Food Web Studies Research Articles

Seasonal trends in Subantarctic plankton δ13C and δ15N are driven by phytoplankton dynamics and nutrient preference

AbstractThe carbon and nitrogen isotope ratios (δ13C and δ15N) of marine plankton record biogeochemical processes at the base of the food web. In the Southern Ocean, such data, predominantly from summer, have been used to infer surface CO2 concentrations and the potential for biological carbon export. However, variability in plankton δ13C and δ15N remains poorly understood, with the lack of seasonal measurements from the Southern Ocean emerging as a particular limitation. Here, we investigate the δ13C and δ15N of suspended particulate matter (SPM) and zooplankton collected from the Subantarctic Ocean in winter, summer, and autumn. The low summertime δ13CSPM and δ15NSPM can be explained by strong diatom reliance on nitrate supplied during winter mixing. A subsequent increase in δ13CSPM and decrease in δ15NSPM by autumn is consistent with iron limitation in mid‐ to late‐summer favoring diatom succession by nano‐phytoplankton that consume mainly recycled ammonium. By winter, bacterial decomposition of biomass outpaces new biomass production, decreasing δ13CSPM and raising δ15NSPM. The δ13C and δ15N of contemporaneously sampled zooplankton generally reflect the variability in δ13CSPM and δ15NSPM, with one autumn mismatch suggesting that in situ SPM is not always the dominant zooplankton food source. Our study shows how nutrient dynamics and phytoplankton community composition shape the seasonality of the Subantarctic's isotopic baselines, emphasizing a key role for iron availability. This work has implications for isotope‐based food web studies, highlighting the need to consider seasonal variability in δ13CSPM and δ15NSPM, as well as the different turnover times of phytoplankton (i.e., SPM) vs. zooplankton biomass.

Molecular characters of free-living nematode Panagrolaimus (Nematoda: Rhabditida) using 28S rDNA from South Africa

Bacterivorous nematodes are a type of nematodes that live in the soil. Free-living bacterivores are crucial for the soil food web, biological control, and ecological studies. Within the free-living bacterivorous nematodes, Panagrolaimus is a species that feeds on pathogenic bacteria, making it important in the transmission of plant-pathogenic bacteria. Additionally, Panagrolaimus plays a role in the nitrification of minerals in the soil. In 2023, a molecular study was conducted at the University of Limpopo to identify bacterivorous nematodes in South Africa's soils using the 28S rDNA marker. The nematode was extracted using the tray method, and its DNA was extracted using the Chelex method. It was then identified as Panagrolaimus. To further identify the nematode, 18S rDNA was amplified using specific primers. The Nblast analysis based on the small subunit ribosomal DNA showed that South African Panagrolaimus has a 95% similarity (EU543176) with the Belgian population. Based on a phylogenetic analysis using maximum likelihood, this species has been placed within the same clade as other species identified as Panagrolaimus, with highly supported (100) bootstrap values. Therefore, this species has been properly identified using 18S rDNA. However, to gain a better understanding of the phylogeny of Panagrolaimus, it is recommended to use other DNA markers as well.

Stable isotopes (15N) facilitate non-invasive labelling of large quantities of macroinvertebrates across different species and feeding types.

While macroinvertebrate dispersal operates at the individual level, predictions of their dispersal capabilities often rely on indirect proxies rather than direct measurements. To gain insight into the dispersal of individual specimens, it is crucial to mark (label) and capture individuals. Isotopic enrichment with 15N is a non-invasive method with the potential of labelling large quantities of macroinvertebrates. While the analysis of 15N is widely utilised in food web studies, knowledge on the specific utility of isotopic enrichment with 15N for mass labelling of macroinvertebrate individuals across different taxa and feeding types is limited. Previous studies have focused on single species and feeding types, leaving gaps in our understanding of the broader applicability of this method. Therefore, this study aimed to test and compare isotopic mass enrichment across several macroinvertebrate taxa and feeding types. We released 15NH4Cl at five stream reaches in North-Rhine Westphalia, Germany, and successfully enriched 12 distinct macroinvertebrate taxa (Crustacea and Insecta). Significant enrichment was achieved in active and passive filter feeders, grazers, shredders and predators, and predominantly showed positive correlations with the enrichment of the taxa's main food sources phytobenthos and particulate organic matter. Enrichment levels rose rapidly and peaked at distances between 50 m and 300 m downstream of the isotopic inlet; significant enrichment occurred up to 2000 m downstream of the isotopic inlet in all feeding types. Macroinvertebrate density estimates on the stream bottom averaged to a total of approximately 3.4 million labelled individuals of the 12 investigated taxa, thus showing the high potential of isotopic (15N) enrichment as a non-invasive method applicable for mass labelling across different macroinvertebrate feeding types. Hence, isotopic enrichment can greatly assist the analysis of macroinvertebrate dispersal through mark-and-recapture experiments, as it allows to measure the movement at the level of individual specimens.

Fatty acid profiles and stable isotope composition of Antarctic macroalgae: a baseline for a combined biomarker approach in food web studies

AbstractThe Western Antarctic Peninsula supports a diverse assemblage of > 100 described macroalgal species that contribute to the base of coastal food webs, but their contribution to local nearshore food webs is still uncertain across larger spatial scales. The analysis of biomarkers, specifically fatty acids and stable isotopes, offers a tool to clarify the trophic role of Antarctic macroalgae. The aim of this study was to describe the fatty acid profiles and stable isotope values of 31 algal species from three divisions (Chlorophyta—1, Ochrophyta—8, Rhodophyta—22) collected at the same sites for both biomarkers. Of these, 13 species had no previously published fatty acid profiles. Most species were rich in polyunsaturated fatty acids (PUFAs), suggesting they are highly nutritious and could be a resource of essential fatty acids for consumers. This was specifically noticeable in the overall low PUFA ratio (∑ω6/∑ω3 ratio), with low ratios being an indicator of high nutritional quality for consumers. Fatty acid profiles of macroalgae grouped strongly by phylogeny (at the levels of division, order, and family), while stable isotope groupings were more driven by the physiological properties of the species. Specifically, some closely related red algal species exhibited very different stable isotope values based on their carbon concentrating mechanisms, with highly 13C-depleted values in several Rhodophyta species. The fact that the two biomarker approaches created different groupings of Antarctic macroalgae collected at the same locations emphasizes that their combined application can be a powerful tool in Antarctic coastal food web studies.

Why we do science—marine ecosystems in context

Abstract Any scientific career is a mix of planning and stochastic events, often with a fair share of the latter. I illustrate this by the evolution of my own career. Ecosystem studies of food webs under the impact of eutrophication (Master), and carbon cycling in DOC (Dissolved Organic Carbon)-rich lakes (PhD) led me to elemental ratios in organisms and the establishment of ecological stoichiometry. The role of phosphorus (P) in cellular processes again led to research on the evolution and regulation of genome size. As climate came higher on the agenda, it was time to apply the basic research on the C-cycle and climate in a wider context. As natural scientists, we should also engage in even wider contexts, and I have enjoyed discussions and co-operation with philosophers, psychologist, and social scientists. This helps seeing our own work in context. We should also reflect on why we do science. I have always felt that science should also add purpose to life by giving something back to society, and I have devoted much time to outreach, public talks, debates, and writing popular science books. It takes some time, but it is also rewarding and important—perhaps even more so than yet another paper.

Newly identified nematodes from the Great Salt Lake are associated with microbialites and specially adapted tohypersaline conditions.

Extreme environments enable the study of simplified food-webs and serveas models for evolutionary bottlenecks and early Earth ecology. Weinvestigated the biodiversity of invertebrate meiofauna in the benthic zone of the Great Salt Lake (GSL), Utah, USA, one of the most hypersaline lake systems in the world. The hypersaline bays within the GSL are currently thought to support only two multicellular animals: brine fly larvae andbrine shrimp. Here, we report the presence, habitat, and microbial interactions of novel free-living nematodes. Nematode diversity drops dramatically along a salinity gradient from a freshwater river into the south arm of the lake. In Gilbert Bay, nematodes primarily inhabit reef-like organosedimentary structures built by bacteria called microbialites. These structures likely provide a protective barrier to UV and aridity, and bacterial associations within them may support life in hypersaline environments. Notably, sampling from Owens Lake, another terminal lake in the Great Basin that lacks microbialites, did not recover nematodes from similar salinities. Phylogenetic divergence suggests that GSL nematodes represent previously undescribed members of the family Monhysteridae-one of the dominant fauna of the abyssal zone and deep-sea hydrothermal vents. These findings update our understanding of halophile ecosystems and the habitable limit of animals.

Simple bulk pigment analysis suggests microphytobenthos contributions to food webs may be underestimated due to isotopic contamination by remineralized wetland carbon

Stable isotope studies have revealed the importance of microphytobenthos (MPB) in coastal food webs. Microalgae typically have δ13C values between depleted C3 wetland/terrestrial macrophytes, and enriched C4 macrophytes and seagrasses. However, the challenges of obtaining clean samples of microalgae from sediments means they are often represented by limited sampling in many food web studies; consequently, we have a limited understanding of spatial and temporal variation in their δ13C values. We tested a simple technique to measure the δ13C of bulk pigments extracted from surficial sediments to represent MPB and applied it to quantify fine scale spatial variation in MPB δ13C around salt marshes. The bulk extraction method is logistically simple, and drives substantial but relatively consistent fractionation in δ13C of −3.5 ± 0.13 ‰ (mean ± 1 S.E., range = 2.3–4.4 ‰, n = 18 paired comparisons) compared to whole cell values. The consistency in fractionation suggests that spatial and temporal δ13C patterns measured in field samples should reflect real variation in source values, and that measured values could be corrected and incorporated into isotope mixing models. In 88 MPB samples among four marsh sites over two summers, MPB δ13C in marsh creeks was lower by an average of 4.4 ± 0.72 ‰ and up to 8.4 ‰ compared to sites along the outer marsh-open water fringe 10's of m away. Few food web studies incorporate this magnitude of variation in their MPB source estimates into mixing models. Over three weekly samplings at one marsh creek site, low tide dissolved inorganic carbon (DIC) δ13C was similarly lower by 4.8 ± 0.36 ‰ and up to 6.2 ‰ compared to high tide and adjacent open water DIC values. The significant small-scale variability in MPB δ13C appears to be driven by remineralized marsh carbon which depletes the DIC δ13C in the marsh creeks, a phenomenon that has long been recognized but is rarely considered in food web studies. Mixing models that assume a narrower range in MPB source values will erroneously attribute isotopically variable MPB contributions to end-member production sources thereby clouding our understanding of energy flows through coastal seascapes.

Testing the effectiveness of different wash protocols to remove body surface contaminants in invertebrate food web studies.

Molecular gut content analysis via diagnostic PCR or high-throughput sequencing (metabarcoding) of consumers allows unravelling of feeding interactions in a wide range of animals. This is of particular advantage for analyzing the diet of small invertebrates living in opaque habitats such as the soil. Due to their small body size, which complicates dissection, microarthropods are subjected to whole-body DNA extraction-step before their gut content is screened for DNA of their food. This poses the problem that body surface contaminants, such as fungal spores may be incorrectly identified as ingested food particles for fungivorous species. We investigated the effectiveness of ten methods for body surface decontamination in litter-dwelling oribatid mites using Steganacarus magnus as model species. Furthermore, we tested for potential adverse effects of the decontamination techniques on the molecular detection of ingested prey organisms. Prior to decontamination, oribatid mites were fed with an oversupply of nematodes (Plectus sp.) and postmortem contaminated with fungal spores (Chaetomium globosum). We used diagnostic PCR with primers specific for C. globosum and Plectus sp. to detect contaminants and prey, respectively. The results suggest that chlorine bleach (sodium hypochloride, NaClO, 5%) is most efficient in removing fungal surface contamination without significantly affecting the detection of prey DNA in the gut. Based on these results, we provide a standard protocol for efficient body surface decontamination allowing to trace the prey spectrum of microarthropods using molecular gut content analysis.

Plant communities and food webs

Recent theoretical work has provided major new insights into the ways that species interactions in food webs are organized in ways that permit the coexistence of significant numbers of species. But, we seem to have forgotten about trees! Not the phylogenetic ones that are increasingly important for dissecting the evolutionary structure of food webs, but the trees, shrubs and grasses that are the basal species in all terrestrial ecosystems. Many of the food webs available for analysis over the last 30 years were based on freshwater or marine systems where algae were the main plants. Trees are very different from algae; they can live for centuries, while annually producing leaves, fruits and seeds that provide nutrients for a diversity of species on higher trophic levels. In sharp contrast to algae, they are only partly consumed by herbivores and usually compensate or recover from herbivory. Most of the biomass in terrestrial systems is in the plants, this again contrasts with aquatic systems, where most of the biomass is in primary and secondary consumers. Moreover, each individual tree supports its own food web of species that are only partially coupled to those of surrounding trees. If we are going to apply our theoretical understanding of food-web structure to species-rich terrestrial ecosystems in ways that are insightful for conservation, then we need a deeper examination of the role that higher plants play in food webs. While community ecology has developed an increasingly detailed understanding of the ways plant communities are organized, this seems to have evolved almost independently of the food-web literature. In this article, we make a plea to more sharply consider higher plants in food webs and to do this by combining recent theoretical work on food webs, with recent empirical and theoretical work on plant communities. Ultimately, we argue for a deeper integration of plant community ecology into studies of food webs.

A missing trophic link: Contribution of the microbial loop to the estimation of the trophic position of pelagic consumers

AbstractTrophic 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.

Energy flow across soil food webs of different ecosystems: Food webs with complex structures support higher energy flux

Energy flux in food webs is a fundamental feature of ecosystems and an important link between biodiversity and ecosystem function. Studies of energy flow through the soil nematode food web have emerged in recent year; however, it is not clear whether and how the nematode food web structure affects the energy flow. In this study, the structures and carbon flux of soil nematode food webs in four different ecosystems, namely, forageland, cropland, secondary forest, and grass-shrubland, were assessed. The food web structure was represented by nematode community indices such as maturity index, structure index, genus and trophic group diversity, and slope of size spectra. The results showed that carbon flux through the nematode food webs was the highest and lowest in the secondary forest and forageland, respectively. The total carbon flux of nematode food web was higher in the cropland than that in the grass-shrubland. The magnitudes of bacterial and fungal energy channels were basicallyequal in the forageland and secondary forest. The energy flow through the bacterial channel was greater than that through the fungal channel in the grass-shrubland, whereas the pattern was reversed in the cropland. The total carbon flux through the nematode food web, the carbon fluxes through the bacterial or fungal channel, and the energy flow uniformity were positively correlated with the nematode structure index, maturity index, and genus diversity index. The trophic group diversity and size spectra slope of nematode communities were not significantly correlated with the carbon flux. The results indicate that mature and complex soil communities support high energy flux across soil food webs.

Cascading carrion: Opportunistic predation at deer gut piles

Carrion is increasingly recognized as an important part of food web ecology that impacts multiple trophic levels and creates an arena for multiple species interactions. The pulsed nature of hunter-derived carrion, in the form of gut piles, is a form of carrion that has been overlooked in the study of food webs and scavenger interaction. The Offal Wildlife Watching project aims to better understand scavenger species that benefit from offal and how they interact with each other and this resource. Through citizen science efforts by hunters and Zooniverse volunteers, we have collected and analyzed over 230,000 images of scavengers at white-tailed deer gut piles. At some gut piles, we have observed barred owls and bobcats both scavenging at hunter provided gut piles and preying on rodents that were also attracted to the gut pile. This interaction, made possible by a food subsidy that is historically novel in time and space, may have implications for rodent populations, predator survival and fecundity, and the activity patterns of both. Continued research and investigation will shed light on the impacts of cascading carrion on species interaction.

Round goby (Neogobius melanostomus) δ13C/δ15N discrimination values and comparisons of diets from gut content and stable isotopes in Oneida Lake.

Gut content analyses have found that round gobies (Neogobius melanostomus) are highly dependent on dreissenid mussels but stable isotope analysis has often suggested that the dreissenid contribution is lower. However, estimation of dietary contributions with stable isotopes relies on accurate discrimination factors (fractionation factors). To test if discrimination values commonly used in aquatic food web studies are suitable for round gobies, we collected round gobies from Oneida Lake, raised them for 63 days under four different diets (Chironomus plumosus, Mytilus chilensis, Dreissenia spp., Euphausia superba) and measured the change in white muscle δ13C and δ15N. Gobies were also collected throughout Oneida Lake for gut content and stable isotope analysis. Diets changed as round gobies grew, with small round gobies (17-42mm) feeding mostly on cladocera and chironomids, intermediate sized gobies (43-94mm) transitioning from chironomid to dreissenid consumption, and larger gobies (95-120mm) predominantly consuming dreissenids, similar to findings in other studies. Discrimination factors were obtained by fitting a commonly used asymptotic regression equation describing changes in fish δ13C and δ15N as a function of time and diet stable isotope ratios. The discrimination factor determined for δ13C (-0.4‰ ± 0.32, SE) was lower than the "standard" value of 0.4‰, while that of δ15N (4.0‰ ± 0.32, SE) was higher than the standard value of 3.4‰. Turnover rates for both δ13C and δ15N were estimated as 0.02 ‰*day-1. The use of experimentally determined discrimination factors rather than "standard" values resulted in model estimates that agree more closely with the observed increasing importance of dreissenids in gut content of larger gobies. Our results suggest that the importance of dreissenid mussels inferred from stable isotope studies may be underestimated when using standard isotopic discrimination values.

SystemLink: Moving beyond Aquatic–Terrestrial Interactions to Incorporate Food Web Studies

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Understanding Paleoclimate Through Southern California’s Packrat Middens

Documenting how ecosystems responded to climate change in the past can inform models for the future relevant to conservation decision-making. Packrat middens, or giant nests consisting of gathered plant material made by rodents of the genus Neotoma, represent one of the best data sources for understanding how plant communities change over time. These nests can be preserved for thousands of years, spanning episodes of glacial-interglacial variation, such as the Pleistocene-Holocene transition, or more recent fluctuations, such as the Little Ice Age and Medieval Warm Period. We used the USGS-NOAA paleoclimate open-access database to develop spatiotemporal reconstructions of plant species in Southern California, incorporating more than 242 individual middens with a total of 193 available radiocarbon dates spanning around 44,493 years cal BP. As a result, 374 plant species were identified within the SoCal middens, with Joshua Tree (Yucca brevifolia), Joint Pine (Ephedra species), and California Juniper (Juniperus californica) being the most abundant. We then applied these data to reconstruct past vegetation communities and contextualize a recently discovered asphaltic midden recovered from the La Brea Tar Pits (Los Angeles, California). The La Brea midden represents both the oldest and most coastal midden in our dataset in California and contains vegetation spanning Marine Isotope Stages 2-3. Ultimately, this research allows us to paint a picture of changing vegetation over time as a baseline for palaeoecological food web studies and can be compared with present day packrat nests found in this urban biodiversity hotspot. These data will be curated through the Conservation Paleobiology Network’s urban vegetation working group (based in Los Angeles, California) and made available for use by conservation and management projects throughout the region. With this information, CPN can implement habitat restoration projects in SoCal.

Mercury bioaccumulation and speciation in coastal invertebrates: Implications for trophic magnification in a marine food web

Studies on mercury bioaccumulation and biomagnification in coastal invertebrates in eastern Canada are limited, but these data are necessary to determine risk of mercury exposure effects in upper trophic level organisms. We quantified methylmercury (MeHg), total mercury (THg), and stable isotopes of δ13C and δ15N in 14 species of invertebrates in the Minas Basin. The overall mean concentration of MeHg (12.78 ± 11.23 ng/g dw) was approximately 10 times below the Canadian guideline for the protection of wildlife consumers like fish and birds of 157.20 ng/g dry weight (dw). Invertebrates at higher trophic positions (δ15N) had greater THg and particularly MeHg. The Trophic Magnification Factors (TMF) for MeHg and THg (1.59 and 1.21 respectively) were similar to others reported in studies of food webs containing higher trophic level organisms.

From metabarcoding time series to plankton food webs: The hidden role of trophic hierarchy in providing ecological resilience

AbstractThe advent of metabarcoding (metaB) in aquatic ecology has provided a huge amount of information on plankton biodiversity worldwide. However, the large datasets obtained with that approach are still partially explored, especially for what concerns the study of trophic interactions and food webs. In this study, we analysed a metaB time series from the Long‐Term Ecological Research station MareChiara (LTER‐MC) in the Gulf of Naples, Mediterranean Sea, Italy, to describe the link between plankton diversity and food‐web structure. We derived co‐occurrence networks from metaB time series, identified putative trophic interactions among co‐occurrences based on biological information (body size and trophic habit) available for planktonic organisms detected by metaB, and converted co‐occurrence networks into conceptual models of food webs. The latter showed structural properties resembling ecological processes, because network modularity (the presence of semi‐independent sub‐networks) paralleled trophic hierarchy (the dimensional difference between predator and prey). We also analysed the role of planktonic organisms in maintaining network modularity. The largest predators occupied distinct modules, suggesting niche partitioning, whereas the smallest preys worked as fundamental connectors between larger predators (and different modules). Overall, the presence of trophic hierarchy and modularity shown herein supports the view of the high ecological resilience of plankton, pursued via food‐web rewiring, to environmental shifts.

Food web structure and microenvironment affect Chagas disease vector infection and abundance in a rural landscape

AbstractVector‐borne pathogen transmission is shaped by multiple abiotic and biotic factors. Understanding the relative importance of these factors on vector abundance and infection is important for developing vector‐borne disease control strategies. The crown of the Attalea butyracea palm provides a natural arboreal mesocosm suitable for studying how food web relations and microclimate affect Chagas disease vectors of the genus Rhodnius, which feed on vertebrate blood and interact with many vertebrate and invertebrate species, vegetation, and detritus within the palm crown. We performed a cross‐sectional, observational study of A. butyracea crowns using a community ecology network approach to evaluate abiotic and biotic conditions associated with Rhodnius pallescens abundance and infection with Trypanosoma cruzi, the Chagas disease etiologic agent. We collected 1098 R. pallescens from 105 A. butyracea crowns in rural landscapes of Panama. In a palm subset (N = 49), we recorded microclimate and habitat conditions and counted vertebrate and invertebrate species in order to characterize palm crown food webs. We used food web metrics, namely Generality (average number of prey per predator) and Vulnerability (average number of predators per prey) to evaluate associations between palm community trophic structure, vector predators, vector blood meal species composition, vector abundance, and vector T. cruzi infection. Field data analyzed with generalized linear models showed that vector abundance and infection in a given palm crown were influenced by a combination of geographic location, land‐use type, palm crown animal community composition, and microhabitat conditions. Vector abundance was negatively associated with increased overall palm crown community diversity. However, vector abundance was positively associated with the invertebrate predator and mammal community. R. pallescens Vulnerability score, which measures predation pressure on the vector within the palm crown, was positively associated with vector abundance. Vector infection with T. cruzi was positively associated with mammal presence; vectors in palms with mammals were about 25 times more likely to be infected with T. cruzi compared with vectors in palms without mammals. Our findings suggest that a holistic study of food webs and environmental variables at the local scale can provide useful information for vector‐borne disease management across a wide habitat range.

Differential food utilization of benthic amphipods of a tropical estuary: a stable isotopic (δ13C and δ15N) investigation.

Globally, stable isotopic methods are rapidly being employed in food web studies to quantify the relative proportions of diverse primary carbon sources in an animal diet. Tropical estuarine ecosystems, one of the vital marine habitats, harbor complex food webs due to the multiplicity of habitats and resources. The complex system of inputs and exchanges of organic matter sometimes complicate identification of the carbon sources that support the higher trophic levels. To assess the primary food sources of four abundant amphipod species in the anthropogenically influenced upper Vashishti estuary, Carbon and Nitrogen stable isotopes were employed. Seasonal variations were significant for δ13C and δ15N of suspended particulate organic matter (SPOM) and also for δ13C of sediment organic matter (SOM), but not for the isotopic values of individual amphipod species. Significant variations in the δ13C and δ15N values between amphipods suggested species-specific diets. Mixing model output indicated discrepancies in seasonal variations of food preferences between species. Trophic level values of all amphipod species indicated seasonality with lower values during post-monsoon. The low δ15N values of SPOM, SOM, and amphipod species were possibly due to the anthropogenic input of nitrogen in the estuary.

The combined effects of land use and seasonal environmental factors on stream food web structure

Land use and seasonal changes in environmental conditions influence biological communities and their trophic interactions in riverine ecosystems. However, how land use and the seasonality of environmental conditions jointly influence the food web structure of riverine ecosystems remains unclear. Here, we conducted a comparative study on basal resources, macroinvertebrates, and fish at woodland and urban sites to explore the combined effects of land use and environmental conditions during spring, autumn, and winter on the food web structure of a subtropical river in China. We used δ13C and δ15N to trace consumers’ diets and calculate community-level metrics within food webs. At woodland sites, we found that allochthonous sources contributed significantly more to consumers’ diets in the high flow season (53%) than in the low flow season (around 30%), but allochthonous sources contributed less than 30% at urban sites. The seasonal flooding facilitated the acquisition of terrestrial sources at woodland sites, while the impervious surface cover at urban sites cut off the influx of terrestrial inputs. The isotopic niche space of basal resources at both woodland and urban sites was significantly higher in the high flow season. However, the isotopic niche space of consumers at urban sites was not significantly different between seasons. Trophic length, niche width, and trophic redundancy showed no seasonal differences at urban sites, but trophic length increased significantly during the high flow season at woodland sites. Our temporal food web study at urban and woodland rivers illustrated the combined effects of land use and seasonal environmental conditions on the food web structure, and highlighted the role of allochthonous carbon in supporting biological communities and the importance of lateral and longitudinal connectivity in river ecosystems.