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Assuming that dispersal modes or abilities can explain the different responses of organisms to geographic or environmental distances, the distance-decay relationship is a useful tool to evaluate the relative role of local environmental structuring versus regional control in community composition. Based on continuing the current theoretical framework on metacommunity dynamics and based on the predictive effect of distance on community similarity, we proposed a new framework that includes the effect of spatial extent. In addition, we tested the validity of our proposal by studying the community similarity among three biotic groups with different dispersal modes (macrofaunal active and passive dispersers and plants) from two pond networks, where one network had a small spatial extent, and the other network had an extent that was 4 times larger. Both pond networks have similar environmental variability. Overall, we found that environmental distance had larger effects than geographical distances in both pond networks. Moreover, our results suggested that species sorting is the main type of metacommunity dynamics shaping all biotic groups when the spatial extent is larger. In contrast, when the spatial extent is smaller, the observed distance-decay patterns suggested that different biotic groups were mainly governed by different metacommunity dynamics. While the distance-decay patterns of active dispersers better fit the trend that was expected when mass effects govern a metacommunity, passive dispersers showed a pattern that was expected when species sorting prevails. Finally, in the case of plants, it is difficult to associate their distance-decay patterns with one type of metacommunity dynamics.

Dispersal is one of the main determinants of metacommunity assembly, yet its interaction with the landscape network structure has not been fully elucidated. Large-scale regional factors such as the centrality–isolation gradient influence community abundance and richness, but small-scale regional factors, such as the direction or the distance among water bodies, might also influence them. Furthermore, the specific dispersal ability of each of the taxa will also determine this interaction. All these drivers are generally difficult to analyse together, but mesocosms usage combined with network analysis can help disentangle their relevance. We used these approaches to analyse how large and small regional factors influence dispersal dynamics. We set up several mesocosms around two source ponds with divergent locations within the network centrality–isolation gradient, at different distances and in different directions. We analysed macroinvertebrates composition and temporal trends throughout the hydroperiod considering the two regional factors: large (centrality–isolation gradient) and small (distance or direction separately) and dispersal ability (i.e. weak or strong). The large scale, being central or isolated, determined abundance and richness across the hydroperiod, with greater values in central locations. However, small regional factors, especially direction, modulated this trend making it similar to the central locations. Contrastingly, the small regional scales did not affect community composition, which differed along the large scale. Our results suggest that centrality–isolation plays a marked role in assembly processes favouring greater dispersal in central locations. However, this general pattern can be modulated by smaller regional factors and by intrinsic taxa dispersal ability, something that must be accounted within the metacommunity framework.

Zooplankton assemblages in the confined coastal lagoons of La Pletera salt marshes (Baix Ter wetlands, Girona, Spain) are dominated by two species: one calanoid copepod (Eurytemora velox) and the other rotifer (Brachionus gr. plicatilis). They alternate as the dominant species (more than 80% of total zooplankton biomass), with the former being dominant in winter and the latter in summer. Shifts between these taxa are sudden, and intermediate situations usually do not last more than 1 month. Although seasonal shifts between zooplankton dominant species appear to be related with temperature, other factors such as trophic state or oxygen concentration may also play an important role. Shifts between species dominances may be driven by thresholds in these environmental variables. However, according to the alternative stable states theory, under conditions of stable dominance a certain resistance to change may exist, causing that gradual changes might have little effect until a tipping point is reached, at which the reverse change becomes much more difficult. We investigated which are the possible factors causing seasonal zooplankton shifts. We used high-frequency temperature and oxygen data provided by sensors installed in situ to analyse if shifts in zooplankton composition are determined by a threshold in these variables or, on the other hand, some gradual change between stable states occur. Moreover, following the postulates of the alternative stable states theory, we looked at possible hysteresis to analyse if these seasonal zooplankton shifts behave as critical transitions between two different equilibriums. We also examined if top-down or bottom-up trophic interactions affect these zooplankton shifts. Our results show that shifts between dominant zooplankton species in La Pletera salt marshes are asymmetric. The shift to a Eurytemora situation is mainly driven by a decrease in temperature, with a threshold close to 19 °C of daily average temperature, while the shift to Brachionus does not. Usually, the decrease in water temperature is accompanied by a decrease in oxygen oscillation with values always close to 100% oxygen saturation. Moreover, oxygen and temperature values before the shift to calanoids are different from those before the reverse shift to Brachionus, suggesting hysteresis and some resistance to change when a critical transition is approaching. Top-down and bottom-up forces appear to have no significant effect on shifts, since zooplankton biomass was not negatively correlated with fish biomass and was not positively related with chlorophyll, in overall data or within shifts.

The application of graph theory to metacommunity ecology allows a deeper analysis of the effect of network structure on diversity patterns. Here, we set out to test the role of network centrality metrics and environmental characteristics in diversity patterns of pond macroinvertebrate metacommunities. We tested two approaches to construct the networks: one used the percolation distance, whereas the other was based on a community‐contingent distance. The role of each patch within the network was then analyzed using its centrality value. Later, we analyzed the relationships between the macroinvertebrate diversity and centrality metrics for four study sites. The calculated diversity metrics cover different facets of biodiversity at two scales: pond and pondscape. Environmental characteristics of the studied ponds were also included. All relationships were tested considering the entire macroinvertebrate dataset, but also differentiated by dispersal mode (i.e., active vs. passive) and considering the two types of network approaches analyzed. The results were mostly consistent when comparing the network approaches used. Centrality metrics tended to be positively related to alpha and negatively to beta diversity. Environmental uniqueness showed a positive effect on beta diversity metrics, regardless of the dispersal mode. We only observed a weak negative relationship between eutrophication and species richness of active dispersers. Pond size showed a positive effect on both alpha and beta diversity, but was detected more frequently on alpha diversity metrics. We could not find evidence for a clear negative effect of habitat degradation on diversity. We found a greater importance of environmental characteristics versus the centrality metrics for both alpha and beta diversity of active dispersers, while a combination of their contributions for passive dispersers. An unexpected importance of centrality was observed for alpha diversity of passive dispersers. Using empirical data, we demonstrate that the centrality of a patch in an undirected network affects diversity regardless of the approach used to construct the networks, with a higher influence at local scale regardless of the dispersal mode. This study broadens knowledge of the relationships between environmental features and network centrality, demonstrating the important role of centrality as a determinant of diversity in metacommunities.

Salt marshes are considered among the ecosystems with the highest capacity for carbon sequestration. However, studies have mainly focused on tidal salt marshes, while knowledge about non-tidal salt marshes, which prevail in the Mediterranean Basin, is currently scarce. Consequently, we aimed to (1) estimate the carbon stocks of the vegetation, litter, and soil of three natural Habitats of Community Interest of a Mediterranean non-tidal salt marsh–a halophilous scrub, a salt meadow, and a glasswort sward–and (2) determine how differences in floristic composition and community structure among habitats over 1 year are related to the amount and distribution of carbon stored in plants and soils. Results show that the halophilous scrub and the salt meadow were constituted by mature plant communities with overall stable carbon stocks in the vegetation, litter, and soil (0–20 cm), which were approximately 4 and 2.5 times larger, respectively, than those found in the glasswort sward (1013 ± 40 g C m−2). However, while the salt meadow and the glasswort sward had more carbon stored in the soil than in the vegetation (approximately 3 and 7.5 times more, respectively), the halophilous scrub had similar amounts in both compartments (2248 ± 388 and 1566 ± 179 g C m−2 in the soil and vegetation, respectively). Sarcocornia fruticosa represented 93.5% of the total plant biomass of the halophilous scrub, which highlights the importance of this species in maintaining carbon stocks in this Mediterranean non-tidal salt marsh.

Estimations of ecosystem metabolism have rarely been used to quantify productivity in structural reductionist approaches for the description of phytoplankton composition. However, estimations of ecosystem metabolism could contribute to a better understanding of the relationship between phytoplankton composition and ecosystem functioning. To examine this, we investigated the community structure of phytoplankton in a set of Mediterranean coastal lagoons (natural and artificial) during a hydrological cycle to identify the most important environmental variables determining phytoplankton species composition. The focus of the study was on the quantification of productivity-related variables using estimations of ecosystem metabolism, such as different proxies for the estimation of the production-to-biomass ratio and of the relative importance of K- and r-strategies, which are commonly used conceptually but not quantified. Our results demonstrated differences in phytoplankton composition between seasons, due to the dominant hydrological pattern of flooding confinement in the salt marsh, and between lagoons that were caused by different levels of nutrient availability. Moreover, there was a notable decrease in the production/biomass ratio and a prevalence of K-strategists with seasonal succession, as predicted by Margalef’s mandala. Thus, the results showed that estimations of ecosystem metabolism are useful for the higher frequency quantification of important ecological variables, and contribute to a better understanding of planktonic assemblages, and physical and chemical changes, in these fluctuating ecosystems.

Changes in zooplankton microhabitat use in lakes through diel migrations (vertical and horizontal) have been related to habitat heterogeneity and to predation pressure, among other factors. However, there is a controversy concerning the effectiveness of diel migrations in temporary ponds, due to the shallowness of these systems and their distinct aquatic predator communities (mainly macroinvertebrates and amphibians). In order to test if diel microhabitat selection patterns described from lakes hold in temporary ponds, we developed a study using funnel traps at different zones of the pond (high and low vegetation density; surface and bottom of the pond) that were checked during the day and at night. Additionally, we assessed predation risk by sampling the macroinvertebrate community at the same time. In the studied Mediterranean temporary pond, zooplankton exhibited diel patterns of microhabitat selection: cladocerans showed a diel horizontal pattern, whereas copepods showed a diel vertical pattern. Results suggest that microhabitat selection and the associated diel pattern may be explained by both biological (potential predation) and environmental drivers (habitat heterogeneity and protection against ultraviolet radiation).

Body size has been widely recognised as a key factor determining community structure in ecosystems. We analysed size diversity patterns of phytoplankton, zooplankton and fish assemblages in 13 data sets from freshwater and marine sites with the aim to assess whether there is a general trend in the effect of predation and resource competition on body size distribution across a wide range of aquatic ecosystems. We used size diversity as a measure of the shape of size distribution. Size diversity was computed based on the Shannon-Wiener diversity expression, adapted to a continuous variable, i.e. as body size. Our results show that greater predation pressure was associated with reduced size diversity of prey at all trophic levels. In contrast, competition effects depended on the trophic level considered. At upper trophic levels (zooplankton and fish), size distributions were more diverse when potential resource availability was low, suggesting that competitive interactions for resources promote diversification of aquatic communities by size. This pattern was not found for phytoplankton size distributions where size diversity mostly increased with low zooplankton grazing and increasing nutrient availability. Relationships we found were weak, indicating that predation and competition are not the only determinants of size distribution. Our results suggest that predation pressure leads to accumulation of organisms in the less predated sizes, while resource competition tends to favour a wider size distribution.

Feeding of the different developmental stages of Calanipeda aquaedulcis on natural particles (bacterio-, phyto- and microzooplankton) was measured in a Mediterranean salt marsh (Emporda wetlands, NE Iberian Peninsula). Bottle incubations were performed in the field both in autumn and spring. The results showed differences in the diet of the different developmental stages due to both prey type and size. In general, the size of the ingested prey increased with increasing size of the C. aquaedulcis stage. While C. aquaedulcis adults had high ingestion rates and selection coefficients for large prey (micro- and nanoplankton), nauplii preferentially consumed smaller prey items (picoplankton). Copepodites showed the widest prey size range, including pico-, nano- and microplankton. Nevertheless, the lower size limit for particle capture was similar for all stages, i.e. between 1.7 and 2.1 [mu]m. Omnivory was observed in all stages of C. aquaedulcis. Heterotrophic prey (picoplankton, dinoflagellates and ciliates) were the most ingested items. The ability to partition the available food among the different developmental stages could represent an advantage in times of food scarcity because it may reduce intraspecific competition. This may explain how C. aquaedulcis is able to predominate in the zooplankton community for several weeks during spring and summer even in situations of low food availability.

In situ 24-h incubation experiments were performed to analyse the grazing effects of Daphnia magna on a planktonic microbial community. Three field grazing experiments under different nutrient concentrations were carried out on treated effluents of a wastewater treatment plant. The grazing effects of three different D. magna size classes (small (0.6 1.6 mm), medium (1.7 2.5 mm) and large individuals (2.6 3.7 mm)) were compared. The different sizes classes had similar effects on the plankton community. However, our results showed big differences in effects among experiments. Our findings suggest that in spite of D. magna s is nonselective feeding behaviour and the fact that different developmental stages (i.e. its size) had similar effects on the microbial planktonic community, these effects can differ according to the initial structure and composition of the community and the resulting cascading trophic interactions. Moreover, D. magna effects can be direct through grazing (as is the case with ciliates), or indirect through trophic cascade interactions (as is the case with bacteria).