Inés G. Viana

Small pelagic fishes play a crucial role in structuring marine food webs, yet their trophic flexibility and feeding interactions remain poorly understood in highly dynamic, nutrient-rich systems such as upwelling regions. This study combines bulk stable isotope analysis (δ¹³C and δ¹⁵N), compound-specific nitrogen isotope analysis of amino acids (CSIA-AA), and fatty acid (FA) profiles to assess feeding patterns and trophic overlap among four planktivorous species in the North Iberian shelf (Galicia and the Cantabrian Sea). The combined trophic indicators reveal clear niche partitioning between clupeids (sardine and anchovy) and scombrids (mackerel and chub mackerel), with the latter exhibiting higher CSIA-AA-derived trophic positions and distinct FA composition. Our analysis further shows that fatty acid biomarkers are essential for resolving finer-scale differences within each group, driven by variable contributions of phytoplankton and zooplankton to species diets. Although all species shared resources within the same food web, isotopic baselines indicate a decoupling between carbon and nitrogen pathways. For sardine, we compared neutral and polar lipid fractions with the total lipid extract in terms of FA composition and key dietary markers. Our results suggest that separating lipid fractions can improve dietary resolution, especially when lipid content is low. Overall, our findings highlight the value of integrating classical and emerging biochemical tracers to better resolve trophic dynamics in coexisting planktivorous fishes.

All organisms contain carbon, nitrogen, and phosphorus in widely ranging amounts and proportions. Integrating existing datasets enables quantification of this variation at global scales. Such efforts could leverage ecological stoichiometry theory, the study of elemental supply and imbalances in ecological interactions, to connect ecological drivers and taxonomic constraints to ecosystem structure and function. Towards this goal, we developed the Limnology Stoichiometric Traits of Organisms In their Chemical Habitats (Limno‐STOICH) database. The Limno‐STOICH database includes 51,576 observations of organismal elemental stoichiometry from >3100 rivers, lakes, wetlands, and other aquatic ecosystem sites on seven continents, derived from 190+ sources. It also includes extensive spatial and temporal metadata to link elemental stoichiometry with ecosystem type, trophic status, etc., and information on organismal data (body size, taxonomic classifications, stable isotope composition) and water physicochemical parameters. The Limno‐STOICH database sets the stage for significant applications across food web ecology, evolutionary ecology, biogeochemistry, and other disciplines.

Upwelling is one of the major mechanisms responsible for the input of nutrients sustaining high levels of marine primary production. As a consequence of global change, variations in upwelling intensity may affect nutrient supply thus impacting marine food webs. In this study, we examine the effects of decadal variability of upwelling strength on nitrate supply and its influence on the nitrogen stable isotope composition at the base of the marine food web at the northern boundary of the Canary Current upwelling system (NW Spain) between 1989 and 2023. The study focused on the early upwelling season each year (March-June) to minimize the effects of nitrogen remineralization. Intertidal macroalgae (Phaeophyceae) and mussels (Mytilus galloprovincialis) were used as proxies for temporally integrated isotopic signals of nitrogen sources (δ¹⁵N). While no significant temporal trends for either upwelling strength, nitrate concentrations, or stable isotopes were found, three periods with characteristic upwelling and nutrient regimes were identified. A linear increase in δ¹⁵N, particularly in Fucus spp., associated with a decreasing contribution of upwelling-derived nitrogen suggest the influence of additional sources, likely of anthropogenic origin. Thus, no net change in productivity would be expected in this region despite quasi-decadal shifts in upwelling dynamics. Further insights on the origin and relevance of these sources can be gained through the investigation of river and runoff inputs and the use of more sensitive tracers, such as amino acid δ¹⁵N analysis in mussels.

Trophic position estimation has been a central issue in food web studies. Estimates using compound‐specific isotopic analyses of amino acids were generally based on the trophic enrichment of glutamic acid. However, this trophic position estimation mainly accounts for the trophic steps involving metazoa, as the isotopic fractionation of this amino acid is minimal at trophic steps within the microbial food web. Alternatively, trophic position estimates based on alanine seem to also reflect microbial trophic steps. The difference between these estimations can provide an index of the microbial food web contribution to the trophic position of a particular consumer. Here, we examined the differences between both trophic position estimates through a meta‐analysis of the published data over a large range of marine consumers. The results suggest that the microbial contribution to trophic position decreases as trophic position increases in the marine pelagic food web, showing the lowest values in top consumers. Ureotelic organisms showed the highest microbial contributions. The current data point to an upper limit for the estimation of this index at δ¹⁵N values of glutamic acid or alanine of ~ 18.5‰, or trophic positions of 3.9 or 4.7 for fish and ammoniotelic organisms, respectively. Under the stable isotope lens, bacteria and protists may play a key role in the estimation of trophic positions of consumers. However, uncertainties that related both to the microbial loop (e.g., metabolic bacterial pathways) and to metazoan consumers (e.g., influence of the gut microbiome) still obscure the establishment of clear quantitative links between microbial and metazoan food webs.

The nutrient filter function is an important ecosystem service of seagrass meadows that mitigates the consequences of coastal eutrophication. In northeast Hainan in China, large seagrass areas were lost due to chronic eutrophication induced by untreated pond aquaculture effluents. However, in adjacent areas, seagrasses could survive due to seasonal exposure (i.e., not chronic) to eutrophication only. In a way, the conditions in these areas represent a transitional environment which allows investigating the effect of eutrophication on seagrass performance and their nitrogen uptake capacity. We tested how a 4‐week in situ nutrient enrichment affected inorganic nitrogen uptake rates of a multispecies seagrass meadow in eutrophic and non‐eutrophic seasons, in light and in darkness. All species maintained nitrogen uptake in the dark and preferred ammonium over nitrate. In the eutrophic season, the seagrass leaf biomass and growth were lower resulting in a lower nitrogen filter capacity. Among the species present, Cymodocea rotundata and Cymodocea serrulata covered 48% and 45%, respectively, of their daily nitrogen demand for leaf growth through leaf uptake from the water column, while it was only 30% for Thalassia hemprichii, the last remaining species in meadows degraded by eutrophication, as deduced from previous studies. It indicates that a multispecies seagrass meadow has a higher nitrogen filter capacity than a monospecific T. hemprichii meadow. By reducing seagrass diversity and, hence, the nitrogen filter function, eutrophication triggers a self‐reinforcing process. Once the nitrogen filtering capacity of a seagrass meadow is exhausted, further eutrophication and seagrass loss are expected.

Wetland plants form blue carbon ecosystems, which are key for climate change mitigation yet threatened by multiple local and global stresses. The capacity of blue carbon ecosystems to store large amounts of carbon relies on ecological processes, many of them regulated by plant traits that are shaped by the stress that plants encounter. This manuscript provides a perspective to investigate the effects of stress on carbon storage and sequestration capacity of blue carbon ecosystems based on the plant traits. A first step was to identify the biochemical, physiological, morphological and growth and development traits involved in the processes controlling carbon storage and sequestration in blue carbon ecosystems. Second, the existing knowledge on the stress effects on blue carbon processes and ecosystems was evaluated. Finally, the application of the trait‐based approach to understand and predict carbon dynamics in blue carbon ecosystems under different stresses' feedback was discussed. The conclusions provide key recommendations for a trait‐based approach in which the trinomial local and global stresses, plant traits and blue carbon are embraced together to identify the feedbacks between them, along with specific attention to the plant traits across the seascape, the stress interactions and the need of plant trait standardization. Read the free Plain Language Summary for this article on the Journal blog.

Understanding species-specific trait responses under future global change scenarios is of importance when doing conservation efforts and to make informed decisions within management projects. The combined and single effects of seawater acidification and warmer average temperature were investigated by means of the trait responses of Cymodocea serrulata, a tropical seagrass, under experimental conditions. After a 35-days exposure period, biochemical, morphological and photo-physiological trait responses were measured. Overall, biochemical traits mildly responded under the individual exposure of high temperature and increasing pCO₂ values. The response of C. serrulata was limited to a decrease in %C and an increase in the sucrose content in the rhizome under the high temperature treatment, 32°C. This suggests that this temperature was lower than the maximum tolerance limit for this species. Increasing pCO₂ levels increased %C in the rhizome, and also showed a significant increase in leaf δ ¹³C values. The effects of all treatments were sub-lethal, however, small changes in their traits could affect the ecosystem services they provide. Particularly, changes in tissue carbon concentrations may affect carbon storage capacity, one key ecosystem service. The simultaneous study of different types of trait responses contribute to establish a holistic framework of seagrass ecosystem health under climate change.