Along the northern Mid-Atlantic Ridge (nMAR), in habitats under moderate (<10 °C) hydrothermal influence on the Snake Pit vent field (SP), large assemblages dominated by Bathymodiolin mussels remain poorly characterised, contrary to those in warmer habitats dominated by gastropods and alvinocaridid shrimps that were recently described. In this study, we assessed and compared the population structure, biomass, diversity and trophic interactions of two Bathymodiolus puteoserpentis assemblages and their associated fauna at SP. Three sampling units distanced by 30 cm were sampled in 2014 during the BICOSE cruise at the top of the Moose site (\'\'Elan\'\' site), while few meters further down three others, distanced by ∼1 m were obtained in 2018 during the BICOSE 2 cruise at the edifice\'s base. We observed a micro-scale heterogeneity between these six sampling units partially explained by temperature variations, proximity to hydrothermal fluids and position on the edifice. Meiofauna dominate or co-dominate most of the sampling units, with higher densities at the base of the edifice. In terms of macrofauna, high abundance of Pseudorimula midatlantica gastropods was observed at the top of the vent edifice, while numerous Ophioctenella acies ophiuroids were found at the base. Contrary to what was expected, the apparent health and abundance of mussels seems to indicate a current climax stage of the community. However, the modification of B. puteoserpentis isotopic signatures, low number of juveniles decreasing over the two years and observations made during several French cruises in the study area raise questions about the fate of the B. puteoserpentis population over time, which remains to be verified in a future sampling campaign.

Knowledge about how ecological networks vary across global scales is currently limited given the complexity of acquiring repeated spatial data for species interactions. Yet, recent developments in metawebs highlight efficient ways to first document possible interactions within regional species pools. Downscaling metawebs towards local network predictions is a promising approach to using the current data to investigate the variation of networks across space. However, issues remain in how to represent the spatial variability and uncertainty of species interactions, especially for large-scale food webs. Here, we present a probabilistic framework to downscale a metaweb based on the Canadian mammal metaweb and species occurrences from global databases. We investigated how our approach can be used to represent the variability of networks and communities between ecoregions in Canada. Species richness and interactions followed a similar latitudinal gradient across ecoregions but simultaneously identified contrasting diversity hotspots. Network motifs revealed additional areas of variation in network structure compared with species richness and number of links. Our method offers the potential to bring global predictions down to a more actionable local scale, and increases the diversity of ecological networks that can be projected in space. This article is part of the theme issue \'Connected interactions: enriching food web research by spatial and social interactions\'.

Estuaries are essential habitats for recreational and commercial fish that are shaped by both natural and anthropogenic processes. In Louisiana a combination of climate change and planned coastal restoration actions is predicted to increase freshwater introduction to coastal estuaries. As such there is a need to quantify the relationships between estuarine fish ecology and salinity to aid in predicting how species will respond to shifts in salinity. We investigated the relative abundance and dietary niches of adult (24.5 ± 5.4 cm standard length) spotted seatrout Cynoscion nebulosus across varying salinity regimes (oligohaline, mesohaline, and polyhaline) within Barataria Bay, Louisiana, using a combination of net sampling and gut content and stable isotopes analysis. We found that the relative abundance of C. nebulosus was lowest at the oligohaline site, translating to approximately five fewer fish captured for every single psu decrease in a site\'s average annual salinity. In contrast, we found that diets and, to a lesser extent, isotopic niches had a high degree of overlap across sites with differing salinity regimes. Fish and penaeid shrimp were the most common and important prey taxa recovered from guts at all sites. The small isotopic differences found among sites were likely due to spatial variation in hydrogeochemical baselines, and the observed isotopic overlap provides support for the idea that C. nebulosus move between adjacent salinity regimes and forage throughout Barataria Bay. Our results contribute to a greater understanding of the salinity preference and trophic ecology of C. nebulosus that can aid in predicting their responses to future salinity and habitat changes within Barataria Bay associated with predicted climate change and planned coastal restoration actions.

Food webs are typically defined as being macro-organism-based (e.g., plants, mammals, birds) or microbial (e.g., bacteria, fungi, viruses). However, these characterizations have limits. We propose a multilayered food web conceptual model where microbial food webs are nested within food webs composed of macro-organisms. Nesting occurs through host-microbe interactions, which influence the health and behavior of host macro-organisms, such that host microbiomes likely alter population dynamics of interacting macro-organisms and vice versa. Here, we explore the theoretical underpinnings of multilayered food webs and the implications of this new conceptual model on food web ecology. Our framework opens avenues for new empirical investigations into complex ecological networks and provides a new lens through which to view a network\'s response to ecosystem changes.

The composition of fatty acids in the muscle tissue of the unique Central Asian carp-like fish, Potanin Altai osman Oreoleuciscus potanini, was studied for the first time. The populations of these fish in the reservoirs of the semiarid zone (Durgun and Taishir) during the period of their formation are considered. It was shown that the content of eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids in O. potanini corresponds to the median of this value in the order Cypriniformes. It was established that the basis of the food web of the herbivorous form of this species consists of microalgae (diatoms, Euglena and, possibly, chrysophytes), as well as bacteria. At the same time, the levels of bacterial biomarkers, 15-17BCFA and 17:0 were significantly higher in fish in the Durgun reservoir, whereas the level of EPA (diatom biomarker) in O. potanini was higher in the Taishir reservoir. The established higher values of the heavy nitrogen isotope content in the muscles of O. potanini from the Taishir reservoir are most likely associated with the yet unformed benthic communities and with the incomplete diversification of the riverine form of the Potanin Altai osman into lacustrine forms.

Assessments of ecosystem functioning are a fundamental ecological challenge and an essential foundation for ecosystem-based management. Species trophic position (TP) is essential to characterize food web architecture. However, despite the intuitive nature of the concept, empirically estimating TP is a challenging task due to the complexity of trophic interaction networks. Various methods are proposed to assess TPs, including using different sources of organic matter at the base of the food web (the \'baseline\'). However, it is often not clear which methodological approach and which baseline choices are the most reliable. Using an ecosystem-wide assessment of a tropical reef (Marquesas Islands, with available data for 70 coral reef invertebrate and fish species), we tested whether different commonly used TP estimation methods yield similar results and, if not, whether it is possible to identify the most reliable method. We found significant differences in TP estimates of up to 1.7 TPs for the same species, depending on the method and the baseline used. When using bulk stable isotope data, the choice of the baseline significantly impacted TP values. Indeed, while nitrogen stable isotope (δ15N) values of macroalgae led to consistent TP estimates, those using phytoplankton generated unrealistically low TP estimates. The use of a conventional enrichment factor (i.e. 3.4‰) or a \'variable\' enrichment factor (i.e. according to feeding guilds) also produced clear discrepancies between TP estimates. TPs obtained with δ15N values of source amino acids (compound-specific isotope analysis) were close to those assessed with macroalgae. An opposite seasonal pattern was found, with significantly lower TPs in winter than in summer for most species, with particularly pronounced differences for lower TP species. We use the observed differences to discuss possible drivers of the diverging TP estimates and the potential ecological implications.

Nutrients can shape ecological interactions but remain poorly integrated into ecological networks. Concepts such as nutrient-specific foraging nevertheless have the potential to expose the mechanisms structuring complex ecological systems. Nutrients also present an opportunity to predict dynamic processes, such as interaction rewiring and extinction cascades, and increase the accuracy of network analyses. Here, we propose the concept of nutritional networks. By integrating nutritional data into ecological networks, we envisage significant advances to our understanding of ecological processes from individual to ecosystem scales. We show that networks can be constructed with nutritional data to illuminate how nutrients structure ecological interactions in natural systems through an empirical example. Throughout, we identify fundamental ecological hypotheses that can be explored in a nutritional network context, alongside methods for resolving those networks. Nutrients influence the structure and complexity of ecological networks through mechanistic processes and concepts including nutritional niche differentiation, functional responses, landscape diversity, ecological invasions and ecosystem robustness. Future research on ecological networks should consider nutrients when investigating the drivers of network structure and function.

Dissolved inorganic carbon (DIC) provides a substrate for primary production in the lotic ecosystems, yet carbon\'s biogeochemical origination in the lotic food webs is still poorly constrained. Here, we assembled a global dataset of isotopic composition (i.e., 13C/12C or δ13C) of DIC and periphyton (algae being the primary producers) in river waters, and carried out a field study in two catchments respectively with carbonate and silicate dominated lithologies on the Tibetan Plateau. A two-endmember mixing model based on the datasets indicated that δ13C and concentrations of DIC in the river waters were largely determined by the catchment-scale chemical weathering of different lithologies. Meanwhile, a significant correlation was obtained between δ13C-DIC and δ13C-periphyton in the datasets, strongly implying that the origination of periphyton carbon was largely regulated by the catchment lithologies. The δ13C-periphyton compositions are also affected by isotopic fractionations during algal primary production, which, in turn, were closely related to the relationships between primary productivity and DIC availability in the rivers. The study advances our understanding of the origination and transfer of carbon biogeochemically bridging the geosphere and biosphere in the lotic ecosystems.

Nutrient dilution (ND) - the decrease in the concentration of nutritional elements in plant tissue - arises from an increase in the mass of carbohydrates and/or a decrease in the 20+ essential elements. Increasing CO2 levels and its promotion of biomass are linked to nutrient dilution. We build a case for nutrient dilution as a key driver in global declines in herbivore abundance. Herbivores must build element-rich animal tissue from nutrient-poor plant tissue, and their abundance commonly increases with fertilization of both macro- and micronutrients. We predict the global impacts of nutrient dilution will be magnified in some of Earth\'s most biodiverse, highly productive, and/or nutrient-poor ecosystems and should favor specific traits of herbivores, including sap-feeding and ruminant microbiomes.

Understanding the factors that determine the occurrence and strength of ecological interactions under specific abiotic and biotic conditions is fundamental since many aspects of ecological community stability and ecosystem functioning depend on patterns of interactions among species. Current approaches to mapping food webs are mostly based on traits, expert knowledge, experiments, and/or statistical inference. However, they do not offer clear mechanisms explaining how trophic interactions are affected by the interplay between organism characteristics and aspects of the physical environment, such as temperature, light intensity or viscosity. Hence, they cannot yet predict accurately how local food webs will respond to anthropogenic pressures, notably to climate change and species invasions. Herein, we propose a framework that synthesises recent developments in food-web theory, integrating body size and metabolism with the physical properties of ecosystems. We advocate for combination of the movement paradigm with a modular definition of the predation sequence, because movement is central to predator-prey interactions, and a generic, modular model is needed to describe all the possible variation in predator-prey interactions. Pending sufficient empirical and theoretical knowledge, our framework will help predict the food-web impacts of well-studied physical factors, such as temperature and oxygen availability, as well as less commonly considered variables such as wind, turbidity or electrical conductivity. An improved predictive capability will facilitate a better understanding of ecosystem responses to a changing world.