Predicting nutrient excretion of aquatic animals with metabolic ecology and ecological stoichiometry: a global synthesis

Temporal and spatial differences in nitrogen and phosphorus biogeochemistry and ecosystem functioning of a hypertrophic lagoon (Curonian Lagoon, SE Baltic Sea) revealed via Ecological Network Analysis

In coastal lagoons, eutrophication and hydrology are interacting factors that produce distortions in biogeochemical nitrogen (N) and phosphorus (P) cycles. Such distortions affect nutrient relative availability and produce cascade consequences on primary producer's community and ecosystem functioning. In this study, the seasonal functioning of a coastal lagoon was investigated with a multielement approach, via the construction and analysis of network models. Spring and summer networks, both for N and P flows, have been simultaneously compiled for the northern transitional and southern confined area of the hypertrophic Curonian Lagoon (SE Baltic Sea). Ecological Network Analysis was applied to address the combined effect of hydrology and seasonality on biogeochemical processes. Results suggest that the ecosystem is more active and presents higher N and P fluxes in summer compared to spring, regardless of the area. Furthermore, larger internal recycling characterizes the confined compared to the transitional area, regardless of the season. The two areas differed in the fate of available nutrients. The transitional area received large riverine inputs that were mainly transferred to the sea without the conversion into primary producers' biomass. The confined area had fewer inputs but proportionally larger conversion into phytoplankton biomass. In summer, particularly in the confined area, primary production was inefficiently consumed by herbivores. Most phytoplanktonic N and P, in the confined area more than in the transitional area, were conveyed to the detritus pathway where P, more than N, was recycled, contributing to the unbalance in N:P stoichiometry and favouring N-fixing cyanobacteria over other phytoplankton groups. The findings of this study provide a comprehensive understanding of N and P circulation patterns in lagoon areas characterized by different hydrology. They also support the importance of a stoichiometric approach to trace relative differences in N and P recycling and abundance, that promote blooms, drive algal communities and whole ecosystem functioning.

Species trait diversity sustains multiple dietary nutrients supplied by freshwater fisheries

Species, through their traits, influence how ecosystems simultaneously sustain multiple functions. However, it is unclear how trait diversity sustains the multiple contributions biodiversity makes to people. Freshwater fisheries nourish hundreds of millions of people globally, but overharvesting and river fragmentation are increasingly affecting catches. We analyse how loss of nutritional trait diversity in consumed fish portfolios affects the simultaneous provisioning of six essential dietary nutrients using household data from the Amazon and Tonlé Sap, two of Earth's most productive and diverse freshwater fisheries. We find that fish portfolios with high trait diversity meet higher thresholds of required daily intakes for a greater variety of nutrients with less fish biomass. This beneficial biodiversity effect is driven by low redundancy in species nutrient content profiles. Our findings imply that sustaining the dietary contributions fish make to people given declining biodiversity could require more biomass and ultimately exacerbate fishing pressure in already-stressed ecosystems.

SEED: A framework for integrating ecological stoichiometry and eco-evolutionary dynamics

Characterising the extent and sources of intraspecific variation and their ecological consequences is a central challenge in the study of eco-evolutionary dynamics. Ecological stoichiometry, which uses elemental variation of organisms and their environment to understand ecosystem patterns and processes, can be a powerful framework for characterising eco-evolutionary dynamics. However, the current emphasis on the relative content of elements in the body (i.e. organismal stoichiometry) has constrained its application. Intraspecific variation in the rates at which elements are acquired, assimilated, allocated or lost is often greater than the variation in organismal stoichiometry. There is much to gain from studying these traits together as components of an 'elemental phenotype'. Furthermore, each of these traits can have distinct ecological effects that are underappreciated in the current literature. We propose a conceptual framework that explores how microevolutionary change in the elemental phenotype occurs, how its components interact with each other and with other traits, and how its changes can affect a wide range of ecological processes. We demonstrate how the framework can be used to generate novel hypotheses and outline pathways for future research that enhance our ability to explain, analyse and predict eco-evolutionary dynamics.

A 34-year survey under phosphorus decline and warming: Consequences on stoichiometry and functional trait composition of freshwater macroinvertebrate communities

Worldwide, freshwater systems are subjected to increasing temperatures and nutrient changes. Under phosphorus and nitrogen enrichment consumer communities are often thought to shift towards fast-growing and P-rich taxa, supporting the well-known link between growth rate and body stoichiometry. While these traits are also favoured under warming, the temperature effect on stoichiometry is less clear. As recently shown, there is a general link between functional traits and body stoichiometry, which makes the integration of stoichiometric traits a promising tool to help understanding the mechanisms behind taxonomic and functional community responses to nutrient changes and/or warming. Yet, such approaches have been scarcely developed at community level and on a long-term perspective. In this study, we investigated long-term responses in stoichiometry and functional trait composition of macroinvertebrate communities to nutrient changes (decreasing water P; increasing water N:P) and warming over a 34-year period in the Middle Loire River (France), testing the potentially opposing responses to these drivers. Both drivers should cause shifts in species composition, which will alter the overall community stoichiometry and functional composition following assumptions from ecological stoichiometry theory. We found that the macroinvertebrate community shifted towards P-poor taxa, causing significant trends in overall community stoichiometry which indicates long-term changes in the nutrient pool provided by these consumers (i.e. decrease in %N and %P, increase in N:P). Further, while the former high-P conditions favoured traits associated to detritus feeding and fast development (i.e. small maximum body size, short life duration), recent conditions favoured predators and slow-developing taxa. These results suggest nutrients to be a more important driver than temperature over this period. By providing a pivotal link between environmental changes and functional trait composition of communities, approaches based on stoichiometric traits offer sound perspectives to investigate ecological relationships between multiple drivers operating at various scales and ecosystem functioning.

The significance of partial migration for food web and ecosystem dynamics

Migration is ubiquitous and can strongly shape food webs and ecosystems. Less familiar, however, is that the majority of life cycle, seasonal and diel migrations in nature are partial migrations: only a fraction of the population migrates while the other individuals remain in their resident ecosystem. Here, we demonstrate different impacts of partial migration rendering it fundamental to our understanding of the significance of migration for food web and ecosystem dynamics. First, partial migration affects the spatiotemporal distribution of individuals and the food web and ecosystem-level processes they drive differently than expected under full migration. Second, whether an individual migrates or not is regularly correlated with morphological, physiological, and/or behavioural traits that shape its food-web and ecosystem-level impacts. Third, food web and ecosystem dynamics can drive the fraction of the population migrating, enabling the potential for feedbacks between the causes and consequences of migration within and across ecosystems. These impacts, individually and in combination, can yield unintuitive effects of migration and drive the dynamics, diversity and functions of ecosystems. By presenting the first full integration of partial migration and trophic (meta-)community and (meta-)ecosystem ecology, we provide a roadmap for studying how migration affects and is affected by ecosystem dynamics in a changing world.

The role of vital dietary biomolecules in eco-evo-devo dynamics

The physiological dependence of animals on dietary intake of vitamins, amino acids, and fatty acids is ubiquitous. Sharp differences in the availability of these vital dietary biomolecules among different resources mean that consumers must adopt a range of strategies to meet their physiological needs. We review the emerging work on omega-3 long-chain polyunsaturated fatty acids, focusing predominantly on predator-prey interactions, to illustrate that trade-off between capacities to consume resources rich in vital biomolecules and internal synthesis capacity drives differences in phenotype and fitness of consumers. This can then feedback to impact ecosystem functioning. We outline how focus on vital dietary biomolecules in eco-eco-devo dynamics can improve our understanding of anthropogenic changes across multiple levels of biological organization.

Marine heatwaves of differing intensities lead to distinct patterns in seafloor functioning

Marine heatwaves (MHWs) are increasing in frequency and intensity due to climate change. Several well-documented effects of heatwaves on community structure exist, but examples of their effect on functioning of species, communities or ecosystems remain scarce. We tested the effects of short-term, moderate and strong MHWs on macrofauna bioturbation and associated solute fluxes as examples of ecosystem functioning. We also measured macrofaunal excretion rates to assess effects of temperature on macrofauna metabolism. For this experiment, we used unmanipulated sediment cores with natural animal communities collected from a muddy location at 32 m depth in the northern Baltic Sea. Despite the mechanistic effect of bioturbation remaining unchanged between the treatments, there were significant differences in oxygen consumption, solute fluxes and excretion. Biogeochemical and biological processes were boosted by the moderate heatwave, whereas biogeochemical cycling seemed to decrease under a strong heatwave. A prolonged, moderate heatwave could possibly lead to resource depletion if primary production cannot meet the demands of benthic consumption. By contrast, decreased degradation activities under strong heatwaves could lead to a build-up of organic material and potentially hypoxia. The strong variability and the complexity of the response highlight the context dependency of these processes complicating future predictions.

Life stage and taxonomy the most important factors determining vertebrate stoichiometry: A meta-analysis

Whole-body elemental composition is a key trait for determining how organisms influence their ecosystems. Using mass-balance, ecological stoichiometry predicts that animals with higher concentrations of element X will selectively retain more X and will recycle less X in their waste than animals with lower X concentrations. These animals will also store high quantities of X during their lives and after their deaths (prior to full decomposition). Vertebrates may uniquely impact nutrient cycling because they store high quantities of phosphorus (P) in their bones. However, vertebrates have diverse body forms and invest variably in bone. Current analyses of vertebrate elemental content predominately evaluate fishes, typically neglecting other vertebrates and leaving much of the diversity unexplored. We performed a systematic review and identified 179 measurements of whole-body percent phosphorus (%P), percent nitrogen (%N), and N to P ratio (N:P) from 129 unique species of non-fish vertebrates (amphibians: 39 species; reptiles: 19 species; birds: 27 species; mammals: 46 species). We found that %P (mean: 1.94%; SD [standard deviation] = 0.77) and N:P (mean: 12.52) varied with taxonomy and life stage, while %N (mean: 10.51%; SD = 3.25) varied primarily with taxonomy. Habitat, diet, and size had small and inconsistent effects in different groups. Our study highlights two research gaps. Life stage, which is frequently neglected in stoichiometric studies, is an important factor determining vertebrate %P. Furthermore, amphibians dominate our dataset, while other vertebrate taxa are poorly represented in the current literature. Further research into these neglected vertebrate taxa is essential.

Body size has primacy over stoichiometric variables in nutrient excretion by a tropical stream fish community

Ecological Stoichiometry (ES) and the Metabolic Theory of Ecology (MTE) are the main theories used to explain consumers’ nutrient recycling. ES posits that imbalances between an animal’s body and its diet stoichiometry determine its nutrient excretion rates, whereas the MTE predicts that excretion reflects metabolic activity arising from body size and temperature. We measured nitrogen, phosphorus and N:P excretion, body N:P stoichiometry, body size, and temperature for 12 fish species from a Brazilian stream. We fitted competing models reflecting different combinations of ES (body N:P, armor classification, diet group) and MTE (body size, temperature) variables. Only body size predicted P excretion rates, while N excretion was predicted by body size and time of day. N:P excretion was not explained by any variable. There was no interspecific difference in size-scaling coefficients neither for N nor for P. Fitted size scaling coefficients were lower than the MTE prediction of 0.75 for N (0.58), and for P (0.56). We conclude that differences in nutrient excretion among species within a shared environment primarily reflect contrasts in metabolic rates arising from body size, rather than disparities between consumer and resource stoichiometry. Our findings support the MTE as the primary framework for predicting nutrient excretion rates.

Stressors Increase the Impacts of Coastal Macrofauna Biodiversity Loss on Ecosystem Multifunctionality

There is substantial evidence that biodiversity underpins ecosystem functioning, but it is unclear how these relationships change with multiple stressors in complex real-world settings. Coastal zones are affected by numerous stressors (for example, sediment input and nutrient runoff from land) and the cumulative effects of these stressors may result in pronounced and unexpected changes in the functioning of ecosystems. To investigate the cumulative effects of turbidity and elevated nutrients on coastal biodiversity-ecosystem functioning relationships, we performed a large-scale field experiment manipulating in situ sediment porewater ammonium concentrations and measured multiple ecosystem functions related to carbon fixation and mineralisation in 15 estuaries with varying levels of turbidity. The results indicated that the benthic macrofauna diversity (species richness, abundance, and functional richness) declined with increased porewater ammonium concentrations and there were clear thresholds in light levels at the seafloor in relation to the biodiversity-ecosystem function relationships. Multifunctionality indices (an integrated index of all measured functions) in moderately turbid and turbid estuaries (daily mean seafloor PAR < 420 µmol m−2 s−1) decreased with the loss of macrofauna biodiversity. Functioning in low-turbidity estuaries (daily mean PAR > 420 µmol m−2 s−1) however remained relatively constant, suggesting that they were more resilient against the nutrient-induced biodiversity loss. Our results demonstrate that ecosystems already stressed by stressors that alter functional performance (turbidity) may be more prone to loss of overall functioning if biodiversity is reduced by another stressor (nutrient enrichment), highlighting the potential snowballing effects of cumulative change.

Integrating physiology and environmental dynamics to operationalize environmental DNA (eDNA) as a means to monitor freshwater macro-organism abundance

Research has demonstrated consistent positive correlations between organism abundance and absolute environmental DNA (eDNA) concentrations. Robust correlations in laboratory experiments indicate strong functional links, suggesting the potential for eDNA to monitor organism abundance in nature. However, correlations between absolute eDNA concentrations and organism abundance in nature tend to be weaker because myriad biotic and abiotic factors influence steady-state eDNA concentrations, decoupling its direct functional link with abundance. Additional technical challenges can also weaken correlations between relative organism abundance and relative eDNA data derived from metabarcoding. Future research must account for these factors to improve the inference of organism abundance from eDNA, including integrating the effects of organism physiology on eDNA production, eDNA dynamics in lentic/lotic systems, and key environmental parameters that impact estimated steady-state concentrations. Additionally, it is critical to manage expectations surrounding the accuracy and precision that eDNA can provide - eDNA, for example, cannot provide abundance estimates comparable to intensively managed freshwater fisheries that enumerate every individual fish. Recent developments, however, are encouraging. Current methods could provide meaningful information regarding qualitative conservation thresholds and emergent research has demonstrated that eDNA concentrations in natural ecosystems can provide rough quantitative estimates of abundance, particularly when models integrate physiology and/or eDNA dynamics. Operationalizing eDNA to infer abundance will probably require more than simple correlations with organism biomass/density. Nevertheless, the future is promising - models that integrate eDNA dynamics in nature could represent an effective means to infer abundance, particularly when traditional methods are considered too "costly" or difficult to obtain.

Phylogenetic conservatism drives nutrient dynamics of coral reef fishes

The relative importance of evolutionary history and ecology for traits that drive ecosystem processes is poorly understood. Consumers are essential drivers of nutrient cycling on coral reefs, and thus ecosystem productivity. We use nine consumer "chemical traits" associated with nutrient cycling, collected from 1,572 individual coral reef fishes (178 species spanning 41 families) in two biogeographic regions, the Caribbean and Polynesia, to quantify the relative importance of phylogenetic history and ecological context as drivers of chemical trait variation on coral reefs. We find: (1) phylogenetic relatedness is the best predictor of all chemical traits, substantially outweighing the importance of ecological factors thought to be key drivers of these traits, (2) phylogenetic conservatism in chemical traits is greater in the Caribbean than Polynesia, where our data suggests that ecological forces have a greater influence on chemical trait variation, and (3) differences in chemical traits between regions can be explained by differences in nutrient limitation associated with the geologic context of our study locations. Our study provides multiple lines of evidence that phylogeny is a critical determinant of contemporary nutrient dynamics on coral reefs. More broadly our findings highlight the utility of evolutionary history to improve prediction in ecosystem ecology.

A methodological roadmap to quantify animal-vectored spatial ecosystem subsidies

Energy, nutrients and organisms move over landscapes, connecting ecosystems across space and time. Meta-ecosystem theory investigates the emerging properties of local ecosystems coupled spatially by these movements of organisms and matter, by explicitly tracking exchanges of multiple substances across ecosystem borders. To date, meta-ecosystem research has focused mostly on abiotic flows-neglecting biotic nutrient flows. However, recent work has indicated animals act as spatial nutrient vectors when they transport nutrients across landscapes in the form of excreta, egesta and their own bodies. Partly due to its high level of abstraction, there are few empirical tests of meta-ecosystem theory. Furthermore, while animals may be viewed as important mediators of ecosystem functions, better integration of tools is needed to develop predictive insights of their relative roles and impacts on diverse ecosystems. We present a methodological roadmap that explains how to do such integration by discussing how to combine insights from movement, foraging and ecosystem ecology to develop a coherent understanding of animal-vectored nutrient transport on meta-ecosystems processes. We discuss how the slate of newly developed technologies and methods-tracking devices, mechanistic movement models, diet reconstruction techniques and remote sensing-that when integrated have the potential to advance the quantification of animal-vectored nutrient flows and increase the predictive power of meta-ecosystem theory. We demonstrate that by integrating novel and established tools of animal ecology, ecosystem ecology and remote sensing, we can begin to identify and quantify animal-mediated nutrient translocation by large animals. We also provide conceptual examples that show how our proposed integration of methodologies can help investigate ecosystem impacts of large animal movement. We conclude by describing practical advancements to understanding cross-ecosystem contributions of animals on the move. Understanding the mechanisms by which animals shape ecosystem dynamics is important for ongoing conservation, rewilding and restoration initiatives around the world, and for developing more accurate models of ecosystem nutrient budgets. Our roadmap will enable ecologists to better qualify and quantify animal-mediated nutrient translocation for animals on the move.

Nutrient excretion by fish supports a variable but significant proportion of lake primary productivity over 15 years

Animals play an important and sometimes overlooked role in nutrient cycling. The role of animals in nutrient cycling is spatially and temporally variable, but few studies have evaluated the long-term importance of animal-mediated nutrient cycling in meeting nutrient demand by primary producers. We quantified the proportion of phytoplankton nutrient (phosphorus, P) demand met by excretion by gizzard shad (Dorosoma cepedianum) in a eutrophic reservoir where this species dominates fish biomass. From 2000 to 2014, gizzard shad excretion supported a variable proportion of phytoplankton P demand, averaging 7-27% among years over the growing season (spring and summer). Temporal patterns emerged, as gizzard shad consistently supported a higher proportion of demand during summer (mean 31%) than spring (8%). In spring, the proportion of demand met from gizzard shad excretion was best predicted by gizzard shad population biomass, stream discharge, and temperature. In summer, this proportion was best predicted only by biomass of the young-of-year (YOY) gizzard shad. Thus, variation in YOY shad biomass significantly alters nutrient supply, and future studies should explore the long-term role of animal population dynamics in nutrient cycling. Our study shows that several years of data are needed to perform a critical evaluation of the importance of animals in meeting ecosystem nutrient demand.

The Effect of Activity, Energy Use, and Species Identity on Environmental DNA Shedding of Freshwater Fish

The quantitative measurement of environmental DNA (eDNA) from field-collected water samples is gaining importance for the monitoring of fish communities and populations. The interpretation of these signal strengths depends, among other factors, on the amount of target eDNA shed into the water. However, shedding rates are presumably associated with species-specific traits such as physiology and behavior. Although such differences between juvenile and adult fish have been previously detected, the general impact of movement and energy use in a resting state on eDNA release into the surrounding water remains hardly addressed. In an aquarium experiment, we compared eDNA shedding between seven fish species occurring in European freshwaters. The investigated salmonids, cyprinids, and sculpin exhibit distinct adaptions to microhabitats, diets, and either solitary or schooling behavior. The fish were housed in aquaria with constant water flow and their activity was measured by snapshots taken every 30 s. Water samples for eDNA analysis were taken every 3 h and energy use was determined in an intermittent flow respirometer. After controlling for the effect of fish mass, our results demonstrate a positive correlation between target eDNA quantities as measured with digital PCR, fish activity, and energy use, as well as species-specific differences. For cyprinids, the model based on data from individual fish was only partly transferable to groups, which showed lower activity and higher energy use. Our findings highlight the importance of fish physiology and behavior for the comparative interpretation of taxon-specific eDNA quantities. Species traits should therefore be incorporated into eDNA-based monitoring and conservation efforts.

Effects of grazing on C:N:P stoichiometry attenuate from soils to plants and insect herbivores in a semi-arid grassland

Understanding the processing of limiting nutrients among organisms is an important goal of community ecology. Less known is how human disturbances may alter the stoichiometric patterns among organisms from different trophic levels within communities. Here, we investigated how livestock grazing affects the C:N:P ecological stoichiometry of soils, plants (Leymus chinensis), and grasshoppers (Euchorthippus spp.) in a semi-arid grassland in northeastern China. We found that grazing significantly enhanced soil available N and leaf N content of the dominant L. chinensis grass by 15% and 20%, respectively. Grazing also reduced (soluble) C:N of L. chinensis leaves by 22%. However, grazing did not affect total C, N, or P contents nor their ratios in Euchorthippus grasshoppers. Our results reveal that the effects of grazing disturbances on elemental composition attenuated from lower to higher trophic levels. These findings support the theory that organisms from higher trophic levels have relatively stronger stoichiometric homeostasis compared to those from lower trophic levels. Moreover, grasshopper abundance dropped by 66% in the grazed areas, and they reduced the feeding time on their host L. chinensis grass by 43%, presumably to limit the intake of excess nitrogen from host plants. The energetic costs associated with the maintenance of elemental homeostasis likely reduced grasshopper individual performance and population abundance in the grazed areas. A comprehensive investigation of stoichiometric properties of organisms across trophic levels may enable a better understanding of the nature of species interactions, and facilitate predictions of the consequences of future environmental changes for a community organization.

Evolutionary history drives aspects of stoichiometric niche variation and functional effects within a guild

Functional traits are characteristics of an organism that represents how it interacts with its environment and can influence the structure and function of ecosystems. Ecological stoichiometry provides a framework to understand ecosystem structure and function by modeling the coupled flow of elements (e.g. carbon [C], nitrogen [N], phosphorus [P]) between consumers and their environment. Animals tend to be homeostatic in their nutrient requirements and preferentially sequester the element in shortest supply relative to demand, and release relatively more of the element in excess. Tissue stoichiometry is an important functional trait that allows for predictions among the elemental composition of animals, their diet, and their waste products, with important effects on the cycling and availability of nutrients in ecosystems. Here, we examined the tissue stoichiometric niches (C:N:P) and nutrient recycling stoichiometries (N:P) of several filter-feeding freshwater mussels in the subfamily Ambleminae. Despite occupying the same functional-feeding group and being restricted to a single subfamily-level radiation, we found that species occupied distinct stoichiometric niches and that these niches varied, in part, as a function of their evolutionary history. The relationship between phylogenetic divergence and functional divergence suggests that evolutionary processes may be shaping niche complementarity and resource partitioning. Tissue and excretion stoichiometry were negatively correlated as predicted by stoichiometric theory. When scaled to the community, higher species richness and phylogenetic diversity resulted in greater functional evenness and reduced functional dispersion. Filter-feeding bivalves are an ecologically important guild in freshwater ecosystems globally, and our study provides a more nuanced view of the stoichiometric niches and ecological functions performed by this phylogenetically and ecologically diverse assemblage.

Copper and Gold Nanoparticles Increase Nutrient Excretion Rates of Primary Consumers

Freshwater ecosystems are exposed to engineered nanoparticles through municipal and industrial wastewater-effluent discharges and agricultural nonpoint source runoff. Because previous work has shown that engineered nanoparticles from these sources can accumulate in freshwater algal assemblages, we hypothesized that nanoparticles may affect the biology of primary consumers by altering the processing of two critical nutrients associated with growth and survivorship, nitrogen and phosphorus. We tested this hypothesis by measuring the excretion rates of nitrogen and phosphorus of Physella acuta, a ubiquitous pulmonate snail that grazes heavily on periphyton, exposed to either copper or gold engineered nanoparticles for 6 months in an outdoor wetland mesocosm experiment. Chronic nanoparticle exposure doubled nutrient excretion when compared to the control. Gold nanoparticles increased nitrogen and phosphorus excretion rates more than copper nanoparticles, but overall, both nanoparticles led to higher consumer excretion, despite contrasting particle stability and physiochemical properties. Snails in mesocosms enriched with nitrogen and phosphorus had overall higher excretion rates than ones in ambient (no nutrients added) mesocosms. Stimulation patterns were different between nitrogen and phosphorus excretion, which could have implications for the resulting nutrient ratio in the water column. These results suggest that low concentrations of engineered nanoparticles could alter the metabolism of consumers and increase consumer-mediated nutrient recycling rates, potentially intensifying eutrophication in aquatic systems, for example, the increased persistence of algal blooms as observed in our mesocosm experiment.

Taxonomic identity best explains variation in body nutrient stoichiometry in a diverse marine animal community

Animal-mediated nutrient dynamics are critical processes in ecosystems. Previous research has found animal-mediated nutrient supply (excretion) to be highly predictable based on allometric scaling, but similar efforts to find universal predictive relationships for an organism’s body nutrient content have been inconclusive. We use a large dataset from a diverse tropical marine community to test three frameworks for predicting body nutrient content. We show that body nutrient content does not follow allometric scaling laws and that it is not well explained by trophic status. Instead, we find strong support for taxonomic identity (particularly at the family level) as a predictor of body nutrient content, indicating that evolutionary history plays a crucial role in determining an organism’s composition. We further find that nutrients are “stoichiometrically linked” (e.g., %C predicts %N), but that the direction of these relationships does not always conform to expectations, especially for invertebrates. Our findings demonstrate that taxonomic identity, not trophic status or body size, is the best baseline from which to predict organismal body nutrient content.

The relationship between eDNA particle concentration and organism abundance in nature is strengthened by allometric scaling

Organism abundance is a critical parameter in ecology, but its estimation is often challenging. Approaches utilizing eDNA to indirectly estimate abundance have recently generated substantial interest. However, preliminary correlations observed between eDNA concentration and abundance in nature are typically moderate in strength with significant unexplained variation. Here, we apply a novel approach to integrate allometric scaling coefficients into models of eDNA concentration and organism abundance. We hypothesize that eDNA particle production scales nonlinearly with mass, with scaling coefficients < 1. Wild populations often exhibit substantial variation in individual body size distributions; we therefore predict that the distribution of mass across individuals within a population will influence population-level eDNA production rates. To test our hypothesis, we collected standardized body size distribution and mark-recapture abundance data using whole-lake experiments involving nine populations of brook trout. We correlated eDNA concentration with three metrics of abundance: density (individuals/ha), biomass (kg/ha) and allometrically scaled mass (ASM) (∑(individual mass^0.73 )/ha). Density and biomass were both significantly positively correlated with eDNA concentration (adj. r²  = 0.59 and 0.63, respectively), but ASM exhibited improved model fit (adj. r²  = 0.78). We also demonstrate how estimates of ASM derived from eDNA samples in "unknown" systems can be converted to biomass or density estimates with additional size-structure data. Future experiments should empirically validate allometric scaling coefficients for eDNA production, particularly where substantial intraspecific size distribution variation exists. Incorporating allometric scaling may improve predictive models to the extent that eDNA concentration may become a reliable indicator of abundance in nature.

Water column nutrient concentrations are related to excretion by benthic invertebrates in Lake Taihu, China

Internal release of nutrients is an important contributor to the nutrient dynamics in shallow eutrophic lakes. Zoobenthic organisms may contribute to this release by excreting nutrients to the overlaying water. Based on experiments and using results from previous experimental studies as well as field monitoring density data from 2007 to 2017, we calculated the annual and seasonal nutrient excretions of the two most common macroinvertebrates (Corbicula fluminea and Limnodrilus hoffmeisteri) in Lake Taihu, China. We compared these rates with the concentrations of NH₄-N, total nitrogen (TN), PO₄-P and total phosphorus (TP) in the lake water as well as with previous results of release rates from undisturbed sediments collected in the lake. The spatial distribution of nutrient excretion by the two invertebrate species varied markedly among sites and years. Regression analyses revealed significant relationships between total nutrient excretions by these two species and the concentrations of NH₄-N, TN, PO₄-P and TP in the lake, but with seasonal differences. The relationship was overall strongest in winter, followed by spring, and weakest in summer and autumn. The flux of NH₄-N and PO₄-P released by the two macroinvertebrate species were equivalent to as much as 50% and 66%, respectively, of the sediment release recorded in lab experiments under undisturbed conditions; however, the percentages would be somewhat lower under field conditions where the sediment is subjected to frequent wind-induced resuspension and fish disturbance, enhancing the release rates. The release declined during the study period due to a reduction in the density of macroinvertebrates, perhaps indicating increasing stocking of fish since 2007. Our results indicate that benthic invertebrates are important contributor to the internal loading in shallow eutrophic lakes.

Individual behavior drives ecosystem function and the impacts of harvest

Current approaches for biodiversity conservation and management focus on sustaining high levels of diversity among species to maintain ecosystem function. We show that the diversity among individuals within a single population drives function at the ecosystem scale. Specifically, nutrient supply from individual fish differs from the population average >80% of the time, and accounting for this individual variation nearly doubles estimates of nutrients supplied to the ecosystem. We test how management (i.e., selective harvest regimes) can alter ecosystem function and find that strategies targeting more active individuals reduce nutrient supply to the ecosystem up to 69%, a greater effect than body size-selective or nonselective harvest. Findings show that movement behavior at the scale of the individual can have crucial repercussions for the functioning of an entire ecosystem, proving an important challenge to the species-centric definition of biodiversity if the conservation and management of ecosystem function is a primary goal.

Linkages between flow regime, biota, and ecosystem processes: Implications for river restoration

River ecosystems are highly biodiverse, influence global biogeochemical cycles, and provide valued services. However, humans are increasingly degrading fluvial ecosystems by altering their streamflows. Effective river restoration requires advancing our mechanistic understanding of how flow regimes affect biota and ecosystem processes. Here, we review emerging advances in hydroecology relevant to this goal. Spatiotemporal variation in flow exerts direct and indirect control on the composition, structure, and dynamics of communities at local to regional scales. Streamflows also influence ecosystem processes, such as nutrient uptake and transformation, organic matter processing, and ecosystem metabolism. We are deepening our understanding of how biological processes, not just static patterns, affect and are affected by stream ecosystem processes. However, research on this nexus of flow-biota-ecosystem processes is at an early stage. We illustrate this frontier with evidence from highly altered regulated rivers and urban streams. We also identify research challenges that should be prioritized to advance process-based river restoration.

Interspecific homeostatic regulation and growth across aquatic invertebrate detritivores: a test of ecological stoichiometry theory

Across resource quality gradients, primary consumers must regulate homeostasis and release of nutrients to optimize growth and fitness. Based primarily on internal body composition, the ecological stoichiometry theory (EST) offers a framework to generalize interspecific patterns of these responses, yet the predictions and underlying assumptions of EST remain poorly tested across many species. We used controlled laboratory feeding experiments to measure homeostasis, nutrient release, and growth across seven field-collected aquatic invertebrate detritivore taxa fed wide resource carbon:nitrogen (C:N) and carbon:phosphorus (C:P) gradients. We found that most invertebrates exhibited strict stoichiometric homeostasis (average 1/H = - 0.018 and 0.026 for C:N and C:P, respectively), supporting assumptions of EST. However, the stoichiometry of new tissue production during growth intervals (growth stoichiometry) deviated - 30 to + 54% and - 145 to + 74% from initial body C:N and C:P, respectively, and across species, growth stoichiometry was not correlated with initial body stoichiometry. Notably, smaller non- and hemimetabolous invertebrates exhibited low, decreasing growth C:N and C:P, whereas larger holometabolous invertebrates exhibited high, often increasing growth C:N and C:P. Despite predictions of EST, interspecific sensitivity of egestion stoichiometry and growth rates to the resource gradient were weakly related to internal body composition across species. While the sensitivity of these patterns differed across taxa, such differences carried a weak phylogenetic signal and were not well predicted by EST. Our findings suggest that traits beyond internal body composition, such as feeding behavior, selective assimilation, and ontogeny, are needed to generalize interspecific patterns in consumer growth and nutrient release across resource quality gradients.

Local adaptation reduces the metabolic cost of environmental warming

Metabolism shapes the ecosystem role of organisms by dictating their energy demand and nutrient recycling potential. Metabolic theory (MTE) predicts consumer metabolic and recycling rates will rise with warming, especially if body size declines, but it ignores potential for adaptation. We measured metabolic and nutrient excretion rates of individuals from populations of a globally invasive fish that colonized sites spanning a wide temperature range (19-37°C) on two continents within the last 100 yr. Fish body size declined across our temperature gradient and MTE predicted large rises in population energy demand and nutrient recycling. However, we found that the allometry and temperature dependency of metabolism varied in a countergradient pattern with local temperature in a way that offset predictions of MTE. Scaling of nutrient excretion was more variable and did not track temperature. Our results suggest that adaptation can reduce the metabolic cost of warming, increasing the prospects for population persistence under extreme warming scenarios.

Consumption explains intraspecific variation in nutrient recycling stoichiometry in a desert fish

Consumer-driven nutrient recycling can have substantial effects on primary production and patterns of nutrient limitation in aquatic ecosystems by altering the rates as well as the relative supplies of the key nutrients nitrogen (N) and phosphorus (P). While variation in nutrient recycling stoichiometry has been well-studied among species, the mechanisms that explain intraspecific variation in recycling N:P are not well-understood. We examined the relative importance of potential drivers of variation in nutrient recycling by the fish Gambusia marshi among aquatic habitats in the Cuatro Ciénegas basin of Coahuila, Mexico. There, G. marshi inhabits warm thermal springs with high predation pressure as well as cooler, surface runoff-fed systems with low predation pressure. We hypothesized that variation in food consumption among these habitats would drive intraspecific differences in excretion rates and N:P ratios. Stoichiometric models predicted that temperature alone should not cause substantial variation in excretion N:P, but that further reducing consumption rates should substantially increase excretion N:P. We performed temperature and diet ration manipulation experiments in the laboratory and found strong support for model predictions. We then tested these predictions in the field by measuring nutrient recycling rates and ratios as well as body stoichiometry of fish from nine sites that vary in temperature and predation pressure. Fish from warm, high-predation sites excreted nutrients at a lower N:P ratio than fish from cool, low-predation sites, consistent with the hypothesis that reduced consumption under reduced predation pressure had stronger consequences for P retention and excretion among populations than did variation in body stoichiometry. These results highlight the utility of stoichiometric models for predicting variation in consumer-driven nutrient recycling within a phenotypically variable species.

Increased resource use efficiency amplifies positive response of aquatic primary production to experimental warming

Climate warming is affecting the structure and function of river ecosystems, including their role in transforming and transporting carbon (C), nitrogen (N), and phosphorus (P). Predicting how river ecosystems respond to warming has been hindered by a dearth of information about how otherwise well-studied physiological responses to temperature scale from organismal to ecosystem levels. We conducted an ecosystem-level temperature manipulation to quantify how coupling of stream ecosystem metabolism and nutrient uptake responded to a realistic warming scenario. A ~3.3°C increase in mean water temperature altered coupling of C, N, and P fluxes in ways inconsistent with single-species laboratory experiments. Net primary production tripled during the year of experimental warming, while whole-stream N and P uptake rates did not change, resulting in 289% and 281% increases in autotrophic dissolved inorganic N and P use efficiency (UE), respectively. Increased ecosystem production was a product of unexpectedly large increases in mass-specific net primary production and autotroph biomass, supported by (i) combined increases in resource availability (via N mineralization and N₂ fixation) and (ii) elevated resource use efficiency, the latter associated with changes in community structure. These large changes in C and nutrient cycling could not have been predicted from the physiological effects of temperature alone. Our experiment provides clear ecosystem-level evidence that warming can shift the balance between C and nutrient cycling in rivers, demonstrating that warming will alter the important role of in-stream processes in C, N, and P transformations. Moreover, our results reveal a key role for nutrient supply and use efficiency in mediating responses of primary producers to climate warming.

The influence of dietary and whole-body nutrient content on the excretion of a vertebrate consumer

In many contexts, nutrient excretion by consumers can impact ecosystems by altering the availability of limiting nutrients. Variation in nutrient excretion can be predicted by mass balance models, most of which are premised on two key ideas: (1) consumers maintain fixed whole-body nutrient content (i.e., %N and %P), so-called fixed homeostasis; (2) if dietary nutrients are not matched to whole-body nutrients, excesses of any nutrient are released as excretion to maintain fixed homeostasis. Mass balance models thus predict that consumer excretion should be positively correlated with diet nutrients and negatively correlated with whole-body nutrients. Recent meta-analyses and field studies, however, have often failed to find these expected patterns, potentially because of a confounding influence—flexibility in whole-body nutrient content with diet quality (flexible homeostasis). Here, we explore the impact of flexible homeostasis on nutrient excretion by comparing the N and P excretion of four genetically diverged Trinidadian guppy (Poecilia reticulata) populations when reared on diets of variable P content. As predicted by mass balance, P excretion increased on the high-P diet, but, contrary to the notion of fixed homeostasis, guppy whole-body %P also increased on the high-P diet. While there was no overall correlation between excretion nutrients and whole-body nutrients, when the effect of diet on both whole-body and excretion nutrients was included, we detected the expected negative correlation between whole-body N:P and excretion N:P. This last result suggests that mass balance can predict excretion rates within species, but only if dietary effects on whole-body nutrient content are controlled. Flexible homeostasis can obscure patterns predicted by mass balance, creating an imperative to accurately capture an organism’s diet quality in predicting its excretion rate.

Bridging Food Webs, Ecosystem Metabolism, and Biogeochemistry Using Ecological Stoichiometry Theory

Although aquatic ecologists and biogeochemists are well aware of the crucial importance of ecosystem functions, i.e., how biota drive biogeochemical processes and vice-versa, linking these fields in conceptual models is still uncommon. Attempts to explain the variability in elemental cycling consequently miss an important biological component and thereby impede a comprehensive understanding of the underlying processes governing energy and matter flow and transformation. The fate of multiple chemical elements in ecosystems is strongly linked by biotic demand and uptake; thus, considering elemental stoichiometry is important for both biogeochemical and ecological research. Nonetheless, assessments of ecological stoichiometry (ES) often focus on the elemental content of biota rather than taking a more holistic view by examining both elemental pools and fluxes (e.g., organismal stoichiometry and ecosystem process rates). ES theory holds the promise to be a unifying concept to link across hierarchical scales of patterns and processes in ecology, but this has not been fully achieved. Therefore, we propose connecting the expertise of aquatic ecologists and biogeochemists with ES theory as a common currency to connect food webs, ecosystem metabolism, and biogeochemistry, as they are inherently concatenated by the transfer of carbon, nitrogen, and phosphorous through biotic and abiotic nutrient transformation and fluxes. Several new studies exist that demonstrate the connections between food web ecology, biogeochemistry, and ecosystem metabolism. In addition to a general introduction into the topic, this paper presents examples of how these fields can be combined with a focus on ES. In this review, a series of concepts have guided the discussion: (1) changing biogeochemistry affects trophic interactions and ecosystem processes by altering the elemental ratios of key species and assemblages; (2) changing trophic dynamics influences the transformation and fluxes of matter across environmental boundaries; (3) changing ecosystem metabolism will alter the chemical diversity of the non-living environment. Finally, we propose that using ES to link nutrient cycling, trophic dynamics, and ecosystem metabolism would allow for a more holistic understanding of ecosystem functions in a changing environment.

Comparing the Ecological Stoichiometry in Green and Brown Food Webs - A Review and Meta-analysis of Freshwater Food Webs

The framework of ecological stoichiometry was developed primarily within the context of "green" autotroph-based food webs. While stoichiometric principles also apply in "brown" detritus-based systems, these systems have been historically understudied and differ from green ones in several important aspects including carbon (C) quality and the nutrient [nitrogen (N) and phosphorus (P)] contents of food resources for consumers. In this paper, we review work over the last decade that has advanced the application of ecological stoichiometry from green to brown food webs, focusing on freshwater ecosystems. We first review three focal areas where green and brown food webs differ: (1) bottom-up controls by light and nutrient availability, (2) stoichiometric constraints on consumer growth and nutritional regulation, and (3) patterns in consumer-driven nutrient dynamics. Our review highlights the need for further study of how light and nutrient availability affect autotroph-heterotroph interactions on detritus and the subsequent effects on consumer feeding and growth. To complement this conceptual review, we formally quantified differences in stoichiometric principles between green and brown food webs using a meta-analysis across feeding studies of freshwater benthic invertebrates. From 257 datasets collated across 46 publications and several unpublished studies, we compared effect sizes (Pearson's r) of resource N:C and P:C on growth, consumption, excretion, and egestion between herbivorous and detritivorous consumers. The meta-analysis revealed that both herbivore and detritivore growth are limited by resource N:C and P:C contents, but effect sizes only among detritivores were significantly above zero. Consumption effect sizes were negative among herbivores but positive for detritivores in the case of both N:C and P:C, indicating distinct compensatory feeding responses across resource stoichiometry gradients. Herbivore P excretion rates responded significantly positively to resource P:C, whereas detritivore N and P excretion did not respond; detritivore N and P egestion responded positively to resource N:C and P:C, respectively. Our meta-analysis highlights resource N and P contents as broadly limiting in brown and green benthic food webs, but indicates contrasting mechanisms of limitation owing to differing consumer regulation. We suggest that green and brown food webs share fundamental stoichiometric principles, while identifying specific differences toward applying ecological stoichiometry across ecosystems.

Animal pee in the sea: consumer-mediated nutrient dynamics in the world's changing oceans

Humans have drastically altered the abundance of animals in marine ecosystems via exploitation. Reduced abundance can destabilize food webs, leading to cascading indirect effects that dramatically reorganize community structure and shift ecosystem function. However, the additional implications of these top-down changes for biogeochemical cycles via consumer-mediated nutrient dynamics (CND) are often overlooked in marine systems, particularly in coastal areas. Here, we review research that underscores the importance of this bottom-up control at local, regional, and global scales in coastal marine ecosystems, and the potential implications of anthropogenic change to fundamentally alter these processes. We focus attention on the two primary ways consumers affect nutrient dynamics, with emphasis on implications for the nutrient capacity of ecosystems: (1) the storage and retention of nutrients in biomass, and (2) the supply of nutrients via excretion and egestion. Nutrient storage in consumer biomass may be especially important in many marine ecosystems because consumers, as opposed to producers, often dominate organismal biomass. As for nutrient supply, we emphasize how consumers enhance primary production through both press and pulse dynamics. Looking forward, we explore the importance of CDN for improving theory (e.g., ecological stoichiometry, metabolic theory, and biodiversity-ecosystem function relationships), all in the context of global environmental change. Increasing research focus on CND will likely transform our perspectives on how consumers affect the functioning of marine ecosystems.