Higher Trophic Levels Research Articles

Combining mesocosms with models reveals effects of global warming and ocean acidification on a temperate marine ecosystem.

Ocean warming and species exploitation have already caused large-scale reorganization of biological communities across the world. Accurate projections of future biodiversity change require a comprehensive understanding of how entire communities respond to global change. We combined a time-dynamic integrated food web modeling approach (Ecosim) with previous data from community-level mesocosm experiments to determine the independent and combined effects of ocean warming, ocean acidification and fisheries exploitation on a well-managed temperate coastal ecosystem. The mesocosm parameters enabled important physiological and behavioral responses to climate stressors to be projected for trophic levels ranging from primary producers to top predators, including sharks. Through model simulations, we show that under sustainable rates of fisheries exploitation, near-future warming or ocean acidification in isolation could benefit species biomass at higher trophic levels (e.g., mammals, birds, and demersal finfish) in their current climate ranges, with the exception of small pelagic fishes. However, under warming and acidification combined, biomass increases at higher trophic levels will be lower or absent, while in the longer term reduced productivity of prey species is unlikely to support the increased biomass at the top of the food web. We also show that increases in exploitation will suppress any positive effects of human-driven climate change, causing individual species biomass to decrease at higher trophic levels. Nevertheless, total future potential biomass of some fisheries species in temperate areas might remain high, particularly under acidification, because unharvested opportunistic species will likely benefit from decreased competition and show an increase in biomass. Ecological indicators of species composition such as the Shannon diversity index decline under all climate change scenarios, suggesting a trade-off between biomass gain and functional diversity. By coupling parameters from multilevel mesocosm food web experiments with dynamic food web models, we were able to simulate the generative mechanisms that drive complex responses of temperate marine ecosystems to global change. This approach, which blends theory with experimental data, provides new prospects for forecasting climate-driven biodiversity change and its effects on ecosystem processes.

Clustering evaluation of water quality for various classes of in-flow rivers in connected brackish lakes.

Brackish waters and estuaries at the lower reaches of rivers accumulate organic matter and nutrients from various sources in the watershed. Sufficient light and shallow water depth stimulate phytoplankton growth, resulting in a more diversified ecosystem with higher trophic levels. For effective watershed management, it is crucial to characterize the water quality of all rivers, including small and medium-sized ones. Our field survey assessed water quality parameters in 26 inflow rivers surrounding Lakes Shinji and Nakaumi, two consolidated brackish lakes in Japan. The parameters included water temperature, salinity, chlorophyll-a, and nutrients. The study used hierarchical clustering. The Silhouette Index was used to assess clustering outcomes and identify any difficulties in dispersion across clusters. The 26 rivers surrounding Lakes Shinji and Nakaumi were classified into six groups based on their water quality characteristics. This classification distinguishes itself from earlier subjective methods that relied on geographical factors. The new approach identifies a need for improved management of river water quality. The results of the cluster analysis provide valuable insights for future management initiatives. It is important to consider these findings alongside established watershed criteria.

Atlas of phytoplankton phenology indices in selected Eastern Mediterranean marine ecosystems.

Phytoplankton is a fundamental component of marine food webs and play a crucial role in marine ecosystem functioning. The phenology (timing of growth) of these microscopic algae is an important ecological indicator that can be utilized to observe its seasonal dynamics, and assess its response to environmental perturbations. Ocean colour remote sensing is currently the only means of obtaining synoptic estimates of chlorophyll-a (a proxy of phytoplankton biomass) at high temporal and spatial resolution, enabling the calculation of phenology metrics. However, ocean colour observations have acknowledged weaknesses compromising its reliability, while the scarcity of long-term in situ data has impeded the validation of satellite-derived phenology estimates. To address this issue, we compared one of the longest available in situ time series (20 years) of chlorophyll-a concentrations in the Eastern Mediterranean Sea (EMS), along with concurrent remotely-sensed observations. The comparison revealed a marked coherence between the two datasets, indicating the capability of satellite-based measurements in accurately capturing the phytoplankton seasonality and phenology metrics (i.e., timing of initiation, duration, peak and termination) in the studied area. Furthermore, by studying and validating these metrics we constructed a satellite-derived phytoplankton phenology atlas, reporting in detail the seasonal patterns in several sub-regions in coastal and open seas over the EMS. The open waters host higher concentrations from late October to April, with maximum levels recorded during February and lowest during the summer period. The phytoplankton growth over the Northern Aegean Sea appeared to initiate at least a month later than the rest of the EMS (initiating in late November and terminating in late May). The coastal waters and enclosed gulfs (such as Amvrakikos and Maliakos), exhibit a distinct seasonal pattern with consistently higher levels of chlorophyll-a and prolonged growth period compared to the open seas. The proposed phenology atlas represents a useful resource for monitoring phytoplankton growth periods in the EMS, supporting water quality management practices, while enhancing our current comprehension on the relationships between phytoplankton biomass and higher trophic levels (as a food source).

Effects of joint invasion: How co-invaders affect each other's success in model food webs?

While there has been considerable research on the interactions between invasive and native species, and on the impact of invasive species on the resident community, there has been less focus on exploring the relationship and interactions among invasive species themselves. Nevertheless, it is widely recognised that invasive species can have either positive or negative effects on each other, as well as neutral outcomes. In the present theoretical study, we compared the success of two invasive non-native species in two scenarios: when they invaded the resident food web separately and simultaneously. We examined the correlations between their direct and indirect ecological relationships and their topological positions in the food web, with the varying outcomes of joint invasion. Using the Allometric Bioenergetic Model (ABM) for dynamic simulations, we determined the success of invasion (presence or absence of invaders) and, in the case of successful co-invasion, the direction of their biomass change, comparing separate and simultaneous invasion scenarios. We studied the relationships between these variables after detailed numerical simulations with variable key parameters of the model.We found that direct and indirect ecological relationships between the two invaders can significantly modifiy the outcomes of the invasion scenarios. For example, their predator-prey relationship increases the probability of invasion success for both invaders, but at the same time the equilibrium biomass of at least one of them is likely to be reduced compared to its separate invasion. Trophic cascade or competitive relationship between them during simultaneous invasion also affects their success rate, with the former having a positive effect and the latter a negative one, as they can hinder each other's spread. Further, we found that higher trophic level and lower betweenness centralities of the invaders reduces the likelihood of invasion success regardless of the presence or absence of another invasive species. The results of the study can be tested experimentally in micro- and mesocosms.

Responses of plankton community to threshold metal concentrations of cadmium and lead in a mesocosm experiment at Bay of Bengal

Metals are essential at trace levels to aquatic organisms for the function of many physiological and biological processes. But their elevated levels are toxic to the ecosystem and even brings about shifts in the plankton population. Threshold limits such as Predicted No Effect Concentration (PNEC - 0.6 μg/l of Cd; 2.7 μg/l of Pb), Criterion Continuous Concentration (CCC - 3.0 μg/l of Cd; 4.5 μg/l of Pb) and Criterion Maximum Concentration (CMC - 23 μg/l of Cd; 130 μg/l of Pb) prescribed for Indian coastal waters were used for the study. Short-term mesocosm experiments (96 h) were conducted in coastal waters of Visakhapatnam to evaluate responses of the planktonic community on exposure to threshold concentrations of cadmium and lead for the first time. Four individual experimental bags of 2500 L capacity (Control, PNEC, CCC & CMC) were used for the deployment and ambient water samples were analysed simultaneously to evaluate the impacts of the threshold levels in the natural waters. Chaetoceros sp. were dominant group in the control system whereas, Prorocentrum sp. Ceratium sp. Tintinopsis sp. Chaetoceros sp. and Skeletonema sp. were major groups in the test bags. Throughout the experiment the phytoplankton community did not show any significant differences with increased nutrients and plankton biomass (Chl-a <8.64 mg/m3). Positive response of plankton community was observed in the experimental bags. High abundance of diatoms were observed in PNEC, CCC & CMC bags at 48 h and the abundance decreased with shift in the species at 72–96 h. The catalase activity in phytoplankton (5.99 nmol/min/ml) and the zooplankton (4.77 nmol/min/ml) showed induction after exposure to PNEC. The present mesocosm study is confirmed that short-term exposure to threshold metal concentration did not affects the phytoplankton community structure in PNEC, but CCC and CMC affects the community structure beyond 24 h. The insights from this study will serve as a baseline information and help develop environmental management tools. We believe that long-term mesocosm experiments would unravel metal detoxification mechanisms at the cellular level and metal transfer rate at higher trophic levels in real-world environment.

Higher trophic levels and species with poorer dispersal traits are more susceptible to habitat loss on island fragments.

Ongoing habitat loss and fragmentation caused by human activities represent one of the greatest causes of biodiversity loss. However, the effects of habitat loss and fragmentation are not felt equally among species. Here, we examined how habitat loss influenced the diversity and abundance of species from different trophic levels, with different traits, by taking advantage of an inadvertent experiment that created habitat islands from a once continuous forest via the creation of the Thousand Island Lake, a large reservoir in China. On 28 of these islands with more than a 9000-fold difference in their area (0.12-1154 ha), we sampled plants, herbivorous insects, and predatory insects using effort-controlled sampling and analyses. This allowed us to discern whether any observed differences in species diversity were due to passive sampling alone or to demographic effects that disproportionately influenced some species relative to others. We found that while most metrics of sampling effort-controlled diversity increased with island area, the strength of the effect was exacerbated for species in higher trophic levels. When we more explicitly examined differences in species composition among islands, we found that the pairwise difference in species composition among islands was dominated by species turnover but that nestedness increased with differences in island area, indicating that some species are more likely to be absent from smaller islands. Furthermore, by examining trends of several dispersal-related traits of species, we found that species with lower dispersal propensity tended to be those that were lost from smaller islands, which was observed for herbivorous and predatory insects. Our results emphasize the importance of incorporating within-patch demographic effects, as well as the taxa and traits of species when understanding the influence of habitat loss on biodiversity.

Strongly asymmetric interactions and control regimes in the Barents Sea: a topological food web analysis

IntroductionIncreasing temperature of the global ocean alters the spatial behavior of a number of species. From the northern Atlantic Ocean, species may shift their area towards the poles. This results in the atlantification of the Barents Sea, raising questions about possible changes in species composition, community structure and community control.MethodsWe address the question whether possible changes in community control can be detected and quantified based on simple network analytical measures applied to the food web. Based on unweighted (binary) and undirected (symmetric) data, we quantify the strength of direct and indirect interactions in the network, represent the most asymmetric effects in the asymmetry graph composed of directed and weighted links and study the overlap among trophic niches of organisms.Results and discussionWe support earlier findings suggesting that the ecosystem can possibly be characterized by wasp-waist control. This would mean that focusing management efforts on intermediate trophic levels is of high importance, providing indirect benefit for organisms also at lower and higher trophic levels.

Life strategy of Antarctic silverfish promote large carbon export in Terra Nova Bay, Ross Sea

Antarctic silverfish Pleuragramma antarcticum is the most abundant pelagic fish in the High Antarctic shelf waters of the Southern Ocean, where it plays a pivotal role in the trophic web as the major link between lower and higher trophic levels. Despite the ecological importance of this species, knowledge about its role in the biogeochemical cycle is poor. We determine the seasonal contribution of Antarctic silverfish to carbon flux in terms of faeces and eggs, from samples collected in the Ross Sea. We find that eggs and faeces production generate a flux accounting for 41% of annual POC flux and that the variability of this flux is modulated by spawning strategy. This study shows the important role of this organism as a vector for carbon flux. Since Antarctic silverfish are strongly dependent on sea-ice, they might be especially sensitive to climatic changes. Our results suggest that a potential decrease in the biomass of this organism is likely to impact marine biogeochemical cycles, and this should be factored in when assessing Southern Ocean carbon budget.

Lake food web structure in Teici Nature reserve, Latvia: fish presence shapes functioning of pristine bog lake food webs

AbstractStudies on the effects of fish presence on lake ecosystems are widespread but only a few have been conducted in pristine aquatic environments. We employed Ecopath model for assessing food web structure in two fish-inhabited and one fishless lake in a pristine bog area. We hypothesized that: (a) fish absence will raise trophic positions of macroinvertebrate predators; (b) fish predation will lead to higher overall predation rates on zooplankton; (c) fish predation on large bodied zooplankton will result in top-down cascading effect, increasing phytoplankton biomasses. We found that fish have direct and indirect effects on zoobenthic communities. Chironomid biomass was greater and predatory macroinvertebrate groups had a higher trophic level in the fishless lake than in fish-inhabited lakes. Consumption rates of the benthic consumer fraction were greater than that of the planktonic fraction in the fishless lake; the opposite was found in the two lakes with fish. No effects of fish presence on zooplankton were found and we explain this partly by the low water transparency masking the impact of fish. Terrestrial insects constituted a crucial part of adult fish diet and we conclude that terrestrial secondary production contributes to the trophic support of fish communities in pristine bog lakes.

Nutrient-induced acidification modulates soil biodiversity-function relationships

Nutrient enrichment is a major global change component that often disrupts the relationship between aboveground biodiversity and ecosystem functions by promoting species dominance, altering trophic interactions, and reducing ecosystem stability. Emerging evidence indicates that nutrient enrichment also reduces soil biodiversity and weakens the relationship between belowground biodiversity and ecosystem functions, but the underlying mechanisms remain largely unclear. Here, we explore the effects of nutrient enrichment on soil properties, soil biodiversity, and multiple ecosystem functions through a 13-year field experiment. We show that soil acidification induced by nutrient enrichment, rather than changes in mineral nutrient and carbon (C) availability, is the primary factor negatively affecting the relationship between soil diversity and ecosystem multifunctionality. Nitrogen and phosphorus additions significantly reduce soil pH, diversity of bacteria, fungi and nematodes, as well as an array of ecosystem functions related to C and nutrient cycling. Effects of nutrient enrichment on microbial diversity also have negative consequences at higher trophic levels on the diversity of microbivorous nematodes. These results indicate that nutrient-induced acidification can cascade up its impacts along the soil food webs and influence ecosystem functioning, providing novel insight into the mechanisms through which nutrient enrichment influences soil community and ecosystem properties.

Seasonal diet partition among top predators of a small island, Iriomote Island in the Ryukyu Archipelago, Japan

Small islands tend to lack predators because species at higher trophic levels often cannot survive. However, two exceptional top predators—the Iriomote cat Prionailurus bengalensis iriomotensis, and the Crested Serpent Eagle Spilornis cheela perplexus—live on the small Iriomote Island in the Ryukyu Archipelago. To understand how these predators coexist with limited resources, we focused on their seasonal diets between which conflicts are considered to occur. To compare the diets, we used DNA metabarcoding analysis of faecal samples. In the summer, we identified 16 unique prey items from Iriomote cat faecal samples and 15 unique prey items from Crested Serpent Eagle faecal samples. In the winter, we identified 37 and 14, respectively. Using a non-metric multidimensional scaling and a permutational multivariate analysis of variance, our study reveals significant differences in the diet composition at the order level between the predators during both seasons. Furthermore, although some prey items at the species-to-order level overlapped between them, the frequency of occurrence of most prey items differed in both seasons. These results suggest that this difference in diets is one of the reasons why the Iriomote cat and the Crested Serpent Eagle are able to coexist on such a small island.

Sources and fate of omega-3 polyunsaturated fatty acids in a highly eutrophic lake

Omega-3 polyunsaturated fatty acids (ω3-PUFA) are central to the growth and reproduction of aquatic consumers. Dissolved nutrients in aquatic ecosystems strongly affect algal taxonomic composition and thus the production and transfer of specific ω3-PUFA to consumers at higher trophic levels. However, most studies were conducted in nutrient-poor, oligotrophic lakes, leading to an insufficient understanding of how water nutrients affect algal ω3-PUFA and their trophic transfer in consumers in highly eutrophic lakes.We conducted a field investigation in a highly eutrophic lake and collected basal food sources (phytoplankton, periphyton and macrophytes) and aquatic consumers (invertebrates, zooplankton and fish), and measured their fatty acid (FA) composition.Our results showed that periphyton and phytoplankton were both important sources of ω3-PUFA supporting the highly eutrophic lake food web. High water nutrient levels led to low ω3-PUFA levels in phytoplankton and periphyton, resulting in decreased nutritional quality. Consequently, ω3-PUFA of invertebrates and zooplankton reflected variations in ω3-PUFA of phytoplankton and periphyton, respectively.The ω3-PUFA levels of fish decreased as phytoplankton and periphyton ω3-PUFA decreased. Among fishes, the Redfin Culter (Cultrichthys erythropterus) and Bar Cheek Goby (Rhinogobius giurinus) exhibited significantly higher levels of EPA and DHA compared to the Pond Loach (Misgurnus anguillicaudatus), which may have been caused by their different feeding modes.Decreases in the ω3-PUFA levels of basal food sources may be one of the causes leading to the reduction of trophic links in aquatic food webs.Our study elucidated the sources and fate of ω3-PUFAs in highly eutrophic lakes, complementing previous studies in oligo- and mesotrophic lakes, and emphasizes the role of high-quality food sources, offering new perspectives for the conservation and management of highly eutrophic lake ecosystems.

High temporal resolution data reveal low bat and insect activity over managed meadows in central Europe

Increasing agriculture and pesticide use have led to declines in insect populations and biodiversity worldwide. In addition to insect diversity, it is also important to consider insect abundance, due to the importance of insects as food for species at higher trophic levels such as bats. We monitored spatiotemporal variation in abundance of nocturnal flying insects over meadows, a common open landscape structure in central Europe, and correlated it with bat feeding activity. Our most important result was that insect abundance was almost always extremely low. This was true regardless of management intensity of the different meadows monitored. We also found no correlation of insect abundance or the presence of insect swarms with bat feeding activity. This suggests that insect abundance over meadows was too low and insect swarms too rare for bats to risk expending energy to search for them. Meadows appeared to be poor habitat for nocturnal flying insects, and of low value as a foraging habitat for bats. Our study highlights the importance of long-term monitoring of insect abundance, especially at high temporal scales to identify and protect foraging habitats. This will become increasingly important given the rapid decline of insects.

Comparisons of benthic associated fauna assemblages in seagrass meadows across conservation and non-conservation areas in Bali and Lombok, Indonesia

Benthic ecosystem has been widely considered as an important feature of seagrass associated fauna, and its function as a grazer and linkage between primary producers and higher trophic level is well known. Though the importance of benthic fauna in seagrass ecosystems has been indicated in many studies, its biodiversity in spatial scale has often been poorly studied. This study aimed at examining the assemblages and diversity of benthic associated fauna in conservation areas (CA) and non-conservation areas (NCA) across the seagrass meadows in Bali and Lombok. This study found that the assemblages and diversity of benthic fauna greatly varied between the meadows. A total of 430 individuals associated to benthic fauna from 24 species were identified in Bali and Lombok. Of these, Gastropods were the highest class of taxa recorded in this study, followed by Bivalvia, Echinodermata, Decapoda, and Amphipoda. Permutation multivariate analysis of variance (PERMANOVA) revealed a significantly different benthic fauna diversity between sites. Non-metric Multidimensional Scaling (nMDS) and Bray–Curtis analysis showed a clear distinction of benthic fauna assemblages between CA and NCA, both in Bali and Lombok. These results indicated that from spatial perspective, different characteristics of seagrass meadows may represent different biodiversity of associated fauna. These differences might be driven by different anthropogenic pressure and variation of substrates among the meadows which may affect the composition of the benthic fauna ecosystems. The implication of this study was to providing baseline data on guiding the appropriate approach and strategies for management and conservations of seagrass ecosystems.

Mercury bioaccumulation and its relationship with trophic biomarkers in a Mediterranean elasmobranch mesopredator

Human activity has led to increased concentrations of mercury (Hg) in the world's oceans. Mercury can bioaccumulate and biomagnify in animal tissues via trophic transfer, thus, becoming most pronounced in larger and older predators. Here, we measured Hg concentrations and their relationship with stable isotopes-based proxies of trophic level (δ13C and δ15N values) in multiple tissues of Mustelus spp. from the Mediterranean Sea. We found higher Hg concentrations in muscle than in liver and fin tissues. The relationship between Hg concentrations and δ15N values in muscle suggested repeated foraging for low trophic level and Hg-poor prey, and biomagnification of Hg at higher trophic levels. Seasonal variations in δ13C values could indicate shifts in primary production sources and/or in local prey availability. The HBVSe index suggested no risk to human health, however the safe meal limit recommendations are 4.5 and 2.2 portions per month for adults and children, respectively.

Increased connections among soil microbes and microfauna enhances soil multifunctionality along a long‐term restoration chronosequence

Abstract The functional role of soil microbes in the restoration of degraded lands is greatly influenced by interactions within and between microbial communities, as well as by soil organisms at higher trophic levels (microfauna). However, little is known about how changes in connection patterns among soil microbes and their co‐existing microfauna contribute to associated changes in soil function over the ecosystem restoration process. In this study, the effects of afforestation and natural regeneration approaches on connection patterns among soil microbes and their co‐existing microfauna were assessed in ex‐arable lands on the Loess Plateau in China along a 50‐year restoration chronosequence. The relationships between soil network connections, microbial functional associations, and multiple soil functions (e.g. multifunctionality) were assessed quantitatively. Although both afforestation and natural regeneration approaches enhance connections among soil microbes and their co‐existing microfauna compared to recently abandoned sites, connection trends were opposed between these two approaches across the restoration chronosequence. Afforestation resulted in a higher number of soil microbial and microfauna species with a unimodal connection pattern, while natural regeneration resulted in a continuous increase in soil network connections over the progress of succession. These increases in soil network connections, particularly those of soil microfauna‐microbe and microbial inter‐kingdom connections, corresponded to enhanced microbial functional associations. This result is corroborated by significant correlations between microbial functional genes encoding different central functional pathways, which, in turn, were positively correlated with soil multifunctionality. Synthesis and applications. Changes in the connections among soil microbes and their co‐existing microfauna were positively associated with changes in soil multifunctionality, mediated by microbial functional associations. While in the early restoration stages, soil networks following afforestation contained more connections than those following natural regeneration, in later restoration stages, the number of connections of the afforestation network declined and was eventually surpassed by those following natural regeneration. These findings raise concerns about the ecological sustainability of afforestation measures in historic grasslands on the Loess Plateau, suggesting that natural regeneration is the preferred approach to achieve long‐term ecological restoration success.

Effects of Acacia invasion on water quality, litterfall, aquatic decomposers, and leaf litter decomposition in streams

Abstract Small streams and their riparian vegetation are closely linked ecosystems. Thus, the invasion of native riparian forests with non‐native species can impact stream ecosystems. We assessed the effects of the invasion of broadleaf deciduous forests by evergreen, nitrogen‐fixing Acacia species on seasonal variation of relevant instream environmental variables, litterfall in the riparian area, aquatic decomposers, and leaf litter decomposition, by comparing three streams flowing through native forests (native streams) and three streams flowing through invaded forests (invaded streams) in central Portugal. Invaded streams flow through forests composed (almost) of monospecific stands of Acacia trees. Litterfall in the riparian area was sampled with fabric traps and sorted into five categories: leaf (including phyllodes), flower, fruit and seed, wood litter, and other materials. Aquatic hyphomycete conidia suspended in water were sampled to assess conidia concentration and community composition. Leaf litter of Quercus robur was enclosed in coarse‐mesh bags and incubated in streams to assess decomposition rates and associated macroinvertebrate density and community composition. Samples from each variable were collected monthly from streams over 1 year. Aquatic hyphomycete conidia concentration was higher in invaded streams in spring/summer when litter inputs, water temperature, and aquatic nutrient concentrations were higher. In contrast, conidia concentration was lower in invaded streams in autumn/winter as they received less native deciduous leaf litter in autumn than native streams. Aquatic hyphomycete community structure changed, and species richness was lower in invaded streams because aquatic nutrient concentrations were higher and leaf litter species richness was lower. Macroinvertebrate and shredder density in decomposing leaf litter did not differ between native and invaded streams, but litter bags may have artificially increased densities by providing high quality food and/or refuges in streams with poor‐quality resources. Nevertheless, macroinvertebrate community structure changed, and family richness was lower in invaded streams. Finally, decomposition rates of Q. robur leaf litter in coarse‐mesh bags were similar between stream types, despite differences in aquatic decomposer communities. Overall, Acacia invasion changed water quality, litterfall seasonality and composition, and aquatic decomposer communities (especially aquatic hyphomycetes). However, Acacia effects on macroinvertebrate density and leaf litter decomposition rates were less pronounced, suggesting that higher trophic levels may be more resilient to invasion than basal levels, or the invasion time/extent in our invaded streams was not strong enough to affect macroinvertebrates and associated processes. Instream invasion effects on aquatic hyphomycete communities were more strongly mediated by changes in litter inputs rather than increases in aquatic nutrient concentrations because they remained oligotrophic in invaded streams. Simplification of riparian and aquatic communities may render them less efficient in coping with additional environmental changes. Acacia effects might be mitigated by the maintenance of a riparian corridor composed of native vegetation. The protection of non‐invaded riparian galleries and restoration of invaded ones could protect and restore stream ecosystems.

Effects of multiple stressors on freshwater food webs: Evidence from a mesocosm experiment

Natural and anthropogenic pressures exert influence on ecosystem structure and function by affecting the physiology and behavior of organisms, as well as the trophic interactions within assemblages. Therefore, understanding how multiple stressors affect aquatic ecosystems can improve our ability to manage and protect these ecosystems and contribute to understanding fundamental ecological principles. Here, we conducted a mesocosm experiment to ascertain the individual and combined effects of multiple stressors on trophic interactions within species in freshwater ecosystems. Furthermore, we investigated how species respond to such changes by adapting their food resources. To mimic a realistic food web, we selected fish and shrimp as top predators, gastropods, zooplankton and zoobenthos as intermediate consumers, with producers (macrophytes, periphyton and phytoplankton) and detritus as basal resources. Twelve different treatments included a control, nutrient loading only, herbicide exposure only, and a combination of nutrient loading and herbicide exposure, each replicated under ambient temperature, constant warming and multiple heat waves to simulate environmental stressors. Our results demonstrated that antagonistic interactions between environmental stressors were widespread in trophic interactions, with a more pronounced and less intense impact observed for the high trophic level species. The responses of freshwater communities to environmental stressors are complex, involving direct effects on individual species as well as indirect effects through species interactions. Moreover, our results confirmed that the combinations of stressors, but not individual stressors, led to a shift to herbivory in top predators, indicating that multiple stressors can be more detrimental to organisms than individual stressors alone. These findings elucidate how changes in the resource utilization of species induced by environmental stressors can potentially influence species interactions and the structural dynamics of food webs in freshwater ecosystems.

Food webs in isolation: The food-web structure of a freshwater reservoir with armoured shores in a former coastal bay area

Many shallow coastal bays have been closed off from the sea to mitigate the risk of flooding, resulting in coastal reservoir lakes with artificial armoured shorelines. Often these enclosed ecosystems show a persistent decline in biodiversity and ecosystem services, which is likely reflected in their food-web structure. We therefore hypothesize that the food webs of coastal reservoir lakes with armoured shorelines (1) consist of relatively few species with a low food-web connectance and short food chains, and (2) are mainly fuelled by autochthonous organic matter produced in the pelagic zone. To investigate these two hypotheses, we used stable-isotope analysis to determine the food-web structure of lake Markermeer (The Netherlands), a large reservoir lake with armoured shorelines in a former coastal bay area. Contrary to expectation, connectance of the food web in lake Markermeer was comparable to other lakes, while food-chain length was in the higher range. However, the trophic links revealed that numerous macroinvertebrates and fish species in this constructed lake exhibited omnivorous feeding behaviour. Furthermore, in line with our second hypothesis, primary consumers heavily relied on pelagically derived organic matter, while benthic primary production exerted only a minor and seasonal influence on higher trophic levels. Stable-isotope values and the C:N ratio of sediment organic matter in the lake also aligned more closely with phytoplankton than with benthic primary producers. Moreover, terrestrial subsidies of organic matter were virtually absent in lake Markermeer. These findings support the notion that isolation of the lake through shore armouring and the lack of littoral habitats in combination with persistent resuspension of sediments have affected the food web. We argue that restoration initiatives should prioritize the establishment of land-water transition zones, thereby enhancing habitat diversity, benthic primary production, and the inflow of external organic matter while preserving pelagic primary production.

DNA-metabarcoding supports trophic flexibility and reveals new prey species for the Galapagos sea lion.

Tropical ecosystems are challenging for pinnipeds due to fluctuating food availability. According to previous research, the Galapagos sea lion (GSL, Zalophus wollebaeki) adopts trophic flexibility to face such conditions. However, this hypothesis comes from studies using traditional methods (hard-parts analysis of scat and isotopic analysis from tissue). We studied the diet of five rookeries in the southeastern Galapagos bioregion (which harbors the highest GSL density), via DNA-metabarcoding of scat samples. The DNA-metabarcoding approach may identify consumed prey with a higher taxonomic resolution than isotopic analysis, while not depending on hard-parts remaining through digestion. Our study included five different rookeries to look for evidence of trophic flexibility at the bioregional level. We detected 98 prey OTUs (124 scats), mostly assigned to bony-fish taxa; we identified novel prey items, including a shark, rays, and several deep-sea fish. Our data supported the trophic flexibility of GSL throughout the studied bioregion since different individuals from the same rookery consumed prey coming from different habitats and trophic levels. Significant diet differentiations were found among rookeries, particularly between Punta Pitt and Santa Fe. Punta Pitt rookery, with a more pronounced bathymetry and lower productivity, was distinguished by a high trophic level and consumption of a high proportion of deep-sea prey; meanwhile, Santa Fe, located in more productive, shallow waters over the shelf, consumed a high proportion of epipelagic planktivorous fish. Geographic location and heterogeneous bathymetry of El Malecon, Española, and Floreana rookeries would allow the animals therein to access both, epipelagic prey over the shelf, and deep-sea prey out of the shelf; this would lead to a higher prey richness and diet variability there. These findings provide evidence of GSL adopting a trophic flexibility to tune their diets to different ecological contexts. This strategy would be crucial for this endangered species to overcome the challenges faced in a habitat with fluctuating foraging conditions.