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Predation is known to have both direct and indirect effects on nutrient cycling in terrestrial and aquatic ecosystems, and the general stress paradigm (GSP) has been promoted as a theory for describing predator-mediated indirect effects on nutrient cycling. The GSP predicts that prey exposed to predators will produce glucocorticosteroids, which have a host of physiological effects including gluconeogenesis, increased respiration, excretion of N and P, and increases in body C:N. We tested the nutrient predictions of the GSP using anuran larvae, which exhibit morphological defenses in addition to behavioral defenses for which the GSP was conceived. Genetically similar Hyla versicolor tadpoles were placed in mesocosms either in the presence or absence of a fed predator ( Dytiscus verticalis ), and after two weeks, tadpoles exposed to predators exhibited strong induced defenses with large, tubular bodies, larger tails, and reduced activity. Tadpole body %C and N:P increased with no change in C:N, which is contrary to expectations from the GSP. Statistical models suggested that changes in body morphology (e.g., tail muscle width) rather than behavioral defenses (i.e., reduced activity) were most likely responsible for predator-mediated differences in body stoichiometry. This study suggests that strong morphological defenses may overwhelm or counteract the nutrient predictions of the GSP.

Streams are being subjected to physical, chemical, and biological stresses stemming from both natural and anthropogenic changes to the planet. In the face of limited time and resources, scientists, resource managers, and policy makers need ways to rank stressors and their impacts so that we can prioritize them from the most to least important (i.e., perform 'ecological triage'). We report results from an experiment in which we established a periphyton community from the Huron River (Michigan, USA) in 84 experimental 'flumes' (stream mesocosms). We then dosed the flumes with gradients of six common stressors (increased temperature, taxa extinctions, sedimentation, nitrogen, phosphorus, and road salt) and monitored periphyton structure and function. A set of a priori deterministic functions were fit to each stressor-endpoint response and model averaging based on AICc weights was used to develop concentration-response best-fit predictions. Model predictions from different stressors were then compared to forecasts of future environmental change to rank stressors according to the potential magnitude of impacts. All of the stressors studied altered at least one characteristic of the periphyton; however, the extent (i.e., structural and functional changes) and magnitude of effects expected under future forecasts differed significantly among stressors. Elevated nitrogen concentrations are projected to have the greatest combined effect on stream periphyton structure and function. Extinction, sediment, and phosphorus all had similar but less substantial impact on the periphyton (e.g., affected only structure not function, smaller magnitude change). Elevated temperature and salt both had measurable effects on periphyton, but their overall impacts were much lower than any of the other stressors. For periphyton in the Huron River, our results suggest that, among the stressors examined, increased N pollution may have the greatest potential to alter the structure and function of the periphyton community, and managers should prioritize reducing anthropogenic sources of nitrogen. Our study demonstrates an experimental approach to ecological triage that can be used as an additional line of evidence to prioritize management decisions for specific ecosystems in the face of ecological change.

Dinitrogen (N2) fixation provides bioavailable nitrogen to the biosphere. However, in some habitats (e.g., sediments), the metabolic pathways of organisms carrying out N2 fixation are unclear. We present metabolic models representing various chemotrophic N2 fixers, which simulate potential pathways of electron transport and energy flow, resulting in predictions of whole-cell stoichiometries. By balancing mass, electrons, and energy for metabolic half-reactions, we quantify the electron usage for nine N2 fixers. Our results demonstrate that all modeled organisms fix sufficient N2 for growth. Aerobic organisms allocate more electrons to N2 fixation and growth, yielding more biomass and fixing more N2, while methanogens using acetate and organisms using sulfate allocate fewer electrons. This work can be applied to investigate the depth distribution of N2 fixers based on nutrient availability, complementing field measurements of biogeochemical processes and microbial communities.IMPORTANCEN2 fixation is an important process in the global N cycle. Researchers have developed models for heterotrophic and photoautotrophic N2 fixers, but there is a lack of modeling studies on chemoautotrophic N2 fixers. Here, we built nine biochemical models for different chemoautotrophic N2 fixers by combining different types of half-chemical reactions. We include three sulfide oxidizers using different electron acceptors (O2, NO3−, and Fe3+), contributing to the sulfur, nitrogen, and iron cycles in the sediment. We have two methanogens using different substrates (H2 and acetate) and four methanotrophs using different electron acceptors (O2, NO3−, Fe3+, and SO42−). By modeling these methane producers and users in the sediment and their N2-fixing metabolic pathways, our work can provide insight for future carbon cycle studies. This study outlines various metabolic pathways that can facilitate N2 fixation, with implications for where in the environment they might occur.

In flooded soils and sediments, bioturbating invertebrates rework sediment and convey oxygenated surface water through burrowing, creating a mosaic of adjacent anoxic and oxic patches while simultaneously translocating and transforming nutrients as they feed and excrete. We investigated the impacts of two functionally contrasting bioturbators (gallery-network burrower Lumbriculus variegatus and U-shaped burrower Ephemera simulans ) on oxygen availability and nutrient fluxes in wetland sediments. To assess excretion contributions, we also incubated bioturbators in sand-water microcosms. Fine-scale oxygen measurements combined with flux rates of redox-sensitive and conservative ions reveal that both bioturbators introduced oxygen to sediments. U-shaped burrowers facilitated measurable oxygen introduction into sediments while gallery-network burrowers did not. However, gallery-network burrowers showed evidence of oxidizing reduced solutes in sediments, which suggests that oxygen is being introduced. At high densities, both bioturbators promoted sufficient iron oxidation to sequester phosphorus from surface water into sediments, effectively counteracting phosphorus release from excretion. Conversely, bioturbation caused nitrate release into surface water, likely driven by excretion of ammonia followed by nitrification. Gallery-network burrowers facilitated P retention in sediments but contributed N to surface water, while U-shaped burrowers showed similar, but less pronounced trends. Bioturbators have profound, but variable, effects on sediment-surface water nutrient exchange in wetlands. Sediment characteristics, bioturbator density, and bioturbation mode regulate both the amount of oxygen introduced to normally anoxic sediments and its reactions with sediment substrates, shaping the magnitude and direction of bioturbator-induced nutrient fluxes.

Nutritional and contaminant stressors influence organismal physiology, trophic interactions, community structure, and ecosystem-level processes; however, the interactions between toxicity and elemental imbalance in food resources have been examined in only a few ecotoxicity studies. Integrating well-developed ecological theories that cross all levels of biological organization can enhance our understanding of ecotoxicology. In the present article, we underline the opportunity to couple concepts and approaches used in the theory of ecological stoichiometry (ES) to ask ecotoxicological questions and introduce stoichiometric ecotoxicology, a subfield in ecology that examines how contaminant stress, nutrient supply, and elemental constraints interact throughout all levels of biological organization. This conceptual framework unifying ecotoxicology with ES offers potential for both empirical and theoretical studies to deepen our mechanistic understanding of the adverse outcomes of chemicals across ecological scales and improve the predictive powers of ecotoxicology.

N 2 fixation is an important process in the global N cycle. Researchers have developed models for heterotrophic and photoautotrophic N 2 fixers, but there is a lack of modeling studies on chemoautotrophic N 2 fixers. Here, we built nine biochemical models for different chemoautotrophic N 2 fixers by combining different types of half-chemical reactions. We include three sulfide oxidizers using different electron acceptors (O 2 , NO 3 − , and Fe 3+ ), contributing to the sulfur, nitrogen, and iron cycles in the sediment. We have two methanogens using different substrates (H 2 and acetate) and four methanotrophs using different electron acceptors (O 2 , NO 3 − , Fe 3+ , and SO 4 2− ). By modeling these methane producers and users in the sediment and their N 2 -fixing metabolic pathways, our work can provide insight for future carbon cycle studies. This study outlines various metabolic pathways that can facilitate N 2 fixation, with implications for where in the environment they might occur.

Riparian zones are an important transition between terrestrial and aquatic ecosystems, and they function in nutrient cycling and removal. Non-native earthworms invading earthworm-free areas of North America can affect nutrient cycling in upland soils and have the potential to affect it in riparian soils. We examined how the presence of earthworms can affect riparian nutrient cycling and nutrient delivery to streams. Two mesocosm experiments were conducted to determine how (1) the biomass of earthworms and (2) earthworm species can affect nutrient flux from riparian zones to nearby streams and how this flux can affect streamwater nutrients and periphyton growth. In separate experiments, riparian soil cores were amended with one of four mixed earthworm biomasses (0, 4, 10, or 23 g m⁻² ash-free dry mass) or with one of three earthworm species (Aporrectodea caliginosa, Lumbricus terrestris, L. rubellus) or no earthworm species. Riparian soil cores were coupled to artificial streams, and over a 36-day period, we measured nutrient leaching rates, in-stream nutrient concentrations, and periphyton growth. Ammonium leaching increased with increasing biomass and was greatest from the A. caliginosa treatments. Nitrate leaching increased through time and increased at a greater rate with higher biomass and from cores containing A. caliginosa. We suggest that the overall response of increased nitrate leaching [90% of total nitrogen (N)] was due to a combination of ammonium excretion and burrowing by earthworms, which increased nitrification rates. During both experiments, periphyton biomass increased through time but did not differ across treatments despite high in-stream inorganic N. Through time, in-stream phosphorus (P) concentration declined to <5 μg l⁻¹, and periphyton growth was likely P-limited. We conclude that activities of non-native earthworms (particularly A. caliginosa) can alter biogeochemical cycling in riparian zones, potentially reducing the N-buffering capacity of riparian zones and altering stoichiometric relationships in adjacent aquatic ecosystems.

Exotic earthworms from Europe and Asia have invaded previously earthworm-free areas of North America where they consume leaf litter, mix soil horizons, and alter nutrient cycling. Primarily, earthworm introductions occur through human activities; we hypothesized that the combination of logging (i.e., road construction and soil disturbance) and stream transport (i.e., hydrochory) allows earthworms to invade new ecosystems and spread within watersheds. On Prince of Wales Island, AK, we surveyed riparian zones in 11 watersheds with varying timber harvest intensity for terrestrial oligochaetes. Additionally, common invasive earthworms were experimentally submerged in a local stream to test for tolerance to prolonged immersion: all taxa survived immersion for at least 6 days. Using principal components analysis, watershed and harvest variables describing the watersheds upstream of our sampled riparian areas were reduced to two principal components describing harvest intensity (PC1) and harvest style (PC2). Logistic models successfully predicted earthworm abundance (r ² = 0.70) from PC1, which indicated that watersheds with older, intense upstream timber harvest contained larger earthworm populations. Earthworm species richness was best predicted by PC2 (r ² = 0.39), which suggested that earthworm communities in watersheds containing large clear-cut stands were more species-rich. Collectively, these results suggest that (1) invasive earthworms may use streams for dispersal and (2) upstream introductions via timber harvest can initiate downstream earthworm invasions. Hydrochory would allow invasive earthworms to spread at rates (tens of km d⁻¹) that are much greater than previously reported rates of terrestrial spread (5-10 m y⁻¹). Effective control of exotic earthworms in riparian zones will require watershed-level management and surveillance.

Predation is known to have both direct and indirect effects on nutrient cycling in terrestrial and aquatic ecosystems, and the general stress paradigm (GSP) has been promoted as a theory for describing predator‐mediated indirect effects on nutrient cycling. The GSP predicts that prey exposed to predators will produce glucocorticosteroids, which have a host of physiological effects including gluconeogenesis, increased respiration, excretion of N and P, and increases in body C:N. We tested the nutrient predictions of the GSP using anuran larvae, which exhibit morphological defenses in addition to behavioral defenses for which the GSP was conceived. Genetically similar Hyla versicolor tadpoles were placed in mesocosms either in the presence or absence of a fed predator (Dytiscus verticalis), and after two weeks, tadpoles exposed to predators exhibited strong induced defenses with large, tubular bodies, larger tails, and reduced activity. Tadpole body %C and N:P increased with no change in C:N, which is contrary to expectations from the GSP. Statistical models suggested that changes in body morphology (e.g., tail muscle width) rather than behavioral defenses (i.e., reduced activity) were most likely responsible for predator‐mediated differences in body stoichiometry. This study suggests that strong morphological defenses may overwhelm or counteract the nutrient predictions of the GSP.

Ecosystem functions, which drive global cycles and provide services to humans, can be influenced by losses and gains of nutrients, detritus, and organisms from beyond an ecosystem’s boundaries (i.e., resource subsidies). Anthropogenic disturbances, such as the transport and release of invasive species and chemical contaminants, can disrupt the functioning of ecosystems and I examined how these stressors can influence the flux of resources across ecosystem boundaries. Ionic liquids (ILs) are a novel class of chemicals with great potential for industrial use yet they may pose a threat to aquatic habitats. I found that certain ILs reduce zebra mussel feeding at concentrations well below lethal levels, which, if ILs are released into an invaded lake, may alter the flux of resources from pelagic to benthic habitats. European earthworms, which have been introduced to historically earthworm-free North American temperate forests, can reduce forest biodiversity and alter nutrient cycling. I demonstrated that non-native earthworms can also increase the flux of nitrogen from riparian soils by both increasing nitrate leaching and stimulating microbial denitrification. Additionally, non-native earthworms may alter the flux of detritus into soil food webs by consuming leaf litter at a high rate and producing nutrient-enriched feces. Finally, I present evidence that non-native earthworms may be spreading to new riparian habitats by using stream ecosystems as corridors of invasion. To control how chemical contaminants and non-native species negatively affect ecosystem functions, we must understand that seemingly separate ecosystems can be connected through resource subsidies and, thus, the impact of stressors can move across ecosystem boundaries.