1. Introduction
Ecological connectivity is regulated by ecosystem boundaries, which function as semi-permeable points of interaction between distinct ecosystems [
1]. According to Cadenasso [
2], ecosystem boundaries, defined as zones of transition between contrasting systems, can function as ecological filters for habitats, materials or organisms. However, the role of ecosystem boundaries formed along environmental gradients [
3] depends on the nature of the neighbouring areas or systems that allow only some fraction of materials, energy or organisms to pass [
4]. In our study we treat connectivity between land and sea (land-sea interface) as the introduction of organic matter into the coastal system through soil microarthropods that top-down control the activity of microorganisms in decaying macroalgae.
Coastal transitional zones that occur at the interface of land and sea [
5] provide regulation of the fluxes of nutrients, water, particles and organisms to and from the land and the ocean [
6] and support a critical habitat for a wide range of both marine and terrestrial biodiversity [
7]. Several studies highlight the importance of ecological connectivity and resource transfer from marine to terrestrial ecosystems across ecosystem boundaries for maintaining productivity and diversity at landscape scale [
8], including either the distributions of wrack cover in the coastal recipient ecosystem [
9], the flow of microbially mediated organic material or both [
10].
On the coasts of the polar regions, accumulated onshore macroalgae debris, mainly derived from kelp forests [
11], is a material that crosses habitat boundaries and subsidises coastal ecosystems, including residential populations. This material functions as a semi-permeable point of interaction between marine and terrestrial ecological systems and has an important effect on the diversity and distribution pattern of biota [
10,
12,
13], including a considerable bottom-up effect on detritivorous communities [
14,
15]. Nonetheless, the knowledge of the interconnection and interdependence of marine, coastal and terrestrial ecosystems, including the importance of detritivore communities facilitating the introduction of organic matter into neighbouring ecosystems, remains unclear. In particular, little is known about how deposited onshore macroalgal detritus, which represent cross-boundary subsidies, are either permeable to soil microarthropods, impact their distribution pattern and diversity or both.
Soil microarthropods, by initiating the process of detrital decomposition (shredding, consuming and transforming), accelerate microbial decomposition and nutrient cycling and mediate substantial fluxes between the aboveground and belowground components of terrestrial ecosystems [
16]. The decomposition process is particularly important in the Arctic region, where invertebrate communities create relatively simple trophic nets sensitive to any disturbances [
17,
18]. Among soil microarthropods of the Arctic region, Collembola are the relevant part of the decomposing food web involved in the recycling of organic matter and nutrients in soil [
19]. Belowground food web models usually place Collembola as feeding generalists [
20]; however, fungi, bacteria and algae may prevail in their diet [
19,
21]. This triggers top-down control of the decomposer community (i.e., bacteria and fungi) and indirectly may affect soil processes such as organic matter decomposition [
19,
21] and contribute to soil structure and humus formation [
22]. Collembola are also useful models for ecological studies, as their abundance and distribution pattern of species are strongly dependent on individual tolerance limits to environmental conditions [
23,
24].
Several collembolan species are known to live in either marine wrack material, salt marshes, littoral zones, or a combination [
25,
26,
27]. These terrestrial colonisers of intertidal habitats, resistant to stress and exposed to pronounced environmental fluctuations, are often found in the intertidal environment in very large numbers [
28]. Although collembolan species are terrestrial, some of them have adapted to marine habitats where environmental parameters are constantly changing [
29]. Collembola also represent one of the most abundant and important taxa of soil invertebrates inhabiting tundra soils [
18,
30]. Their adaptations to the harsh climatic conditions of polar and subpolar areas [
31,
32] allow them to survive low winter temperatures [
33] and avoid the negative impact of sub-Arctic climate changes induced by drought through either migrating to wetter sites both vertically, horizontally or both [
34], through limiting evaporative water loss due to their impermeable cuticles or in combination [
35,
36]. However, not much is known about either the distribution pattern and permeability, colonisation of the tundra soil microarthropods, including Collembola, to deposited onshore macroalgae debris in such harsh climatic conditions of polar and sub-polar areas, or both.
The current study focuses on the structure and distribution pattern of the collembolan communities along the environmental gradient from costal tundra to living macroalgae and decaying macroalgae debris deposited across the sub-Arctic coastline (Murman coast). There is well-documented coastal habitat connectivity for Collembola [
37,
38,
39]; however, the colonisation patterns of macroalgae by collembolan species has been poorly investigated in subpolar and polar regions [
30,
40,
41]. It should be emphasised that the importance of macroalgae for the diversity and distribution pattern of soil microarthropods, including Collembola, resulting from the interaction between terrestrial and marine ecosystems of the Barents Sea, is not yet known.
To address this knowledge gap, we investigated colonisation patterns of macroalgae by collembolan communities across environmental gradients from coastal tundra to macroalgae (aged macroalgae debris accumulated onshore and living macroalgae freshly exposed by the outflow). We hypothesised that (i) soil microarthropods of the coastal tundra zone, including terrestrial Collembola, can cross the ecosystem boundary and colonise marine macroalgae; and (ii) various inundation regimes by sea water, microhabitat stability and decaying of macroalgae drive distribution patterns of collembolan species.
2. Materials and Methods
2.1. Study Area and Sampling Processing
The study was conducted on the shoreline of the Murman coast (northwest Russia) of the Barents Sea near the Dalnye Zelentsy settlement (69°7′ N, 36°3′ E) located on the Kola Peninsula (
Figure 1). The Barents Sea is a sub-Arctic ecosystem located between 70 and 80° N, connected with the Norwegian Sea to the west and the Arctic Ocean to the north. The salinity of the offshore waters washed onto the Murman coast is about 34% [
42] A wide development of the polar front phenomenon and vertical water circulation are the basis of high biological productivity in the sea and high richness of its pelagic and bottom life required for dense growth of kelp forests [
43]. They are deposited on the coastal line as spatial subsidies (macroalgae debris) [
44].
The northwest Murman coast is characterised by the presence of rocky areas and fjords and is covered with vegetation characteristic of the sub-Arctic tundra [
45]. The climate of the Murman coast is rather mild because of the Gulf Stream and is characterised by a humid, cool summer and a relatively mild winter, so the frost-free season lasts about 120 days [
45]. The warmest month is July, and the average temperature ranges from 9 to 11 °C; the coldest month is February, which is characterised by an average temperature of about −10 °C [
45]. The annual precipitation ranges from 300 to 400 mm [
45]. The territory is affected by strong winds and drastically changeable weather [
30]. The average temperature, precipitation and wind force in July 2010 and 2013 are given in
Table 1.
The study was carried out in the bays located along the Murmansk coast (
Figure 1). Dalne-Zelenetskaya Bay (B1) is characterised by a relatively mild onshore slope, with a coastal tundra overgrown mainly with black crowberries, mosses, lichens and grasses, dwarf birch and willows. Plohye Chevry (B2) is stony, with fragments of coastal tundra that function as sea meadows with numerous grasses, sedges and perennials resistant to salinity that have penetrated into the tidal zone and are separated from the tidal zone by a steep fjord. Medvezhya Bay and Parchniha Bay (B3 and B4, respectively) are characterised by a gentle onshore slope, with a coastal tundra covered by mosses, grasses and sedges, respectively, and tundra vegetation at higher elevations not inundated by tidal water that consists of lichens, black crowberries and dwarf birch. Yarnyshnaya Bay, unlike all the previous bays, cuts deeply into the land. The study sites were located on the east and west coasts of the bay, due to the differences in the onshore slope and the vegetation overgrowing them. The eastern side of the bay, with a gentle slope, is covered mainly with a thick layer of black crowberries, lichens, mosses and dwarf birches (B5), while the western side is covered mainly with lichens and mosses with the addition of dwarf birch, often visited by sea birds (B6). Within each bay, sampling was performed across environmental gradients from coastal tundra to decaying macroalgae debris not inundated by tidal water to living macroalgae freshly exposed by outflow.
Sampling was carried out in July of 2010 and 2013 (after the spring tide). On each occasion, the samples were collected from the non-inundated sea water coastal tundra zone (T), then, in the old algae (OA), macroalgae debris zone which had been deposited on the shore at least three weeks earlier, and finally in the fresh algae (FA), which is freshly exposed by outflow (one hour before sampling), living macroalgae zone which had been regularly inundated (twice a day) by sea water. In a given site (B1–B6), 10 samples were taken in each of the study zones (total of 30 samples in each site). The samples were taken every 2 m to a depth of 5 cm using a 10 cm side frame. The depth of the samples was connected to the thickness of the macroalgae, which was about 5 cm. Because study sites involved inundated, living macroalgae, the standard frame method with defined surface area was used, which was suitable for all environments. Collembola from the samples were extracted in Tullgren’s apparatus.
The identification of specimens was based on recent comprehensive manuals [
25,
26,
46,
47,
48,
49,
50]. All specimens were deposited at the Cardinal Stefan Wyszynski University in Warsaw, Institute of Biological Sciences (Warsaw, Poland).
2.2. Macroalgal Nutrients
Macro- and micronutrient (phosphorous, P; potassium, K; calcium, Ca; magnesium, Mg; manganese, Mn; iron, Fe) content in the living macroalgae freshly exposed by outflow and decaying macroalgae debris was used as a proxy of fertiliser regime (properties) and nutrient supply for microbial growth and bacterial biofilm formation. This content in macroalgal detritus was quantified in 400 mg of algal material digested in 7 mL of concentrated nitric acid and then in an MLS Turbowave microwave liquid digestion system (Anton-Paar Multiwave 3000 microwave unit; Anton-Paar, Graz, Austria), where deionised H2O was added to the capacity of 100 cm3 and analysed by inductively coupled plasma optical emission spectroscopy (ICP-OES) with an ICPE-9820 simultaneous ICP atomic emission spectrometer (Shimadzu, Kyoto, Japan).
2.3. Data Analysis
We used Kruskal–Wallis one-way ANOVA to assess differences in collembolan density, richness and diversity as well as the composition of ecological traits between coastal tundra and decaying and fresh macroalgal detritus patches. The significance of differences was determined with a multiple comparison post hoc test of mean ranks (Dunn’s test) applied after the Kruskal–Wallis ANOVA. The offshore/onshore habitat matrix permeability to Collembola at the sub-Arctic seashore was evaluated by non-metric multidimensional scaling (NMDS). NMDS is a data reduction technique that projects multivariate data along latent axes in a distance measure-based space and preserves the underlying dissimilarity structure between points [
51]. The NMDS distance matrix was calculated using the square-root transformed Bray–Curtis distance between cases (samples) [
52]. Principal component rotation was employed to maximise the variation of the scores resulting from the NMDS. The NMDS solution was projected in K = 2 dimensions, Kruskal stress algorithm type 2, which uses the sum of squares of the starting differences [
52]. To evaluate the goodness of fit for the final NMDS model, we considered stress values of <0.05 as showing an excellent representation of the data: <0.1 good representation, <0.2 acceptable representation and >0.3 unsatisfactory representation [
53].
The effect of the land-sea interface on springtail community variation and the distribution pattern was assessed by canonical correspondence analysis (CCA). CCA was employed because, in our case, the value of the gradient calculated for the first axis of detrended correspondence analusis (DCA) was 3.45 SD [
54]. The CCA analysis was constrained by factors such as location of site within the bays (B), habitat (H) related to FA, OA and T, and time (T) related to year of study. A partial canonical correspondence analysis (pCCA) was performed to reflect the relative importance of study factors as a group of predictors of Collembola community variation. According to the factors examined, we excluded the possible effect of site (B) in partitioning of the response effect to habitat patch (H) and time (T). The Monte Carlo permutation test was used to quantify the significance of the CCA and pCCA models. In our study, B was the unit of replication (
N = 6); therefore, for the significance level of the effect of the habitat patch and time, we specified a permutation within blocks defined by the covariate B. Raw and adjusted variation using the number of degrees of freedom was estimated in the CCA and pCCA analyses. A generalised linear model (GLM) was performed to analyse the species-specific distribution of Collembola along the environmental (habitat) gradient examined. A regression model was fitted with a second-order polynomial of the predictor variable and quasi−Poisson distribution for the response data and F-test-based selection. The predictor variable was the first DCA axis fitted with the habitat scores. Species’ response curves were fitted for species that occurred at least in the coastal tundra and macroalgal patches. Among species that met this criterion were
Hypogastrura viatica,
Tetracanthella arctica,
Folsomia quadrioculata,
Entomobrya nivalis,
Friesea mirabilis,
Agrenia bidenticulata,
Pseudisotoma sensibilis and
Isotoma anglicana.
Analyses were performed using a data matrix with mean abundance data in which the value in each cell was the mean abundance of species retrieved from 10 soil cores at each plot on each sampling occasion. In the multivariate analysis, time was treated as repeated measures for adjusting p-value data. Before calculation, all data were standardised to m2 basis, and abundance data for each species were log(x10 + 1) transformed prior to ordination. The level of significance in all analyses was at α = 0.05. Calculations were made with Statistica 10.0 and Canoco 5.0 software packages.
2.4. Functional Trait-Based Analysis
The dispersal ability and life form groupings according to morphology-based Gisin’s system [
55], modified by Stebajeva [
56], were used to evaluate the Collembola community penetration between vegetated low-arctic coastal ecosystems and macroalgal patches across the shoreline of the Barents Sea.
The trait-based structure of Collembola communities was analysed on the basis of the sum of density (m
2) of species attributed to each trait in each of 10 cores. In this study, we divided Collembola species into epigeic (including species occurring on the ground, vegetation or on the water surface), hemiedaphic and euedaphic life form traits considered as dwellers of litter surface, litter depth and topsoil, respectively, and according to their dispersal ability (fast dispersion, slow dispersion) [
57]. The assignment of Collembola species determined during the study to individual life form traits was mainly based on the analysis of various literature resources, including a number of specified synopses and identification keys [
25,
26,
46,
47,
48,
50].