Marine Biodiversity in Inútil Bay (Tierra del Fuego): Patterns of Zooplanktonic and Benthic Assemblages

Abstract

Southern Patagonian ecosystems are characterized by high environmental heterogeneity. Within this context, Inútil Bay exhibits a complex geomorphology and only fragmentary information on its biodiversity, despite a long history of resource exploitation and increasing human pressures. The objective of this study was to establish a baseline of biodiversity focusing on three key trophic components: zooplankton, megabenthos, and macrobenthos. Samples were collected using both traditional and non-invasive methods, including a bongo net, ROV, and Van Veen grab. A total of 239 taxa were identified, comprising 32 zooplankton species, 61 megabenthic taxa, and 146 macrobenthic taxa. Alpha diversity indices revealed a spatial gradient, with higher mixed-level taxonomic richness near the Whiteside Channel. In contrast to patterns observed in zooplankton and megabenthos, the macrofauna showed significant differences between assemblages at stations located inside and outside the bay. Moreover, a low representation of meroplankton was recorded compared to the high abundance of adult benthic invertebrates. Overall, these results provide a biodiversity baseline, underscore the ecological vulnerability of Inútil Bay, and support its recognition as a priority area for conservation.

1. Introduction

Over the past century, the study of biodiversity has faced major challenges in both scientific and political spheres. Although significant progress has been made internationally, biodiversity remains a subject of intense debate with persistent knowledge gaps [1]. This situation is particularly critical in the sub-Antarctic regions at the southern tip of South America, where the available information on biodiversity is fragmentary, spatially restricted, and temporally discontinuous [2]. In this context, documenting marine biodiversity in areas under increasing pressure, mainly due to human presence, has become a priority, given the expansion of aquaculture concessions [3,4], the development of large-scale green hydrogen projects [5,6], and fisheries [7], the latter being one of the main economic drivers in the Magallanes region, where Inútil Bay is located.

Biodiversity studies in Inútil Bay are scarce, sporadic, and largely site-specific. For instance, much of the current knowledge of zooplankton originates from the CIMAR program (Marine Research Cruises in Remote Areas, in Spanish), which has been limited to a single station within the bay. These efforts quantified zooplankton biomass and ichthyoplankton [8,9], reporting low abundances (<65 ind·1000 m⁻³) and a limited presence of fish larvae. Additional work has addressed trophic interactions of early fish stages at the bay’s mouth and adjacent sectors [10]. While studies exist on specific zooplankton groups in Inútil Bay [8,9,10,11,12,13], no integrated assessment of zooplankton and ichthyoplankton biodiversity with broad spatial coverage is yet available. Such knowledge is essential to identify species, understand the factors controlling their dynamics, evaluate their role in the food web, particularly as prey for seabirds and marine mammals and inform management actions. This need is underscored by the bay’s long history of marine resource exploitation, which has included species of commercial interest with meroplanktonic phases such as the razor clam (Ensis macha), octopus (Enteroctopus megalocyathus), whelk (Trophon geversianus), and king crab (Lithodes santolla).

Research on benthic fauna, by contrast, has focused primarily on the abundance and richness of macrofauna, especially polychaetes [14,15,16], but also echinoderms [17,18,19], mollusks [15,18], and amphipods [16]. Recent studies have demonstrated ecological connectivity between Inútil Bay and Almirantazgo Sound, with Polychaeta, Mollusca, and Arthropoda as the most representative groups [16]. In recent years, however, the use of underwater imagery (photographs and video) has gained increasing prominence as a scientific tool for ecological assessment, owing to its non-destructive nature [20], reduced costs, and technological improvements [21]. Within southern Chile, important advances have been achieved through photographic surveys across fjords, the Beagle Channel, and the Strait of Magellan [22,23,24]. In northern Patagonia, studies in the Comau Fjord [25] and Puyuhuapi Fjord [26,27] have reported diverse benthic assemblages, while more recent efforts in the Katalalixar National Reserve have provided valuable new data [28]. Yet, to date, no study has applied such approaches in the extensive territory of Inútil Bay. This study therefore seeks to provide novel insights into the composition of its megabenthic communities.

Comparative analyses of sub-Antarctic marine communities involving more than two ecological groups sampled simultaneously remain rare. Most studies have focused on a single fraction of the ecosystem, offering a limited perspective that overlooks the interconnected nature of these complex systems. In this context, conducting an integrated study across a broad spectrum of biodiversity was made possible through the collaboration of experts in multiple marine taxa. Here, we present the results of expeditions conducted in 2024 in Inútil Bay, combining zooplankton surveys, non-invasive underwater video systems for megabenthic observations, and Van Veen grab sampling for benthic macrofauna. Our primary objective was to establish a baseline of marine biodiversity in the area, providing critical data to support a potential marine protected area proposal and to contribute to the conservation of this unique ecosystem within the Magallanes marine ecoregion [29,30].

2. Materials and Methods

2.1. Study Area

Inútil Bay (53°30′ S, 69°30′ W) is located on the northwestern coast of Tierra del Fuego Island, southern Chile. The bay originated during the Late Pleistocene, when deglaciation events about 14,000 years ago led to the retreat of the Darwin Range ice towards the east, forming a broad and topographically uniform basin. Inútil Bay is part of the Magdalena Sound–Puerto del Hambre–Paso Ancho sub-basin, where the combined action of strong tidal currents, persistent winds, and seafloor topography promotes a well-mixed water column that counteracts stratification caused by freshwater inputs [31]. As a result, local water masses are mainly defined by thermohaline properties modulated by continental runoff, tidal regime, and wind patterns.

The bay is semi-enclosed and southwestward oriented, extending approximately 100 km in length and 50 km in average width, with a total surface area of about 4776 km². Its mouth lies at Cape Boquerón, where maximum depths reach nearly 250 m near the Whiteside Channel. The rest of the bay shows a relatively uniform bathymetry, with depths generally around 25 m that gradually increase toward the mouth [15]. Inútil Bay connects directly to the Strait of Magellan through Cape Boquerón and to Almirantazgo Sound via the Whiteside Channel, which separates it from Dawson Island (Figure 1).

Figure 1. Study area maps. Left panel: (A). Geographical location of Inútil Bay in southern Chilean Patagonia. Right panel: detailed view of the study area showing the numbered sampling stations; green dots indicate stations inside the bay, while red dots represent external and adjacent sites. Each symbol denotes the methodology applied to study community assemblages: (B). Zooplankton (Bongo net sampling), (C). Megabenthic fauna (ROV DTG3), (D). Soft-bottom macrofauna (Van Veen grab).

The system receives substantial freshwater inputs from at least ten rivers, most of them located on the eastern side of the fjord, including the Rosario, Esperanza, and Discordia (northern shore); Marazzi and Centenario (southeastern shore); and Torcido, Macklelland, Ana, Blanco, and Woodsend (southern shore).

Sediment composition varies across the bay. Sandy sediments dominate the northern and eastern shores, whereas mixed sediments, sand with mud and muddy sand, are more common along the southern coast. More broadly, the seafloor consists mainly of mud interspersed with sand and gravel, reflecting high structural heterogeneity [32]. Oceanographically, the northern sector of Inútil Bay exhibits surface temperatures exceeding 6.5 °C. Mean surface salinity is 30.46 g kg⁻¹, with values above 31.0 g kg⁻¹ recorded at the head of the bay. At depths greater than 100 m, salinity ranges between 31.0 and 32.0 g kg⁻¹ [33].

Fieldwork was carried out from 10 to 14 July 2024, covering Inútil Bay and adjacent areas (Figure 1). Sampling included: (i) zooplankton collection (Figure 1A); (ii) visual surveys of megabenthic assemblages using remotely operated vehicle (ROV) transects (Figure 1B); and (iii) sampling of soft-bottom-associated macrofauna (Figure 1C).

2.2. Zooplankton Sampling

Zooplankton sampling was conducted along transects at stations located both inside and outside Inútil Bay (Figure 1B). At each station, oblique tows were performed from 100 m depth to the surface, or to 10 m above the seafloor in shallow areas (Table A1), using a 60 cm diameter Bongo net fitted with a 300 µm mesh and a TSK flowmeter to estimate filtered water volume. Tows lasted 20–35 min depending on site depth. To ethically euthanize any fish larvae that might have been present in the samples, benzocaine (BZ-20^®, Veterquímica, Región Metropolitana, Chile) was added to the sample prior to fixation in 5% buffered formalin with sodium borate. After 24 h, the samples were transferred to 70% ethanol for laboratory analysis. Each sample was thoroughly examined under a Leica EZ4 stereomicroscope (Leica Microsystems, Wetzlar, Germany). Zooplankton organisms were counted and identified to the lowest possible taxonomic level using specialized literature [34,35,36] and categorized as either holoplankton or meroplankton [37].

2.3. Megabenthic Sampling

Megabenthic invertebrates (>10.0 mm) [38] were surveyed through transects conducted with remotely operated vehicles (ROVs) at depths ranging from 25 to 150 m (Figure 1C; Table A1). Two ROV models were used: DTG3 (Deep Trekker, Kitchener, ON, Canada) and BlueROV2 (Blue Robotics, Torrance, CA, USA), deployed from separate vessels, each equipped with an echosounder, a portable unit on the auxiliary vessel and a fixed unit on the main vessel. Echosounders were used to determine depth and locate areas with homogeneous substrates. Each transect lasted 10 min at a constant speed of 10 m·min⁻¹, covering approximately 100 m in length, with the ROVs maintained 1 m above the seafloor to ensure focus and stability of the video recordings.

Video footage was processed using the online platform BIIGLE (Bio-Image Indexing and Graphical Labelling Environment) [39]. After uploading the recordings to the “Storage” module (which required approximately 24 h for validation), a main project was created, assigning separate “Volumes” to each locality. Taxonomic identification was performed by configuring a list of “Labels” with standardized nomenclature for each species or group of interest. Videos were systematically reviewed, and every organism observed along the transects was annotated using the platform’s labeling tools and the predefined “Labels” list.

2.4. Macrobenthic Sampling

Sediment samples were collected using a 0.15 m² Van Veen grab. A total of 22 samples were obtained at eight stations, at depths ranging from 28 to 50 m (Figure 1D; Table A1). Each station was sampled once with two replicates, except for station 5, where only one sample could be collected.

For the analysis of benthic macrofauna (>1.0 mm [or >0.5 mm]) [39], samples were sieved on site through a 0.5 mm mesh and preserved in 10% buffered formalin (with borax).

In the laboratory, samples were re-sieved through a 0.5 mm mesh and examined in fractions under an Olympus SZ61 stereomicroscope (Olympus, Tokyo, Japan). Organisms were identified to the lowest possible taxonomic level using specialized literature [40,41,42,43,44,45,46].

All individuals were counted, except those with clonal growth (e.g., sponges and ascidians), which were recorded as presence/absence only and excluded from the statistical analysis.

2.5. Statistical Analyses

Alpha diversity was quantified as mixed-level taxonomic richness, Simpson’s diversity (1—D; hereafter D′), and Pielou’s evenness (J′). D′ considers both mixed-level taxonomic richness and relative abundance, while J′ measures the evenness of individuals among taxa (range: 0 = maximum inequality; 1 = maximum evenness). Calculations were performed in PAST v4.03 [47]. Because taxa were identified at mixed hierarchical levels (from phylum to species), richness was interpreted as mixed-level taxonomic richness following [48], rather than strict species richness.

Zooplankton abundances were standardized to individuals per 1000 m³ (ind. 1000 m⁻³) and log-transformed (log₁₀ (x + 1)) for multivariate analyses, while spatial distributions were expressed as individuals per m² (ind. m⁻²). For megabenthic communities, absolute abundance (MaxN), defined as the maximum number of individuals observed per station in ROV transects, was used. In the case of soft-bottom macrofauna, abundances were standardized to individuals per square meter (ind. m⁻²) and fourth-root transformed (x^0.25).

Based on abundance data of zooplankton and benthic communities, a one-way PERMANOVA using the Bray–Curtis similarity matrix was applied to assess differences in community composition. In addition, non-metric multidimensional scaling (nMDS) using the same Bray–Curtis similarity matrix on abundance data was performed to visualize spatial patterns in zooplankton, megabenthic, and macrobenthic communities. Furthermore, percentage similarity analysis (SIMPER) was applied to decompose variability in community composition and identify the species contributing most to significant differences among groups. All multivariate analyses were conducted in PRIMER-e v7 [49].

3. Results

3.1. Zooplankton Assemblages

A total of 32 taxa were identified from 7 phyla, of which 25 corresponded to holoplankton and 7 to meroplankton (Table A2). Within the holoplankton, the most abundant taxa were crustaceans of the order Ostracoda, representing 51% of the total abundance (median, 25–75% quartiles; 0.055, 0.012–6.653 ind. m⁻³), the copepod Clausocalanus brevipes with 20.8% dominance (0.22, 0.078–0.827 ind. m⁻³), and Clausocalanus arcuicornis with 6.2% dominance (0.017, 0.009–0.351 ind. m⁻³). The high abundance of ostracods was due to a single collection at station 8 (9760 specimens, 13.2 ind. m⁻³).

In contrast, meroplankton exhibited very low abundances. The most representative taxa were zoeae of Grimothea gregaria, with 0.28% dominance (0.002, 0.001–0.02 ind. m⁻³; Table A2), cyphonauta larvae of bryozoans, with 0.20% dominance (0.008, 0.005–0.017 ind. m⁻³), and crustacean larvae in the mysis stage, with 0.10% dominance (0.005, 0.002–0.013 ind. m⁻³). In addition, the only representative of ichthyoplankton was a postflexion larva of the icefish Champsocephalus esox, which constitutes the first record of an early life stage of this species in the literature (The specimen is deposited in the Museo Nacional de Historia Natural, Santiago, Chile (MNHNCL), under catalog number MNHN-ICT 7737; Figure A1).

Mixed-level taxonomic richness ranged from 6 taxa at station E1 to 15 taxa at E2. Simpson diversity varied between 0.4 (E8) and 0.8 (E7), showing an increase toward the inner part of the bay (Figure 2A). The highest abundances (>1000 ind. m⁻²) were concentrated at stations most exposed to the Whiteside Channel (E8, E1, and E3), while the lowest abundances (<100 ind. m⁻²) occurred in scattered sites within Inútil Bay (E6, E4, E10, and E7; Figure 2B). Evenness (J′) fluctuated between 0.41 (E8) and 0.89 (E6), mirroring the pattern of Simpson diversity. Stations in the central-northern sector of the bay showed the highest levels of uniformity (>0.8) and also the greatest diversity values (Figure 2C,D).

Figure 2. Characteristics of the zooplankton assemblage across sampling stations. (A). Mixed-level taxonomic richness, (B). Number of individuals (ind. m⁻²), (C). Simpson diversity, (D). Pielou’s evenness.

The non-metric multidimensional scaling (nMDS) analysis identified three main groups with 60% similarity. Stations located outside the bay were more similar to each other and to those situated at the southern margin of the bay’s entrance. In contrast, stations at the northern margin and the head of the bay showed higher similarity among themselves. Station E8 stood out as the most dissimilar, segregating primarily due to the high abundance of ostracods (Figure 3A,B). The assemblages did not show significant differences associated with areas (PERMANOVA pseudo-F = 1.799, p = 0.0745). The abundance of copepods such as Clausocalanus arcuicornis and Clausocalanus brevipes contributed significantly to the differentiation of these stations, while species such as Themisto gaudichaudii, Candacia sp., and Subeucalanus sp. showed distribution patterns associated with the more exposed stations (Figure 3C,D).

Figure 3. Non-metric multidimensional scaling (nMDS) of zooplankton taxa abundances in Inútil Bay. Abundances were standardized to individuals per cubic meter (ind. m⁻³) and log-transformed [log₁₀ (x + 1)]. Bubble size is proportional to the abundance of each taxon at each station; the values in the side legend indicate abundance (ind. m⁻³). Contours represent Bray–Curtis similarity groups obtained through hierarchical cluster analysis (outer: 40%; inner: 60%). A contour enclosing a single station indicates a unique group (singleton) at that similarity level. Distribution of the species contributing most to the assemblage structure: (A) All stations, (B) Ostracoda, (C) Clausocalanus brevipes (Copepoda), (D) Clausocalanus arcuicornis (Copepoda).

3.2. Megabenthic Assemblages

A total of 61 taxonomic groups were identified; of the total taxa, 32 were sessile and 29 non-sessile, including 12 Crustacea, 11 Mollusca, and 9 Echinodermata (Table A3). The squat lobster Grimothea gregaria was the most dominant species (Figure 4B), representing 55.5% of total abundance (median, 25–75% quartiles; 375.0, 185.25–641.0 ind. 10 m·min⁻¹). The scallop Zygochlamys patagonica ranked second, accounting for 16.4% of dominance (165.0, 8.0–289.5 ind. 10 m·min⁻¹), followed by the tube-dwelling polychaete Chaetopterus variopedatus with 7% dominance (42.5, 3.25–90.25 ind. 10 m·min⁻¹; Table A3).

Figure 4. Remotely operated vehicle (ROV) images of the benthic community. (A). Specimen of Doryteuthis gahi; (B). Aggregation of Grimothea gregaria; (C). Colonies of Actinostola sp.; (D). Specimen of Caridea and Actinaria.

The spatial pattern showed a decline in alpha diversity indices toward the more sheltered and inner sector of the bay. Mixed-level taxonomic richness ranged from 11 to 28 taxa, with the highest values recorded at stations E1 (27 taxa), E5 (26), E4 (25), and E2 (23), located in the Whiteside Channel and the western margin of Inútil Bay (Figure 5A). In contrast, the innermost stations, such as E8, E9, and E10, exhibited the lowest richness (11–15 taxa). Similarly, the highest abundances (>1000 individuals) were concentrated at stations most exposed to the channel (E1–E5), while E8 and E9 registered the lowest abundances (<500 individuals; Figure 5B). Simpson diversity was highest at stations E1, E4, E5, E6, and E10 (D′ > 0.5), reflecting more balanced communities, whereas lower values (D′ < 0.5) were observed at E3, E7, and E9, indicating dominance by a few species (Figure 5C). Overall, diversity also decreased toward the inner sector of the bay. Evenness (J′) showed relatively homogeneous values among stations, ranging from 0.13 (E2, E3) to 0.28 (E4; Figure 5D).

Figure 5. Characteristics of the megabenthic assemblage across sampling stations. (A). Mixed-level taxonomic richness, (B). Number of individuals (MaxN, 10 m·min⁻¹), (C). Simpson diversity (H′), (D). Pielou’s evenness (J′).

The non-metric multidimensional scaling (nMDS) analysis identified three main clusters with 60% similarity. Stations located outside the bay were more similar to each other and to those at the southern entrance. In contrast, stations E6, E7, and E9, situated on the northeastern margin of the bay, formed a distinct group. Station E8 was segregated from the rest, primarily due to the low abundances of Grimothea gregaria and Chaetopterus variopedatus, together with the absence of Zygochlamys patagonica (Figure 6B). Although variations in the occurrence of these species were observed between the inner bay and outer areas, no statistically significant differences were detected in assemblage composition PERMANOVA pseudo-F = 1.432, p = 0.186.

Figure 6. Non-metric multidimensional scaling (nMDS) of the abundance of soft-bottom megabenthic taxa in Inútil Bay. Abundances were expressed as Nmax (maximum counts per taxon across transects). Bubble size is proportional to taxon abundance at each station; side legend values indicate Nmax counts. Contours denote Bray–Curtis similarity groups derived from hierarchical clustering (outer: 40%; inner: 60%). A contour enclosing a single station indicates a singleton group at that similarity threshold. Distribution of the species contributing most to the assemblage structure: (A) All stations, (B) Grimothea gregaria, (C) Zygochlamys patagonica, (D) Chaetopterus variopedatus.

3.3. Macrobenthic Assemblages

A total of 146 taxa were recorded, distributed across 10 phyla. The most diverse and abundant group was Annelida, with 80 taxa (median = 2318.2 ind.m⁻²), followed by Crustacea (31 taxa; median = 224.45 ind.m⁻²) and Mollusca (23 taxa; median = 207.7 ind.m⁻²). Within Annelida, the assemblage was dominated by Aricidea spp., contributing 19.6% of the total abundance (median = 348.4; 25–75% quartiles: 58.6–842.5 ind.m⁻²; Table A4), followed by Tharyx spp. with 13.5% (244.6; 22.5–529.3 ind.m⁻²) and Aphelochaeta spp. with 6.6% (13.4; 6.7–469 ind.m⁻²). Among Crustacea, Monocorophium sp. was the most representative taxon, accounting for 1.8% of the total abundance (6.7; 6.7–13.4 ind.m⁻²), while Fuegiphoxus sp. and Urothoe falcata reached 0.26% (53.6; 6.7–33.5 ind.m⁻²) and 0.23% (6.7; 6.7–33.5 ind.m⁻²), respectively. In Mollusca, the dominant species was Thyasira sp., contributing 3.9% (26.8; 12.8–368.5 ind.m⁻²), followed by Pseudoneilonella sp. with 1.3% (26.8; 13.4–110.6 ind.m⁻²) and Yoldiella sp. with 1.1% (30.2; 5.0–139.0 ind.m⁻²). Additionally, the brachiopod Magellania venosa reached 4.1% dominance (77.1; 3.5–244.6 ind.m⁻²), while Nematoda represented 10.4% of the assemblage (50.3; 26.8–335 ind.m⁻²), highlighting its importance in the innermost and more protected stations.

The spatial pattern showed a decrease in alpha indices toward the more protected and inner sector of Inútil Bay. Mixed-level taxonomic richness (S) ranged from 25 to 61 taxa, with the highest values at E1 (61 taxa), E2 (41), and E3 (39). In contrast, the innermost stations, such as E6 (25), E7 (28), and E8 (29), exhibited the lowest richness (Figure 7A). Total abundance ranged from 978 to 5206 ind.m⁻², with maxima at E1 (5206 ind.m⁻²) and E8 (4663), despite their differences in richness, whereas E5 (978) and E2 (1206) recorded the lowest densities (Figure 7B). Simpson diversity index (D′) remained high, with values close to 1 at almost all stations (>0.79; Figure 7C), reflecting dominance by few species (Figure 7C). Evenness (J′) followed this pattern, with higher values at E2 (0.6), E5 (0.5), and E7 (0.5), and reduced values at E1, E6, and E8 (0.3; Figure 7D).

Figure 7. Characteristics of the soft-bottom macrobenthic assemblage among sampling stations. (A). Mixed-level taxonomic richness, (B). Number of individuals (ind.m⁻²), (C). Simpson diversity, (D). Pielou’s evenness (J′).

The NMDS analysis (stress = 0.02) revealed a clear spatial differentiation in the composition and abundance of dominant taxa among the sampled stations, with well-defined groupings at similarity levels of 60% (blue dashed lines) and 40% (green lines; Figure 8A). The most exposed stations of the bay (E1–E4) clustered with high similarity (≥60%), characterized by high abundances of the polychaete Tharyx sp. (especially in E1; Figure 8C) and the brachiopod Magellania venosa. In these stations, Aricidea sp., Capitella sp., and Nematoda were scarcely represented. In contrast, the more sheltered and inner stations (E6–E8) formed a well-defined cluster at ≥60% similarity, dominated by Capitella sp. and Aricidea sp. (Figure 8B–D), both reaching maximum abundances at E6 and E8, along with a high representation of Nematoda (Figure 8F), suggesting more restricted or environmentally stressed conditions. Station E5 did not cluster with any other under the similarity thresholds considered, indicating a unique faunal composition and possibly a distinct or transitional environmental condition. Significant differences were detected between assemblages at stations located outside and inside the bay (PERMANOVA, pseudo-F = 4.013, p = 0.0186). Dissimilarity was mainly driven by Tharyx spp., Magellania venosa, and Pholoe sp. (SIMPER, Table 1).

Figure 8. Non-metric multidimensional scaling (nMDS) of the abundance of soft-bottom macrobenthic taxa in Inútil Bay. Abundances were standardized to individuals per square meter (ind. m⁻²) and fourth-root transformed (x^0.25). Bubble size is proportional to taxon abundance at each station; side legend values indicate abundance (ind.m⁻²). Contours denote Bray–Curtis similarity groups from hierarchical clustering (outer: 40%; inner: 60%). A contour enclosing a single station indicates a singleton group at that similarity threshold. Distribution of the species contributing most to the assemblage structure: (A) All stations, (B) Aricidea spp. (Polychaeta), (C) Tharyx sp. (Polychaeta), (D) Capitella sp. (Polychaeta), (E) Magellania venosa (Brachiopoda), (F) Nematoda.

Table 1. Similarity Percentage (SIMPER) analysis results for the comparison of dissimilarity (Overall average dissimilarity = 60.03%) of the benthic macrofauna contributing >1.5% to the total dissimilarity inside and outside Inútil Bay, Tierra del Fuego.

Taxa	Av. Dissim	Contrib. %	Cumulative %	Mean Outside	Mean Inside
Tharyx spp.	1.568	2.612	2.612	1.41	4.56
Magellania venosa	1.41	2.348	4.96	3.49	0.536
Pholoe sp.	1.262	2.102	7.063	0	2.55
Kirkegaardia spp.	0.9944	1.657	8.719	4.4	2.78
Lepidozona sp.	0.9777	1.629	10.35	2.01	0
Levinsenia sp.	0.9569	1.594	11.94	3.46	5.17
Yoldiella sp.	0.9061	1.509	13.45	2.22	0.902
Eunireis patagonica	0.9034	1.505	14.96	0	1.82
Nematoda	0.9006	1.5	16.46	2.77	4.19

4. Discussion

The present study is not only the first to provide high-resolution biological information from Inútil Bay but also one of the few to encompass both benthic and zooplanktonic communities in Patagonian bays. This information was derived from the analysis of high-resolution data on taxonomic composition and abundance obtained through simultaneous sampling of mesozooplankton and benthic fauna in Inútil Bay and adjacent areas. The combined use of dredging and ROV-based observations, an approach scarcely applied in the region, allowed for a more comprehensive characterization of benthic assemblages, expanding habitat coverage and reducing the likelihood of underestimating biodiversity. This complementary approach proved particularly valuable for detecting both highly mobile species, which typically evade passive sampling, and those associated with or buried in the substrate [15].

The mesozooplankton community exhibited low abundance but higher diversity and evenness in the northeastern sector of the bay; however, no clear zonation pattern was observed. The mixed-level taxonomic richness totaled 32 taxa, exceeding the 21 taxa previously reported during summer by Zagami et al. (2011) [11]. As in other Patagonian areas, the community was characterized by low diversity and marked dominance [16,50,51,52]. Copepods, mostly of sub-Antarctic distribution, dominated the assemblage, with Clausocalanus brevipes (median: 0.220 ind. m⁻³) as the most abundant holoplanktonic species, typical of coastal systems of Patagonia and Antarctica [52,53]. Other species included Clausocalanus arcuicornis (median: 0.017 ind. m⁻³) and Acartia sp. (median: 0.028 ind. m⁻³). In addition, copepods of the order Monstrilloidea were recorded, parasites of polychaetes and bivalves in early stages (nauplius to juvenile), whose adults detach from the host to reproduce in the water column [54]. Among these, Monstrilla sp. and Cymbasoma sp. were detected, the latter with only a few records in southern Patagonia [55]. Other groups, such as ostracods and siphonophores, reached high densities at a single station, likely favored by local factors such as vertical mixing [56]. Although previous studies proposed that the Magdalena Sound–Puerto del Hambre–Paso Ancho sub-basin supports relatively stable zooplankton assemblages, the dominant species in this study differed from earlier reports [53,57], suggesting that Inútil Bay may exhibit oceanographic conditions that promote distinct planktonic communities. Regarding ichthyoplankton, previous studies [9] reported only fish eggs and no larvae [10], whereas in the present study a single early stage was recorded: a postflexion larva of Champsocephalus esox, representing the first record of a larval stage for this little-known species.

In contrast, both megabenthic and macrobenthic assemblages showed the highest taxonomic richness, diversity, and evenness outside Inútil Bay and at stations more exposed to the Whiteside Channel. These spatial patterns appear to be linked to environmental differences associated with exposure, bathymetry, and sediment type [28,50,57], together with functional traits of the taxa (e.g., trophic preferences, dispersal strategies) [58,59] that enhance settlement success in certain substrates over others. Research on benthic communities in Inútil Bay has been scarce and intermittent, beginning in the 1990s with one of the first analyses of sublittoral macrobenthos, focused exclusively on Polychaeta, with a total of 37 species [14]. Almost a decade later, the first study including the entire macrofaunal assemblage broadened the taxonomic scope, recording 173 taxa [8]. More recently, the macrobenthic community of Inútil Bay and its functional connectivity with Almirantazgo Sound were described in detail [13]. In the present study, 211 benthic taxa typical of soft-bottom habitats were identified, including macrofaunal and megafaunal species, some of high ecological relevance as habitat formers (Chaetopterus variopedatus, Apomatus sp., Magellania venosa, Mytilus sp.), and others of commercial value for local communities (Lithodes santolla, Zygochlamys patagonica, Grimothea gregaria). The latter forms large aggregations [60], that have emerged as a potential fishery resource; however, their harvest also involves at least 44 associated species [61], largely composed of filter feeders, suspension feeders, and detritivores [22].

Previous studies reported lower benthic abundances compared to those observed here. Gambi and Mariani (1999) [14] found an average of 184 individuals considering only polychaetes, dominated by Prionospio (Minuspio) sp. (24 ind.), Onuphis pseudoiridescens (21 ind.), and Ninoe falklandica (19 ind.). Thatje and Brown (2009) [15] estimated mean densities of 1287 ind. m⁻², with Polychaeta as the dominant group at all stations, particularly Aricidea sp. 1 (604 ind. m⁻²), Cauleriella sp. (480 ind. m⁻²), Minuspio patagonico (438 ind. m⁻²), and Tharyx sp. (386 ind. m⁻²). Among mollusks, Mysella sp. (333 ind. m⁻²), Yoldiella valettei (126 ind. m⁻²), and Antistreptus magellanicus (83 ind. m⁻²) were recorded, while Amphipoda included Heterophoxus videns and Urothoe falcata. More recently, Jara et al. (2024) [16] reported a mean abundance of 888.9 ± 26.8 ind. m⁻², with Polychaeta as the most numerous group (440.0 ± 89.0 ind. m⁻²), followed by Mollusca (243.4 ± 304.7 ind. m⁻²) and Arthropoda (93.8 ± 75.4 ind. m⁻²). By comparison, the present study recorded a median of 3283 ind. m⁻², substantially higher than previous estimates, while maintaining the dominance of polychaetes, particularly Aricidea spp. and Tharyx spp., already recognized as dominant taxa and continuing to play a structural role in the macrobenthos of Inútil Bay. The higher benthic abundances observed in this study may be attributed to differences in sampling design and local environmental conditions rather than to real temporal variations. While the studies by Gambi and Mariani (1999) [14] and Thatje and Brown (2009) [15] were conducted in summer, the present sampling was carried out in winter (July 2024). Factors such as the use of a finer mesh (0.5 mm), a larger sampling area, and standardized replication could explain the higher abundances recorded. Their spatial distribution appears to be driven by bathymetric and sedimentological variability, unlike other benthic communities in the region.

Sampling also revealed a high abundance and diversity of adult benthic invertebrates but a scarce representation of early meroplanktonic stages, which likely reflects seasonal reproductive patterns typical of austral winter conditions. Among the main representatives were bryozoan cyphonaut larvae and Grimothea gregaria zoeae at different stages, taxa commonly reported and often dominant in coastal meroplankton during the austral winter in southern Patagonia [53,60,61]. However, no polychaete larvae were recorded, in contrast to the high abundance and taxonomic richness of this group in the local benthos. A likely explanation is that many benthic species exhibit strongly seasonal reproductive cycles, concentrating larval release in austral spring and summer, synchronized with oceanographic processes that ensure food availability during the planktonic phase [62]. The scarcity of meroplankton, together with the absence of emergent benthos at shallow stations (<75 m), suggests limited benthopelagic interaction during the austral winter in Inútil Bay, as reflected by the low representation of larval stages in the plankton relative to the high abundance and diversity of adult invertebrates in the benthos. In this context, anthropogenic impacts during colder periods, when larval recruitment is limited, could have significant consequences for community structure and dynamics.

Although the area currently shows some degree of human disturbance, its proximity to aquaculture-suitable areas, the growing fishing effort in the region [63], interest in large-scale energy projects [64,65], and the development of unregulated tourism [66] expose it to considerable risks of ecological degradation. These activities may cause habitat loss and fragmentation [67,68], the introduction of invasive species [69], pollution [70], overexploitation of resources, and disruption of key ecological processes, affecting both benthic and pelagic communities.

A comprehensive assessment of biodiversity in Inútil Bay is therefore essential to establish conservation baselines, implement adaptive management, and prevent irreversible impacts. Documenting and understanding local biodiversity thus becomes a critical step toward recognizing the bay as a conservation priority and ensuring the provision of socio-ecosystem services.

5. Conclusions

This study provides the first integrated characterization of the marine communities of Inútil Bay, revealing high biodiversity and a marked spatial structure. A total of 211 benthic and 32 zooplanktonic taxa were recorded, dominated by polychaetes (Aricidea spp., Tharyx spp.) and copepods (Clausocalanus spp.). Macrobenthic assemblages showed significant differences between inner and outer stations of the bay. Inner areas exhibited lower richness and diversity, whereas sites more exposed to the Whiteside Channel harbored more diverse and even communities. The low presence of larval stages in the plankton suggests weak benthopelagic connectivity during the austral winter, associated with reduced seasonal reproductive activity. Inútil Bay retains high ecological value but faces increasing pressures from human activities.

For future studies, it would be valuable to repeat surveys with a comparable design during other seasons of the year to contrast seasonal patterns of biodiversity and benthopelagic connectivity. Additionally, incorporating oceanographic and sedimentological data would allow for a more precise understanding of the relationships between biological composition, ecological indices, and the environmental conditions structuring the marine communities of Inútil Bay.