Drivers of variation in seagrass-associated amphipods across biogeographical areas.

Amphipods are one of the dominant epifaunal groups in seagrass meadows. However, our understanding of the biogeographical patterns in the distribution of these small crustaceans is limited. In this study, we investigated such patterns and the potential drivers in twelve Cymodocea nodosa meadows within four distinctive biogeographical areas across 2000 Km and 13° of latitude in two ocean basins (Mediterranean Sea and Atlantic Ocean). We found that species abundances in the assemblage of seagrass-associated amphipods differed among areas following a pattern largely explained by seagrass leaf area and epiphyte biomass, while the variation pattern in species presence/absence was determined by seagrass density and epiphyte biomass. Seagrass leaf area was also the most important determinant of greater amphipod total density and species richness, while amphipod density also increased with algal cover. Overall, our results evidenced that biogeographical patterns of variation in amphipod assemblages are mainly influenced by components of the habitat structure, which covary with environmental conditions, finding that structurally more complex meadows harboring higher abundance and richness of amphipods associated.

Perceived multiple stressor effects depend on sample size and stressor gradient length.

Multiple stressors are continuously deteriorating surface waters worldwide, posing many challenges for their conservation and restoration. Combined effect types of multiple stressors range from single-stressor dominance to complex interactions. Identifying prevalent combined effect types is critical for environmental management, as it helps to prioritise key stressors for mitigation. However, it remains unclear whether observed single and combined stressor effects reflect true ecological processes unbiased by sample size and length of stressor gradients. Therefore, we examined the role of sample size and stressor gradient lengths in 158 paired-stressor response cases with over 120,000 samples from rivers, lakes, transitional and marine ecosystems around the world. For each case, we split the overall stressor gradient into two partial gradients (lower and upper) and investigated associated changes in single and combined stressor effects. Sample size influenced the identified combined effect types, and stressor interactions were less likely for cases with fewer samples. After splitting gradients, 40 % of cases showed a change in combined effect type, 30 % no change, and 31 % showed a loss in stressor effects. These findings suggest that identified combined effect types may often be statistical artefacts rather than representing ecological processes. In 58 % of cases, we observed changes in stressor effect directions after the gradient split, suggesting unimodal stressor effects. In general, such non-linear responses were more pronounced for organisms at higher trophic levels. We conclude that observed multiple stressor effects are not solely determined by ecological processes, but also strongly depend on sampling design. Observed effects are likely to change when sample size and/or gradient length are modified. Our study highlights the need for improved monitoring programmes with sufficient sample size and stressor gradient coverage. Our findings emphasize the importance of adaptive management, as stress reduction measures or further ecosystem degradation may change multiple stressor-effect relationships, which will then require associated changes in management strategies.

Posidonia oceanica L. (Delile) meadows regression: Long-term affection may be induced by multiple impacts.

Coastal development has an undeniable impact on marine ecosystems resulting in the detriment of the more sensible communities. Posidonia oceanica meadows are climax communities which offer a wide variety of ecosystem services both ecological and socio-economic. Human-derived impact on these habitats has been widely assessed although conclusions may vary depending on the area. P. oceanica meadow regression next to the city of Alicante (SE Spain) was analyzed on the long term (1984-2014) using bionomic cartographies and side-scan sonar images and, during the last two decades (2003-2021), using cover percentage and shoot density descriptors in the remaining meadow. Results showed a 25% colonized area reduction since 1984, this process being more rapid during the 1984-1994 period and decreasing with time. Cover and density have suffered a significant decrease in the last 20 years, mainly in the upper limit of the meadow. Dead matte cover was also assessed and have shown a significant increase in the same period following an inverse trend with the other metrics. There are several coastal impacts which have co-occurred in the area in the last few decades (port enlargement, brine and sewage discharges, industrial activity) thus resulting in the regression of the meadow. The existing negative trend of the measured descriptors indicate the necessity of implementing management actions which focus on the present sources of impact and actively reduce their effect on P. oceanica beds.

Partitioning resilience of a marine foundation species into resistance and recovery trajectories.

The resilience of an ecological unit encompasses resistance during adverse conditions and the capacity to recover. We adopted a 'resistance-recovery' framework to experimentally partition the resilience of a foundation species (the seagrass Cymodocea nodosa). The shoot abundances of nine seagrass meadows were followed before, during and after simulated light reduction conditions. We determined the significance of ecological, environmental and genetic drivers on seagrass resistance (% of shoots retained during the light deprivation treatments) and recovery (duration from the end of the perturbed state back to initial conditions). To identify whether seagrass recovery was linearly related to prior resistance, we then established the connection between trajectories of resistance and recovery. Finally, we assessed whether recovery patterns were affected by biological drivers (production of sexual products-seeds-and asexual propagation) at the meadow-scale. Resistance to shading significantly increased with the genetic diversity of the meadow and seagrass recovery was conditioned by initial resistance during shading. A threshold in resistance (here, at a ca. 70% of shoot abundances retained during the light deprivation treatments) denoted a critical point that considerably delays seagrass recovery if overpassed. Seed densities, but not rhizome elongation rates, were higher in meadows that exhibited large resistance and quick recovery, which correlated positively with meadow genetic diversity. Our results highlight the critical role of resistance to a disturbance for persistence of a marine foundation species. Estimation of critical trade-offs between seagrass resistance and recovery is a promising field of research to better manage impacts on seagrass meadows.

Driving factors of biogeographical variation in seagrass herbivory.

Despite the crucial role of herbivory in shaping community assembly, our understanding on biogeographical patterns of herbivory on seagrasses is limited compared to that on terrestrial plants. In particular, the drivers of such patterns remain largely unexplored. Here, we used a comparative-experimental approach in Cymodocea nodosa meadows, across all possible climate types within the seagrass distribution, 2000 km and 13° of latitude in two ocean basins, to investigate biogeographical variation in seagrass herbivory intensity and their drivers during July 2014. Particularly, the density and richness of herbivores and their food resources, seagrass size, carbon and nitrogen content, as well as latitude, sea surface temperature, salinity, chlorophyll, and sediment grain size, were tested as potential drivers. We found that shallow meadows can be subjected to intense herbivory, with variation in herbivory largely explained by fish density, seagrass size, and annual sea temperature range. The herbivorous fish density was the most important determinant of such variation, with the dominant seagrass consumer, the fish Sarpa salpa, absent at meadows from regions with low herbivory. In temperate regions where herbivorous fish are present, annual temperature ranges drive an intense summer herbivory, which is likely mediated not only by increased herbivore metabolic demands at higher temperatures, but also by higher fish densities. Invertebrate grazing (mainly by sea urchins, isopods, amphipods, and/or gastropods) was the dominant leaf herbivory in some temperate meadows, with grazing variation mainly influenced by seagrass shoot size. At the subtropical region (under reduced annual temperature range), lower shoot densities and seagrass nitrogen contents contributed to explain the almost null herbivory. We evidenced the combined influence of drivers acting at geographic (region) and local (meadow) scales, the understanding of which is critical for a clear prediction of variation in seagrass herbivory intensity across biogeographical regions.

Sustainable desalination: Long-term monitoring of brine discharge in the marine environment.

The environmental impact of desalination is the most important concern related to its sustainable development. We present the results of a long-term environmental plan to monitor brine discharge (BD) from a desalination plant located in a high environmental value area in Spain. Generalized additive models were used to analyze the biological parameters of biological communities. Results of 17 years of BD monitoring show how its environmental impact can be minimized through well-planned decision-making between scientists and industry. The brine dilution prior to its discharge into an artificial channel of low ecological value significantly reduced the brine influence area. P. oceanica shoot production and echinoderms abundances were relatively stable across historical series and similar values in control and impacts locations were observed. Conversely, there was a higher abundance and species richness of fishes in the BD area. The important findings reported here should be considered for future applications in similar projects.

Seagrass Cymodocea nodosa across biogeographical regions and times: Differences in abundance, meadow structure and sexual reproduction.

Seagrasses are key habitat-forming species of coastal areas. While previous research has demonstrated considerable small-scale variation in seagrass abundance and structure, studies teasing apart local from large-scale variation are scarce. We determined how different biogeographic scenarios, under varying environmental and genetic variation, explained variation in the abundance and structure (morphology and biomass allocation), epiphytes and sexual reproduction intensity of the seagrass Cymodocea nodosa. Regional and local-scale variation, including their temporal variability, contributed to differentially explain variation in seagrass attributes. Structural, in particular morphological, attributes of the seagrass leaf canopy, most evidenced regional seasonal variation. Allocation to belowground tissues was, however, mainly driven by local-scale variation. High seed densities were observed in meadows of large genetic diversity, indicative of sexual success, which likely resulted from the different evolutionary histories undergone by the seagrass at each region. Our results highlight that phenotypic plasticity to local and regional environments need to be considered to better manage and preserve seagrass meadows.

Recent trend reversal for declining European seagrass meadows.

Seagrass meadows, key ecosystems supporting fisheries, carbon sequestration and coastal protection, are globally threatened. In Europe, loss and recovery of seagrasses are reported, but the changes in extent and density at the continental scale remain unclear. Here we collate assessments of changes from 1869 to 2016 and show that 1/3 of European seagrass area was lost due to disease, deteriorated water quality, and coastal development, with losses peaking in the 1970s and 1980s. Since then, loss rates slowed down for most of the species and fast-growing species recovered in some locations, making the net rate of change in seagrass area experience a reversal in the 2000s, while density metrics improved or remained stable in most sites. Our results demonstrate that decline is not the generalised state among seagrasses nowadays in Europe, in contrast with global assessments, and that deceleration and reversal of declining trends is possible, expectingly bringing back the services they provide.

Lack of Impact of Posidonia oceanica Leaf Nutrient Enrichment on Sarpa salpa Herbivory: Additional Evidence for the Generalist Consumer Behavior of This Cornerstone Mediterranean Herbivore.

The fish Sarpa salpa (L.) is one of the main macroherbivores in the western Mediterranean. Through direct and indirect mechanisms, this herbivore can exert significant control on the structure and functional dynamics of seagrass beds and macroalgae. Past research has suggested nutritional quality of their diet influences S. salpa herbivory, with the fish feeding more intensively and exerting greater top down control on macrophytes with higher internal nutrient contents. However recent findings have questioned this notion and shown that herbivores do not preferentially feed on macrophytes with higher nutrient contents, but rather feed on a wide variety of them with no apparent selectivity. To contribute to this debate, we conducted a field fertilization experiment where we enriched leaves of the seagrass Posidonia oceanica, a staple diet for S. salpa, and examined the response by the herbivore. These responses included quantification of leaf consumption in fertilized and non-fertilized/control plots within the bed, and food choice assays where fertilized and non-fertilized/control leaves were simultaneously offered to the herbivore. Despite the duration of leaf exposure to herbivores (30 days) and abundant schools of S. salpa observed around the plots, leaf consumption was generally low in the plots examined. Consumption was not higher on fertilized than on non-fertilized leaves. Food choice experiments did not show strong evidence for selectivity of enriched leaves. These results add to a recent body of work reporting a broad generalist feeding behavior by S. salpa with no clear selectivity for seagrass with higher nutrient content. In concert, this and other studies suggest S. salpa is often generalist consumers not only dictated by diet nutrient content but by complex interactions between other traits of nutritional quality, habitat heterogeneity within their ample foraging area, and responses to predation risk.

Short-term effect of beach replenishment on a shallow Posidonia oceanica meadow.

Putative perturbations on a Posidonia oceanica meadow produced by recent artificial beach nourishment were evaluated in relation to four undisturbed meadows. Temporal variations of putative impacted location vs. control locations of environmental (light availability and sediment features), plant (associated epiphytes, silt-clay fraction attached to epiphytes, herbivore attack, non-structural carbohydrate reserves) and structural parameters (cover and density) of meadows were tested by asymmetrical analysis of variance beyond BACI (Before/After, Control/Impact). Additionally, two asymmetrical analyses of variance were used to test for differences in vegetative growth of horizontal rhizomes (leaf production, horizontal rhizome growth, biomass production and net secondary rhizome recruitment) before and after beach replenishment. Environmental effects induced by dumping works were only detected in connection with a higher silt-clay deposition rate. This increase was consistent with the increase of silt-clay cover attached to epiphytes. As a consequence of silt-clay smothering, a decrease of filter feeding epiphytes, starch reserves, shoot surface and shoot biomass was observed. The sensitivity of plants to sediment inputs, leads us to recommend avoidance of dumping or sediment movement in the vicinity of P. oceanica meadows.