle sector of the ria, and the subset of samples from the outer ria (Figure 1; Table 1) were considered. Average percentages were recalculated for each group/subgroup of modern samples and compared with the vegetation cover (Figure 11, Figure 12 and Figure 13; Table 4).

4. Results

4.1. Vegetation Cover in the Ría de Vigo Basin

Image analyses produced eleven coverage categories (Figure 1), but only seven may have a characteristic pollen signal (Table 3). Categories lacking a distinctive pollen signal (urban, water masses, sandy beaches) were excluded to recalculate the relative importance of each vegetation class as a percentage of the total coverage (Figure 11, Figure 12 and Figure 13; Table 4). Scrublands, pastures and crops represent the largest coverage, each comprising about 30% of the total area. The remaining surface is covered by Eucalyptus stands (17.5%), conifer/mixed stands (12.6%), native hardwood stands (5.5%), exotic hardwood stands (4.3%) and coastal wetlands (0.3%).

4.2. Pollen Sedimentation in the Open Coastal Lagoon System

CONISS analysis divides the LN samples into two main groups. Group LPAZ-1 includes samples LN/A4, LN/M18, LN/A1 and LN/M21 and is characterised by higher values of cf. Juncus (20–40%), Poaceae (>10%) and Asteraceae Tubuliflorae (>5%) than those found in LPAZ-2 (LN/M20, LN/A5, LN/M7, LN/A3, LN/A6 and LN/A2). LPAZ-2 is characterised by a higher content of diatoms (>30%), tree pollen (Pinus subgenus Pinus > 70%; Quercus robur-type > 5%) and heath (Erica, Corema). Amaranthaceae are consistently present in this group (Figure 5). The total pollen concentration of the sum in the LN samples ranges from 2.2 to 13.4 × 103 grains cm−3, while the terrestrial pollen concentration varies from 1.1 to 12.2 × 103 grains cm−3 (Figure 8B). Specifically, the Pinus subgenus Pinus pollen concentration varies considerably between the two groups of samples, averaging 3.85 × 103 grains cm−3 in the LPAZ-2 samples but only 1.3 × 103 grains cm−3 in LPAZ-1 samples.
The PCA confirms the CONISS classification of the samples (Figure 9), with the first three principal components explaining more than 80% of the total variance of the pollen diversity found in the lagoon. PC1 accounts for 50.62% of the total variance (Figure 9), and groups LN/M21, LN/A1, LN/A4 and LNM18, which are more influenced by the tides (Figure 3) flowing through the western margin connection, positioning them on the negative side of the PC1 axis (Figure 9). In contrast, the PCA places samples LN/M7, LN/A5, LN/A3, LN/A6 and LN/A2, which are further away from the permanent connection with the open sea, into the positive part of the axis, along with sample LN/M20. This sample is relatively close to the western connection to the sea but somewhat removed from the main tidal channel (Figure 3).
In the samples located on the positive part of PC1, the pollen concentrations are higher (>10.5 × 103 grains cm−3 on average; Figure 8B). The samples showing the highest pollen concentrations are dominated by Pinus subgenus Pinus, along with marine-origin elements such as central diatoms, dinoflagellate cysts, dinoflagellate vegetative cells, and, to a lesser extent, foraminiferal linings. Other pollen evidence corresponds to the regional woody vegetation (Quercus, Betula, Olea, Fraxinus, Ulex-type), some elements from low marshes (Amaranthaceae), sandy and wet soils (Corema, Apiaceae, Brassica-type, Ranunculus-type), and other microremains, primarily of terrestrial origin (fungal remains).
On the negative side of PC1, the pollen concentrations are lower, averaging 8.3 × 103 grains cm−3 (Figure 8B). The most relevant pollen elements are broadly of local origin, associated with the shallow lagoon bottom and other characteristic environments of the nearby margins, such as salt marshes and dune environments. These include Cyanobacteria, cf. Juncus, green algae, Asteraceae Tubuliflorae, Asteraceae Liguliflorae, and, to a lesser degree, other species linked to forest repopulations (Eucalyptus, Castanea).
PC2 accounts for 18.47% of the total variance (Figure 9). On the positive axis, it separates the samples from the deeper areas of the basin (LN/A6, LN/A4, LN/A2 and LN/A3), i.e., those samples located in sites with high tidal residence times and water body almost permanent. The greatest weight in these samples corresponds to marine-origin elements such as dinoflagellate vegetative cells, green algae, and, to a lesser extent, central diatoms. Additionally, there is a notable presence of vegetation linked to plantations (Eucalyptus), crops (Olea) and dune systems (Corema, Ulex-type, Apiaceae, Brassica-type, Asteraceae Liguliflorae, Asteraceae Tubuliflorae). On the negative side of the axis (Figure 9), the samples are from the shallowest areas (LN/A5, LN/M7, LN/A1, LN/M20, LN/M18 and LN/M21), which are subjected to more intense tidal oscillation and longer exposure periods (Figure 3). Nonetheless, the last three samples are already close to the deeper areas and show very low (LN/M20, LN/M18) or virtually absent (LN/M21) values for this component. The most significant weight on the negative side of PC2 corresponds to pollen from anemophilous trees (Pinus subgenus Pinus, Quercus, Betula, Fraxinus), some elements linked to freshwater and wet soils (pennate diatoms, Ranunculus-type), vegetation associated with marshes and intertidal muds (Cyanobacteria, cf. Juncus, Amaranthaceae), and some marine-origin palynomorphs (dinoflagellate cysts, foraminiferal linings).
PC3 (Figure 9) accounts for 12.87% of the total variance and separates the samples LN/M18, LN/M20, LN/M7 and LN/A6 on the positive side of the axis from LN/A5, LN/A3, LA/A1, LN/M21, LN/A2 and LN/A4 on the negative side. However, the weight of this component in samples LN/M7, LN/A6 and LN/A4 is very low (Figure 9). The most significant variable on the positive side of the axis is fungal remains. Other relevant components include Cyperaceae, green algae, cf. Juncus, Brassica-type, and some tree elements (Castanea, Eucalyptus, Quercus, Fraxinus, Salix). Additionally, there are other elements with lower weights, such as microremains of marine origin (dinoflagellate cysts and foraminifera) or palynomorphs that may have been transported by marine currents penetrating the lagoon (Erica) [25,41].
Therefore, on the positive side of PC3, there are sampling points where the tidal flooding level fluctuates more due to the configuration of the basin, which also receives a significant lateral contribution from runoff from the surrounding land. In this group of samples (LN/M18, LN/M20, LN/M7 and LN/A6), the terrestrial/marine contribution to the sediment is more balanced. The samples with lower tidal oscillation levels (LN/A5, LN/A3, LA/A1, LN/M21, LN/A2, and LN/A4) are on the negative side of the axis. Samples LN/A5, LN/A1, and LN/M21 are in marginal areas with anoxic intertidal muds (Cyanobacteria, Amaranthaceae) that remain exposed for longer periods. These samples also show a higher representation of Pinus subsp. Pinus (anemophilous) and other elements linked to freshwater (Alnus, Ranunculus-type, pennate diatoms). Samples LN/A3, LNA2, and LNA4, with nearly permanent marine flooding, are rich in marine remains (dinoflagellate vegetative cells, central diatoms) but also reflect a certain contribution from the vegetation on the eastern sandy barrier (Poaceae, Glaux maritima, Apiaceae).

4.3. Pollen Sedimentation in the Upland Ponds

The analysis of the complete set of pollen evidence from the upland ponds reveals significant differences between the average samples from each sedimentary system and even among some samples from the same system (Figure 6). The main pollen contribution in the upland subset of samples corresponds to anemophilous species (Pinus subgenus Pinus, Alnus, Betula), which can produce large amounts of airborne pollen (Figure 6). Additionally, the fluctuating presence of Erica pollen cannot be directly linked to proximity to the nearest heath stands but seems to be associated with other local vegetation, such as bryophytes and ferns. This suggests that most of the heath pollen contribution comes through water flows [50]. The fact that Erica pollen is more efficiently transported by water flows than other pollen types [70], likely due to the better buoyancy and hydrodynamic characteristics of its tetrads [49], supports this hypothesis. Therefore, the abundance of anemophilous pollen (which is directly related to the pond size), the tree coverage around the pond (which is inversely related to anemophilous pollen abundance), and the seasonality of the water table, which strongly determines the percentages of extra-local Erica and regional Pinus subgenus Pinus percentages and concentrations, are the main factors explaining these observed differences [50].
The concentration of the terrestrial pollen and the total pollen sum average 1.7 × 105 and 1.8 × 105 grains cm−3, respectively (Figure 8B). However, the pollen concentration varies significantly depending on the limnetic system. Samples from stational water bodies (S1 to S8) show very high pollen accumulation, averaging 3.9 × 105 grains cm−3, while those from permanently flooded ponds affected by active outflows (all P, L, and O samples, along with S9 and S10 samples) exhibit substantially lower concentrations, averaging 7.5 × 104 grains cm−3 [50].

4.4. Pollen Sedimentation in the Modern Subtidal Seabed

The pollen percentages identified in each surface sample from the Ría de Vigo are summarised in Figure 7. Notable pollen types identified in this complete subset of samples are Pinus subgenus Pinus (>50%) and Poaceae (20 to 40%). Additionally, the CONISS analysis separates the modern seabed samples into two groups: one with aquatic pollen largely below 20% of the total identified remains and the other with aquatic pollen ranging from 20% to more than 40% (Figure 7). Other characteristic pollen types representing deciduous trees (Quercus robur-type, Betula, Corylus, Alnus, etc.), heathlands (Erica), scrub (Helianthemum-type, Ulex-type), upland grassland (mainly Poaceae and Asteraceae) and marshlands (cf. Juncus) are consistently found across most samples, although their relative abundance varies notably between locations (Figure 7 and Figure 10). The average total pollen concentration is 6.4 × 104 grains cm−3, but it varies significantly, being relatively low (<2 × 104 grains cm−3) inside the San Simón Bay and consistently higher (>2 × 106 grains cm−3) outside of the Rande Strait (Figure 8B). This variation is primarily due to the high concentrations of both total tree pollen (almost 8 × 104 grains cm−3) and total aquatic pollen (>8 × 104 grains cm−3), notably peaking in the outer part of the ria (Figure 10).
The percentages and concentrations of many palynomorphs found in seabed sediment follow a normal distribution across the Ría de Vigo. However, the abundance of several pollen types increases exponentially in certain areas, particularly Erica, Alnus, Quercus robur-type, Castanea, Poaceae and cf. Juncus [41]. Key factors explaining the differences in pollen representation along the main axis of the Ría de Vigo have been previously studied, concluding that the anomalous accumulation of some taxa in specific sedimentary environments may be strongly mediated by their transport via river plumes and marine currents. This includes the transport of pollen pellets as staminate catkin remains (deciduous trees), the preferential transport by flows of buoyant tetragonal tetrads (Erica, Corema), and the dragging of pollen from coastal wetlands. Many of those pollen grains are preferentially deposited when their sizes and sinking rates correspond to those of the mineral grains (silt) [41]. Furthermore, Pinus subgenus Pinus appears to be overrepresented in the average pollen signal of sediment from the Ría de Vigo seabed (Figure 7). This may be due to the high pollen productivity of pines (anemophilous species that disseminates saccate pollen through the wind), their excellent dispersal ability [42,71], and their resistance to oxidation and microbial degradation [25,44,72]. In contrast, certain pollen types are underrepresented, particularly those from exotic hardwood (mainly Acacia), legume and Cistaceae tickets, and Eucalyptus, all with predominantly entomophilous pollination syndromes [73,74].

5. Discussion

5.1. Pollen Evidence in the Ría de Vigo Basin: Differences Across Sedimentary Environments

A CONISS re-analysis for all 61 modern pollen samples, excluding those from the local vegetation components specific to each sedimentary system, confirms the grouping of samples according to their original sedimentary environments (Figure 8). The analysis consistently identifies three main clusters that separate the upland systems from the coastal lagoon and the subtidal samples (Figure 8). However, most upland samples are segregated by their respective sampling sites, with the S9 and S10 samples being notable exceptions (Figure 8). These samples, taken from the lowest points of Pond S where the water column is persistent [50], are grouped with the unseasonal, almost permanently flooded ponds P and L. Sample P1 groups more closely with the L samples than with the other P samples, due to its relatively high Erica values (Figure 8). The high heather pollen content likely results from a temporal stream that drains close to the P1 location (Figure 4).
The total percentages of tree pollen identified in the set of 61 modern samples fluctuate significantly, ranging from 90% to 30%, largely depending on the abundance of Pinus subgenus Pinus pollen found at each sampled station (Figure 8). Pinus subgenus Pinus is particularly abundant in sedimentary systems affected by significant water table variations, with oscillations that may be seasonal [50] (i.e., most of the markedly seasonal S samples) or influenced by the tides, as in the case of the LN samples (Figure 8). The pollen representation of deciduous oaks (Quercus robur-type) is relatively uniform (5–15%) across all samples, while other deciduous tree pollen, such as Alnus and Betula, show a slight increase in the upland pond samples (Figure 8). Notably, Alnus peaks occur in the shallowest samples of the San Simón Bay (e.g., sample SM-15; Figure 8). This has been attributed to the fluvial transport of catkins [41]. Erica and fern pollen values increase in subtidal sediments relative to upland pond samples but peak notably in the upland O samples from the drainage channel (Figure 7). Meanwhile, the pollen representation of upland herbs, particularly Poaceae, increases significantly in coastal and subtidal sediment samples compared with the purely terrestrial sedimentation environments (Figure 8).
The average total pollen concentration found across the different sedimentary environments in the Ría de Vigo basin exceeds 1.1 × 105 grains cm−3 (Figure 8B). However, it fluctuates significantly, with maximum values of more than 5 × 105 grains cm−3 in the more seasonally flooded samples from the lower upland Pond S [50] and minimum values of 5 × 103 grains cm−3 in the coastal lagoon samples (Figure 8B), where tidal movement and waves can remove part of the sedimented material (Figure 2). The fact that the lagoon is located on an island system at the mouth of the estuary and exposed to coastal winds may also negatively affect the sedimentation of extra-local anemophilous pollen (e.g., Pinus subgenus Pinus¸ Poaceae and others). Total pollen concentrations in the permanently flooded stations at the upland ponds average 8 × 104 grains cm−3, while the average pollen concentration exceeds 6 × 104 grains cm−3 in the drainage channel and exceeds 9 × 104 grains cm−3 in the modern seabed sediments (Figure 8B). This unequal capacity for pollen accumulation may affect the ability of the sedimentary systems to accurately reflect the nearby presence of vegetation with high pollen production capacity.

5.2. Pollen Samples vs. Vegetation Types: Comparison Between Different Sedimentary Systems

To assess the ability of the different sedimentary environments to capture the extra-local, regional, and basin vegetation composition in the Ría de Vigo basin, we first examined the differences in the sum of percentages recorded in each aquatic system (Table 3 and Table 5). A Student’s t-test was used to compare the pollen percentages calculated for each vegetation category across the various groups/subgroups of samples considered with a parametric distribution, while a Wilcoxon rank-sum test (or Mann–Whitney U test) was used for those with a non-parametric distribution (Table 5).
In the LN dataset, significant differences between samples were observed for all the vegetation units, except for the relationship between Eucalyptus pollen and Eucalyptus stands. In the upland samples, the irregular presence of Acacia pollen is particularly notable. Acacia pollen is absent from the L pond and O channel, and its percentages show no significant differences in the samples from ponds S and P (Table 5). In the P samples, no significant differences were observed for scrublands (mainly Erica pollen), while in the O samples, no significant differences were found for pastures and crops and Eucalyptus (Table 5).
Significant differences were observed across pollen percentages representing each vegetation unit for the complete Ría de Vigo subtidal dataset, except for those associated with exotic hardwood stands, primarily Acacia pollen (Table 5). Similar results were found (Table 5) for the inner subgroup of samples in the San Simón Bay (Table 1). Finally, for the complete set of 61 samples discussed (Table 1), significant differences were observed in the pollen percentages associated with each vegetation unit considered (Table 5).

5.3. Pollen Representation in the Coastal Lagoon vs. Vegetation Cover

The samples collected from the coastal lagoon (Figure 11; Table 4) show high pollen percentages of conifers/mixed stands (>54%), intermediate percentages of coastal wetlands (>22%), pastures and crops (>13%) and native hardwood stands (>7%). Eucalyptus has very low pollen representation (0.6%). Within a <500 m radius from the lagoon, vegetation cover includes 40.5% of scrublands, 22% of pastures and crops (primarily dune vegetation), 15.4% of coastal wetlands, 13% of conifers/mixed stands and ca. 9% of Eucalyptus. Notably, the islands lack significant native hardwood stands (Figure 1). Within a 2 km radius, the coverage percentages shift to 57% of scrublands, 20% of Eucalyptus, >10.5% of pastures and crops (including dune vegetation), >9% of conifers/mixed stands and 3% of coastal wetlands (Figure 11).
The pollen data reflect the nearby occurrence (<500 m) of coastal wetlands and grasslands (pastures and crops, including dune vegetation) but tend to underestimate the occurrence of Eucalyptus plantations and scrublands at <500 m and <2 km. Additionally, pollen data overestimate the importance of pine plantations (Figure 11; Table 4), as there is a marked overrepresentation of Pinus subgenus Pinus pollen in the lagoon compared to the surface area covered by these species both on its margins (Figure 3) and within the 2 km radius (Figure 11). Furthermore, despite the nearby presence of pine plantations, the average concentration of Pinus subgenus Pinus pollen in the lagoon samples is significantly lower than the equivalent concentrations in modern seabed samples and several orders of magnitude smaller than those found in some upland pond samples (Figure 8B). Therefore, for this coastal lagoon dataset, pollen percentages are more useful than pollen concentrations for reconstructing the extra-local vegetation of the insular system (Figure 3).
Notably, a small proportion of Erica pollen was found in the lagoon samples, which does not originate from the local or regional vegetation due to the absence of these species in the Cíes Islands [25]. While the shrub Corema (also an Ericaceae) is present in the islands, its oblate-spheroidal tetrads can generally be distinguished from the prolate-spheroidal Erica tetrads [75]. The nearest Erica-dominated shrub formations in the Ría de Vigo can be found in Cabo Home (4.3 km away) and Cabo Silleiro (12 km away, Figure 1).

5.4. Pollen Representation in Upland Ponds vs. Vegetation Cover

Previous studies conducted in these upland ponds [50] suggest that the averaged pollen records at each site reflect both the extra-local (<500 m) and the regional (<2 km) vegetation cover depending on the sedimentation systems and vegetation types (Figure 12).
For the Pond S samples, the average values align well with the extra-local and regional values for Eucalyptus only (Table 4) but tend to underestimate some open vegetation units (pasture and crops, scrublands) and overestimate the conifers/mixed stands. In contrast, average pollen values from Pond L overestimate scrublands and native hardwood stands while underestimating the coverage of pastures and crops and conifers/mixed stands (Table 4). Average values from Pond P samples align more closely with the extra-local vegetation cover for scrublands and conifers/mixed stands but overestimate native hardwood stands, likely due to the overrepresentation of alluvial forest pollen (Figure 12; Table 4). Finally, average values from the drainage channel (O samples) clearly overestimate the extra-local and regional importance of scrublands (Table 4).
The best fit between the vegetation categories at the extra-local and regional levels and the pollen percentages results from the averaged pollen values of the entire set of samples, regardless of the sedimentary system (Table 4). The average pollen assemblage collected from the upland sedimentary systems (Table 4; Figure 12) is dominated by pollen from conifers/mixed stands (>61%), followed by native hardwood stands (18.5%), pasture and crops (almost 7%), scrublands (12.5%), Eucalyptus (1%) and exotic hardwood stands (<0.05%). Consequently, this averaged pollen evidence overrepresents the regional importance of the native hardwood stands (<1%) and the presence of pine plantations within 500 m (31.2%) and 2 km (43.3%) radii while underestimating the extra-local (45.1%) and regional (50.4%) presence of pastures and crops (Table 4; Figure 12). Additionally, pollen data corresponding to scrublands almost fits (12.5%) the extra-local presence of heath (15.25%) and overrepresents its regional coverage (almost 2.7%). Exotic hardwood stands are underrepresented (5.6% at <500; 0.7% and <2 km), while Eucalyptus pollen data slightly underestimate its extra-local (2,4%) and regional (2,2%) coverage (Table 4; Figure 12).

5.5. Pollen Representation in Modern Seabed Sediments vs. Vegetation Cover

The full pollen dataset, consisting of 26 seabed pollen samples from the Ría de Vigo (Figure 13), reveals an over-representation of pollen associated with conifers/mixed stands (i.e., Pinus subgenus Pinus), while the pollen from exotic hardwoods stands (i.e., Acacia), Eucalyptus and shrublands (Ulex-type, including Ulex, Cytisus and Genista) are underrepresented compared to the surface area occupied by these vegetation units (Table 4; Figure 13).
The overrepresentation of Pinus subgenus Pinus may be attributed to its high pollen productivity and excellent dispersal ability. Additionally, the high sporopollenin content in the exine of Pinus pollen makes it resistant to oxidation and microbial degradation, enhancing its preservation in sediments. In contrast, the underrepresentation of exotic hardwoods in the pollen record is likely due to their entomophilous pollination strategy (i.e., low pollen productivity), with pollen being dispersed as dense polyads. This also applies to Eucalyptus and legume thickets pollen, which share entomophilous pollination mode.
Native hardwood stands are notably overrepresented in the inner part of the ria (the San Simón Bay), but the correspondence between pollen evidence and vegetation cover is more accurate in the outer and middle parts of the ria (Figure 13; Table 4). Exotic hardwood stands have no pollen representation except for samples from San Simón Bay. Conifers/mixed stands are overrepresented in all subgroups of samples (Figure 13; Table 4), but the pollen evidence is more accurate in the middle part of the ria (Figure 13; Table 4). In contrast, the pollen representation of pastures and crops is accurate across all the sample groups, with a slight reduction in accuracy in the outer part of the ria.
Eucalyptus spp. is underrepresented in all seabed sample groups, while coastal wetlands are overrepresented in the inner, middle, and outer groups of samples, particularly in all the subsets outside of the Rande Strait (Figure 13; Table 4). The pollen representation of coastal wetlands is more accurate for the area covered by marshes within San Simón Bay (Figure 1). Finally, scrublands are underrepresented in all seabed sample groups, but their pollen representation is more accurate in the middle and outer parts of the ria (Figure 13; Table 4).

5.6. Lessons for the Interpretation of Holocene Pollen Records from Shallow Seabed Sediments

The results obtained indicate that vegetation reconstructions based on pollen content from shallow subtidal sediments are not substantially different from those derived from other upland aquatic systems. Therefore, pollen data from seabed cores recovered in incised valleys may be useful for the reconstruction of the Holocene environmental changes upland. Moreover, these sedimentary systems can capture key ephemeral ecotones, such as coastal wetlands. Variations in this type of evidence (Figure 2) may be associated with marine transgression [26,41]. Nevertheless, those sedimentary systems also have certain particularities related to the mechanisms of pollen transport, accretion, preservation and remobilisation (Figure 2 and Figure 10) [25,41]. Previous studies in comparable fluviomarine sedimentary contexts have reported variations in the representation of different types of palynomorphs at different water depths [53,76,77]. Additionally, other studies comparing modern vegetation with pollen evidence have reported an overestimation of certain arboreal taxa and an underestimation of herbaceous elements, as well as differences across upstream-downstream gradients. The transport of pollen from the alluvial species is affected by the watershed effect [70,78]. Our results (Figure 8, Figure 10, Figure 11, Figure 12 and Figure 13) align with many of those observations but also highlight that important differences may exist in pollen records depending on the relative position of the site studied in the submarine basin, the type of sedimentary system in which the sedimentation primarily occurred (e.g., seabed, open coastal lagoons or confined ponds) and the depositional (e.g., upwelling/downwelling regimes, rainy periods, marine currents) or post-depositional processes (e.g., upland and subtidal pollen reworked or rebedded) that influenced the sedimentation process (Figure 2).

6. Conclusions

This study analyses a comprehensive dataset of 61 pollen samples collected from various sedimentary environments within the same coastal basin of the Ría de Vigo (NW Iberia). The aquatic systems studied include an open coastal lagoon, three nearby upland ponds with differing local configurations, the drainage channel of one pond, and 26 subtidal sediment stations distributed across the inner, middle and outer sectors of the Ría de Vigo. The potential of seabed pollen in the reconstruction of upland vegetation was assessed by comparing it to the ability of lacustrine sediments to capture the extra-local (<500 m), regional (<2 km) and complete basin vegetation composition.
Pollen evidence from all the aquatic systems investigated in this study allows the identification of the major vegetation types within the basin. However, the accuracy of reconstructed vegetation units in each case is influenced by the overrepresentation of certain anemophilous pollen types, the underrepresentation of entomophilous species and the specific taphonomy of each site.
We have found that the percentages and concentrations of the total pollen counts and most palynomorph types in the sediment of the ria follow linear growth patterns. However, several exceptions exist, notably the exponential increase in certain taxa in specific sedimentary environments (Figure 10). After comparing upland and seabed pollen evidence, we interpret this phenomenon as being strongly mediated by the transport of pollen via river plumes and marine currents [41]. The high pollen concentrations in some subtidal zones are likely driven by fluvial transport from the entire basin and specific sedimentation processes in the ria. Tidal action, which promotes the outflow of buoyant saccate pollen grains, is the main factor responsible for the significant differences in pollen concentration between the open lagoon sediment and the seabed and upland pond samples. This buoyancy phenomenon also likely drags tetrahedral Erica pollen into the lagoon despite the absence of this species on the Cíes Islands.
The ability of the seabed pollen evidence to represent modern deciduous and alluvial forests developed upland, as well as saltmarsh vegetation, is improved when only the pollen samples from the San Simón Bay (i.e., sites shallower than 10 m below modern sea level) are considered (Figure 1). The representation of upland pastures and croplands in the Ría de Vigo is reasonably good across the main axis of the ria pollen samples (shallower than 40 m below modern sea level), with particularly accurate pollen records in sediments from the inner and middle section of the ria (sites <30 m below modern sea level). Upland scrublands, especially heathlands, are better represented in the middle and outer parts of the ria (pollen sites deeper than 20 m below modern sea level). However, entomophilous taxa (Eucalyptus, Acacia, and legume shrubs) and anemophilous taxa (pines) stands are both underrepresented and overrepresented, respectively, in pollen samples across all the pollen stations and groups of pollen stations studied (Figure 13).
Therefore, shallow seabed pollen can provide a useful picture of the environmental conditions of the emerged basin, including the main types of vegetation cover. However, the selection of the most suitable subtidal sites for coring, combined with pollen data from several environmental contexts, is critical for achieving an accurate reconstruction of the changing conditions of the emerged basin over time. Additionally, it is important to consider that ancient upland pollen evidence may be remobilised during the marine transgression, particularly during stages of upwelling intensification, and subsequently redeposited in the seabed. To address this, differential pollen dating [25] may be necessary to assess unexpected pollen trends in the sedimentary records.

Author Contributions

Conceptualization, C.M.S.; Data curation, A.C.-P., N.C. and C.M.S.; Formal analysis, A.C.-P., N.C. and C.M.S.; Funding acquisition, S.G.-G. and C.M.S.; Investigation, C.M.S., A.C.-P. and N.C.; Methodology, V.C., N.M.-C., F.J.F.d.C. and N.C.; Project administration, C.M.S. and S.G.-G.; Software, A.C.-P., V.C. and F.J.F.d.C.; Supervision, C.M.S. and V.C.; Validation, C.M.S., S.G.-G. and A.C.-P.; Visualisation, A.C.-P. and N.C.; Writing—original draft, C.M.S.; Writing—review and editing, V.C., A.C.-P., N.M.-C., F.J.F.d.C., S.G.-G. and N.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the Spanish Ministry of Science Innovation and Universities project PID2023-147147OB-I00, financed by MICIU/AEI/10.13039/501100011033 and FEDER, UE, and Spanish Ministry of Science and Innovation the European NextGeneration EU Funds TED2021-131141B-I00; and the Axudas Propias a Investigación da Universidade de Vigo 2024.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors would like to thank I. Alejo and J.A. Fernández Bouzas for their assistance in accessing some samples and information included in this work.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Location of the Galician Rías Baixas and the Ría de Vigo on the northwest coast of the Iberian Peninsula and the surface samples analysed. The extra-local (<500 m, green) and regional (<2 km, red) areas considered for the coastal lagoon and upland ponds are shown. Inner, middle, and outer seabed samples are shown in different colours. Images extracted from Google Earth Pro 7.3.6.10201.
Figure 1. Location of the Galician Rías Baixas and the Ría de Vigo on the northwest coast of the Iberian Peninsula and the surface samples analysed. The extra-local (<500 m, green) and regional (<2 km, red) areas considered for the coastal lagoon and upland ponds are shown. Inner, middle, and outer seabed samples are shown in different colours. Images extracted from Google Earth Pro 7.3.6.10201.
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Figure 2. (A) High-resolution seismic profile (3.5 kHz) across the location of core MVR-3 and the seismic stratigraphic interpretation [25]. Seismic units are bounded by sedimentary discontinuities. U1 to U5 correspond to old Pleistocene sediments, while the Holocene is mainly represented by units U6 and U7. The thickness and distribution of each unit vary significantly along the main axis of the ria. The cloudy area (acoustic masking) in U7 denotes the presence of gas in the sediment. (B) Radiocarbon dates available and the age model produced for the core MVR-3 [25]. Dates selected for the age-depth model are highlighted in blue, and outliers in red. Pentagons indicate pollen events related to historical evidence. Squares indicate radiocarbon dates from shells, and circles represent radiocarbon dates using pollen extracted from sediment. Some outliers (e.g., 14 and 15) may be related to the accretion of upland material redeposited, inverted, in the seabed. These levels are particularly rich in Pinus subgenus Pinus (= Diploxylon) pollen (dark stripes). The secondary curve of percentages (dotted) represents the values recalculated after discarding the rebedded pollen underestimated. Other outliers indicate pollen material rebedded during erosive stages. The marine dinoflagellate Lingulodinium is independent of the redeposition of ancient upland materials but related to wet periods, as revealed by the changing Total Aquatics concentrations (grains cm−3) in the seabed.
Figure 2. (A) High-resolution seismic profile (3.5 kHz) across the location of core MVR-3 and the seismic stratigraphic interpretation [25]. Seismic units are bounded by sedimentary discontinuities. U1 to U5 correspond to old Pleistocene sediments, while the Holocene is mainly represented by units U6 and U7. The thickness and distribution of each unit vary significantly along the main axis of the ria. The cloudy area (acoustic masking) in U7 denotes the presence of gas in the sediment. (B) Radiocarbon dates available and the age model produced for the core MVR-3 [25]. Dates selected for the age-depth model are highlighted in blue, and outliers in red. Pentagons indicate pollen events related to historical evidence. Squares indicate radiocarbon dates from shells, and circles represent radiocarbon dates using pollen extracted from sediment. Some outliers (e.g., 14 and 15) may be related to the accretion of upland material redeposited, inverted, in the seabed. These levels are particularly rich in Pinus subgenus Pinus (= Diploxylon) pollen (dark stripes). The secondary curve of percentages (dotted) represents the values recalculated after discarding the rebedded pollen underestimated. Other outliers indicate pollen material rebedded during erosive stages. The marine dinoflagellate Lingulodinium is independent of the redeposition of ancient upland materials but related to wet periods, as revealed by the changing Total Aquatics concentrations (grains cm−3) in the seabed.
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Figure 3. Lagoa dos Nenos, Cíes Islands, Ría de Vigo, showing bathymetry, tidal residence times and the pine plantation existing at its southwestern margin. The extra-local (<500 m) and regional (<2 km) vegetation composition (see Figure 1) in the islands is largely described in [19]. Images extracted from Google Earth Pro 7.3.6.10201.
Figure 3. Lagoa dos Nenos, Cíes Islands, Ría de Vigo, showing bathymetry, tidal residence times and the pine plantation existing at its southwestern margin. The extra-local (<500 m) and regional (<2 km) vegetation composition (see Figure 1) in the islands is largely described in [19]. Images extracted from Google Earth Pro 7.3.6.10201.
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Figure 4. Detailed aerial image of the As Lagoas-Marcosende Campus, with the location of the three ponds (S, L, P) studied and the extra-local (<500 m, red) and regional (<2 km, green) areas considered. Sample locations in all the sedimentary systems analysed: Pond S, Pond L and its drainage channel (O samples), and Pond S. Table 1 indicates the main characteristics of each lacustrine system studied. Images extracted from Google Earth Pro.
Figure 4. Detailed aerial image of the As Lagoas-Marcosende Campus, with the location of the three ponds (S, L, P) studied and the extra-local (<500 m, red) and regional (<2 km, green) areas considered. Sample locations in all the sedimentary systems analysed: Pond S, Pond L and its drainage channel (O samples), and Pond S. Table 1 indicates the main characteristics of each lacustrine system studied. Images extracted from Google Earth Pro.
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Figure 5. Summarised pollen percentages of the samples from Lagoa do Nenos (LN). Two groups of samples were identified. Sky blue shows samples at points closest to the permanent connection to sea. Orange shows samples at points furthest from the permanent connection to sea. The secondary pollen curves indicate ×10 exaggeration. Within each sample group, dotted lines separate subgroups of samples that are more similar to each other.
Figure 5. Summarised pollen percentages of the samples from Lagoa do Nenos (LN). Two groups of samples were identified. Sky blue shows samples at points closest to the permanent connection to sea. Orange shows samples at points furthest from the permanent connection to sea. The secondary pollen curves indicate ×10 exaggeration. Within each sample group, dotted lines separate subgroups of samples that are more similar to each other.
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Figure 6. Summarised pollen percentages samples from the upland ponds (P, S, L) and the drainage channel (O). Samples associated with drainage channels (purple), permanently flooded points (sky blue) or strongly seasonal sites (orange) are shown. The secondary pollen curves indicate ×10 exaggeration. Within each sample group, the dotted line separates subgroups of samples that are more similar to each other.
Figure 6. Summarised pollen percentages samples from the upland ponds (P, S, L) and the drainage channel (O). Samples associated with drainage channels (purple), permanently flooded points (sky blue) or strongly seasonal sites (orange) are shown. The secondary pollen curves indicate ×10 exaggeration. Within each sample group, the dotted line separates subgroups of samples that are more similar to each other.
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Figure 7. Summarised pollen percentages of the Ría de Vigo seabed sediment samples. The secondary pollen curves indicate ×10 exaggeration. The colours indicate the vegetation units with which the pollen assemblages are associated. Horizontal line separates the two main groups of marine samples discussed in the text. Within each sample group, dotted lines separate subgroups of samples that are more similar to each other.
Figure 7. Summarised pollen percentages of the Ría de Vigo seabed sediment samples. The secondary pollen curves indicate ×10 exaggeration. The colours indicate the vegetation units with which the pollen assemblages are associated. Horizontal line separates the two main groups of marine samples discussed in the text. Within each sample group, dotted lines separate subgroups of samples that are more similar to each other.
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Figure 8. (A) Summarised pollen percentages and CONISS clustering for all the pollen samples analysed from the Ría de Vigo basin. (B) Pollen concentrations (grains cm−3) for selected pollen types identified across all the Ría de Vigo basin samples. The coloured bars indicate upland (green), lagoon (purple) and seabed (sky blue) samples. Within each sample group, dotted lines separate subgroups of samples that are more similar to each other.
Figure 8. (A) Summarised pollen percentages and CONISS clustering for all the pollen samples analysed from the Ría de Vigo basin. (B) Pollen concentrations (grains cm−3) for selected pollen types identified across all the Ría de Vigo basin samples. The coloured bars indicate upland (green), lagoon (purple) and seabed (sky blue) samples. Within each sample group, dotted lines separate subgroups of samples that are more similar to each other.
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Figure 9. PCA for the pollen samples from Lagoa dos Nenos, Cíes Islands, Ría de Vigo: (A) The first and second principal components explain 50.62% and 18.46% of the total variance. (B) The second and third principal components, the latter explaining 12.87% of the total variance.
Figure 9. PCA for the pollen samples from Lagoa dos Nenos, Cíes Islands, Ría de Vigo: (A) The first and second principal components explain 50.62% and 18.46% of the total variance. (B) The second and third principal components, the latter explaining 12.87% of the total variance.
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Figure 10. Percentages (left) and concentrations (right; grains/cm3) of the aquatic (above) and terrestrial (bellow) total pollen in modern sediments of the Ría de Vigo. Images extracted from Google Earth Pro.
Figure 10. Percentages (left) and concentrations (right; grains/cm3) of the aquatic (above) and terrestrial (bellow) total pollen in modern sediments of the Ría de Vigo. Images extracted from Google Earth Pro.
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Figure 11. Average pollen samples from Lagoa dos Nenos, Cíes Islands, compared with the extra-local (<500 m) and regional (<2 km) vegetation coverage (see Figure 1 and [19]).
Figure 11. Average pollen samples from Lagoa dos Nenos, Cíes Islands, compared with the extra-local (<500 m) and regional (<2 km) vegetation coverage (see Figure 1 and [19]).
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Figure 12. Average pollen samples from each upland pond analysed, averaged pollen samples for all the upland samples studied, and their comparison with the extra-local (<500 m) and regional (<2 km) vegetation coverage (see Figure 1 and Figure 3 and [50]) and with the vegetation cover in the Ría de Vigo basin.
Figure 12. Average pollen samples from each upland pond analysed, averaged pollen samples for all the upland samples studied, and their comparison with the extra-local (<500 m) and regional (<2 km) vegetation coverage (see Figure 1 and Figure 3 and [50]) and with the vegetation cover in the Ría de Vigo basin.
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Figure 13. Average pollen samples for the complete set of subtidal samples analysed from the Ría de Vigo Ría de Vigo, the different subgroups of samples discussed (see Table 1), and their comparison with vegetation coverage within the Ría de Vigo basin (see Figure 1).
Figure 13. Average pollen samples for the complete set of subtidal samples analysed from the Ría de Vigo Ría de Vigo, the different subgroups of samples discussed (see Table 1), and their comparison with vegetation coverage within the Ría de Vigo basin (see Figure 1).
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Table 1. Modern samples from the Ría de Vigo basin with their altitude/depth, location in the Ría de Vigo (inner, middle or outer), groups/subgroups of samples considered for analyses, sampling year and references. For the seabed sediment samples, the water depth is expressed as m NMMA, which corresponds to the Spanish orthometric datum (mean sea level in Alicante). Positive values indicate points that emerge during low tides.
Table 1. Modern samples from the Ría de Vigo basin with their altitude/depth, location in the Ría de Vigo (inner, middle or outer), groups/subgroups of samples considered for analyses, sampling year and references. For the seabed sediment samples, the water depth is expressed as m NMMA, which corresponds to the Spanish orthometric datum (mean sea level in Alicante). Positive values indicate points that emerge during low tides.
SampleAltitude m asl/
Depth m NMMA		Environment of
Sedimentation		Group of
Samples		Subgroup
of Samples		Subgroup
of Samples		Sampling
Year		References
LN/A10LagoonLagoon 2016This study
LN/A20LagoonLagoon 2016This study
LN/A3−0.5LagoonLagoon 2016This study
LN/A4−0.5LagoonLagoon 2016This study
LN/A50.5LagoonLagoon 2016This study
LN/A60LagoonLagoon 2016This study
LN/M70.5LagoonLagoon 2016This study
LN/M180.5LagoonLagoon 2016This study
LN/M20−0.25LagoonLagoon 2016This study
LN/M210LagoonLagoon 2016This study
P1458Upland PondUplandPond P 201949
P2458Upland PondUplandPond P 201949
P3458Upland PondUplandPond P 201949
P4458Upland PondUplandPond P 201949
P5458Upland PondUplandPond P 201949
P6458Upland PondUplandPond P 201949
L1433Upland PondUplandPond L 201949
L2433Upland PondUplandPond L 201949
L3433Upland PondUplandPond L 201949
L4433Upland PondUplandPond L 201949
L5433Upland PondUplandPond L 201949
O1432Drainage channelUplandChannel 201949
O2432Drainage channeUplandChannel 201949
O3431Drainage channeUplandChannel 201949
O4431Drainage channeUplandChannel 201949
S1387Upland PondUplandPond S 201949
S2387Upland PondUplandPond S 201949
S3387Upland PondUplandPond S 201949
S4387Upland PondUplandPond S 201949
S5387Upland PondUplandPond S 201949
S6387Upland PondUplandPond S 201949
S7387Upland PondUplandPond S 201949
S8387Upland PondUplandPond S 201949
S9387Upland PondUplandPond S 201949
S10387Upland PondUplandPond S 201949
SM123Seabed sedimentSubtidalInner 201240
SM-153Seabed sedimentSubtidalInner 201240
SM53Seabed sedimentSubtidalInner 201240
SM202Seabed sedimentSubtidalInner 201240
SM222Seabed sedimentSubtidalInner 201240
SM26−2Seabed sedimentSubtidalInner 201240
B-8 (0-1)−3Seabed sedimentSubtidalInner 201240
B-1 (0-1)−4Seabed sedimentSubtidalInner 201240
VG6-06-4-1 (0-2)−13Seabed sedimentSubtidalMiddleOuter Rande Strait200640
B-5 (0-1)−18Seabed sedimentSubtidalMiddleOuter Rande Strait201240
MET4/17 (0-1)−20Seabed sedimentSubtidalMiddleOuter Rande Strait201140
MVR-4 (0-1)−22Seabed sedimentSubtidalMiddleOuter Rande Strait201140
VGS-06_7-1 (0-2)−24Seabed sedimentSubtidalMiddleOuter Rande Strait200640
MET-1-14 (0-2)−26Seabed sedimentSubtidalMiddleOuter Rande Strait201040
MET-1/12 (0-1)−28Seabed sedimentSubtidalMiddleOuter Rande Strait201040
MVR-5 (0-1)−28Seabed sedimentSubtidalMiddleOuter Rande Strait201140
MVR-3 (0-1)−30Seabed sedimentSubtidalOuterOuter Rande Strait201124
MVR-2 (0-1)−33Seabed sedimentSubtidalOuterOuter Rande Strait201140
V14-VC2 (0-1)−35Seabed sedimentSubtidalOuterOuter Rande Strait201440
MET-1/11 (0-1)−38Seabed sedimentSubtidalOuterOuter Rande Strait201040
MET-2/10 (0-1)−38Seabed sedimentSubtidalOuterOuter Rande Strait201040
VGS-06_1-1 (0-2)−38Seabed sedimentSubtidalOuterOuter Rande Strait200640
VGS-06-9-3 (0-2)−38Seabed sedimentSubtidalOuterOuter Rande Strait200640
MET-1-10 (0-1)−40Seabed sedimentSubtidalOuterOuter Rande Strait201040
MET-4/19 (0-1)−40Seabed sedimentSubtidalOuterOuter Rande Strait201040
MET-4/16 (0-1)−42Seabed sedimentSubtidalOuterOuter Rande Strait201140
Table 2. Synthesis of the main characteristics of the three upland lacustrine systems studied [50].
Table 2. Synthesis of the main characteristics of the three upland lacustrine systems studied [50].
Pond SPond LPond P
Surface covered by tree-canopy (%)801005
Altitude (m a.s.l.)387433458
ShapeElongatedEllipticIrregular
Total Surface (m2)25001004500
Position in the sub-basinLowIntermediateHigh
SeasonalityMarkedNoneLittle
Active OutflowWinter and springAll the yearNone
Local vegetationTypha angustifolia L., Lemna minor L., Callitriche stagnalis Scop., Alisma plantago-aquatica L., Juncus effusus L., Iris pseudacorus L., Alnus glutinosa (L.) Gaertn., Betula pubescens Ehrh., Fraxinus angustifolia Vahl, Salix atrocinerea Brot.Bryophytes, Osmunda regalis L., Dryopteris spp., Athyrium filix-femina (L.) Roth, Typha angustifolia L., Juncus effusus L. Potamogeton natans L.Salix alba L., Salix atrocinerea Brot., Betula alba L., Fraxinus excelsior L., Alnus glutinosa (L.) Gaertn., Typha angustifolia L., Phragmites australis (Cav.) Trin. Former Steud., Juncus effusus L., Taxodium distichum (L.)
Table 3. Pollen types included in each vegetation unit discussed.
Table 3. Pollen types included in each vegetation unit discussed.
Vegetation UnitsPollen Types Included
ScrublandsCalluna vulgaris; Erica; Corema; Helianthemum-type; Hedera; Ulex-type
Pastures and crops landsVitis; Alchemilla; Anchusa; Armeria maritima-type; Artemisia; Asphodelus; Asteraceae_Liguliflorae; Asteraceae_Tubuliflorae; Brassicaceae; Caryophyllaceae; Centaurea scabiosa; Campanula-type; Amaranthaceae; Echium; Glaux maritima; Geranium-type; Humulus lupulus-type; Allium-type; Lotus-type; Mentha-type; Pentaglotis sempervirens; Plantago; Polygonum amphibium; Potentilla-type; Rumex acetosa-type; Urtica; Poaceae; Cerealia-type; Zea; Sedum; Apiaceae; Umbilicus; Callitriche, Lythrum salicara
Eucalyptus standsEucalyptus
Coastal wetlandsCyperaceae; Isoetes; Myriophyllum; Ranunculus-type; Typha latifolia; Sphagnum; Iris pseudacorus-type; cf. Juncus; Anthoceros; cf. Ruppia
Conifers/mixed standsPinus subgenus Pinus
Exotic hardwoods standsAcacia
Native hardwoods standsAlnus; Betula; Castanea; Corylus; Platanus; Fraxinus; Ilex-type; Juniperus –type; Arbutus; Olea; Populus; Quercus robur-type; Salix; Sambucus nigra-type; Tilia
Table 4. Average pollen percentages for all the groups and subgroups of the studied samples (see Table 1) and percentages of vegetation coverages for different radii (<500 m; 2 km) and the complete Ría de Vigo (RdV) basin.
Table 4. Average pollen percentages for all the groups and subgroups of the studied samples (see Table 1) and percentages of vegetation coverages for different radii (<500 m; 2 km) and the complete Ría de Vigo (RdV) basin.
Lagoa dos NenosUpland PondsSeabed Samples
PollenCoveragePollenCoveragePollenCoverage
Vegetation Units%<500 m<2 km% S % L% P% O% ALL<500 m<2 km% All% Inner% Middle% Outer% Out
R. Strait		RdV
Basin
Scrublands2.0240.4956.914.4111.448.1026.1212.5215.252.668.827.319.299.859.6129.4
Pastures and crops 13.322.0610.644,134.599.688.066.6245.1050.4125.1827.6826.7321.1223.5330.5
Eucalyptus stands0.588.8820.180.981.071.630.401.022.452.190.490.650.590.220.3817.5
Coastal wetlands22.7915.433.10------ ------------16.408.1118.2322.8720.880.3
Conifers/mixed stands54.1113.149.1672.8957.6657.5257.3161.3531.2043.3137.0839.2434.1937.8236.2612.6
Native hardwood stands7.19------17.5625.2323.048.1018.480.380.7311.9716.8210.978.119.345.5
Exotic hardwood stands---------0.03---0.04---0.025.610.700.070.20---------4.3
Table 5. Analyses of differences in pollen percentages representing each vegetation unit across the different sample groups/subgroups. Significant differences (p < 0.05) are shown in orange, and no significant differences (p > 0.05) are in white. Non-normally distributed data (tested using Shapiro–Wilk test) is presented in bold. Lack of representation shown in gray.
Table 5. Analyses of differences in pollen percentages representing each vegetation unit across the different sample groups/subgroups. Significant differences (p < 0.05) are shown in orange, and no significant differences (p > 0.05) are in white. Non-normally distributed data (tested using Shapiro–Wilk test) is presented in bold. Lack of representation shown in gray.
% PollenLNUpland P PondS PondL PondO ChannelAll
RdV		InnerMiddleOuterOut
R. Strait		All 61 Samples
Scrublands<0.05<0.050.03<0.05<0.050.01<0.05<0.05<0.05<0.05<0.05<0.05
Pastures and crops<0.05<0.05<0.05<0.05<0.050.125<0.05<0.05<0.05<0.05<0.05<0.05
Eucalyptus stands0.02<0.05<0.05<0.050.030.09<0.05<0.050.050.125<0.05<0.05
Coastal wetlands<0.05---------------<0.05<0.050.01<0.05<0.05<0.05
Conifers/mixed stands<0.05<0.05<0.05<0.05<0.05<0.05<0.05<0.05<0.05<0.05<0.05<0.05
Exotic hardwood stands<0.050.251.000.05------0.050.25---------<0.05
Native hardwood stands<0.05<0.05<0.05<0.05<0.050.01<0.050.01<0.05<0.05<0.05<0.05
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Castro-Parada, A.; Cazás, N.; Cartelle, V.; Ferreiro da Costa, J.; Martínez-Carreño, N.; García-Gil, S.; Muñoz Sobrino, C. Potentials and Limitations of Fluviomarine Pollen Records to Reconstruct Spatiotemporal Changes in Coastal Ecosystems During the Holocene: A Case of Study from Ría de Vigo (NW Iberia). Land 2025, 14, 540. https://doi.org/10.3390/land14030540

AMA Style

Castro-Parada A, Cazás N, Cartelle V, Ferreiro da Costa J, Martínez-Carreño N, García-Gil S, Muñoz Sobrino C. Potentials and Limitations of Fluviomarine Pollen Records to Reconstruct Spatiotemporal Changes in Coastal Ecosystems During the Holocene: A Case of Study from Ría de Vigo (NW Iberia). Land. 2025; 14(3):540. https://doi.org/10.3390/land14030540

Chicago/Turabian Style

Castro-Parada, Alberto, Nerea Cazás, Víctor Cartelle, Javier Ferreiro da Costa, Natalia Martínez-Carreño, Soledad García-Gil, and Castor Muñoz Sobrino. 2025. "Potentials and Limitations of Fluviomarine Pollen Records to Reconstruct Spatiotemporal Changes in Coastal Ecosystems During the Holocene: A Case of Study from Ría de Vigo (NW Iberia)" Land 14, no. 3: 540. https://doi.org/10.3390/land14030540

APA Style

Castro-Parada, A., Cazás, N., Cartelle, V., Ferreiro da Costa, J., Martínez-Carreño, N., García-Gil, S., & Muñoz Sobrino, C. (2025). Potentials and Limitations of Fluviomarine Pollen Records to Reconstruct Spatiotemporal Changes in Coastal Ecosystems During the Holocene: A Case of Study from Ría de Vigo (NW Iberia). Land, 14(3), 540. https://doi.org/10.3390/land14030540

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