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Ecosystem engineers can generate hotspots of ecological structure and function by facilitating the aggregation of both resources and consumers. However, nearly all examples of such engineered hotspots come from long-lived foundation species, such as marine and freshwater mussels, intertidal cordgrasses, and alpine cushion plants, with less attention given to small-bodied and short-lived animals. Insects often have rapid life cycles and high population densities and are among the most diverse and ubiquitous animals on earth. Although these taxa have the potential to generate hotspots and heterogeneity comparable to that of foundation species, few studies have examined this possibility. We conducted a mesocosm experiment to examine the degree to which a stream insect ecosystem engineer, the net-spinning caddisfly (Tricoptera:Hydropsychidae), creates hotspots by facilitating invertebrate community assembly. Our experiment used two treatments: (1) stream benthic habitat with patches of caddisfly engineers present and (2) a control treatment with no caddisflies present. We show that compared to controls, caddisflies increased local resource availability measured as particulate organic matter (POM) by 43%, ecosystem respiration (ER) by 70%, and invertebrate density, biomass, and richness by 96%, 244%, and 72%, respectively. These changes resulted in increased spatial variation of POM by 25%, invertebrate density by 76%, and ER by 29% compared to controls, indicating a strong effect of caddisflies on ecological heterogeneity. We found a positive relationship between invertebrate density and ammonium concentration in the caddisfly treatment, but no such relationship in the control, indicating that either caddisflies themselves or the invertebrate aggregations they create increased nutrient availability. When accounting for the amount of POM, caddisfly treatments increased invertebrate density by 48% and richness by 40% compared to controls, suggesting that caddisflies may also enhance the nutritional quality of resources for the invertebrate assemblage. The caddisfly treatment also increased the rate of ecosystem respiration as a function of increasing POM compared to the control. Our study demonstrates that insect ecosystem engineers can generate heterogeneity by concentrating local resources and consumers, with consequences for carbon and nutrient cycling., (© 2023 The Ecological Society of America.)

Human impacts, particularly nutrient pollution and land-use change, have caused significant declines in the quality and quantity of freshwater resources. Most global assessments have concentrated on species diversity and composition, but effects on the multifunctionality of streams and rivers remain unclear. Here, we analyse the most comprehensive compilation of stream ecosystem functions to date to provide an overview of the responses of nutrient uptake, leaf litter decomposition, ecosystem productivity, and food web complexity to six globally pervasive human stressors. We show that human stressors inhibited ecosystem functioning for most stressor-function pairs. Nitrate uptake efficiency was most affected and was inhibited by 347% due to agriculture. However, concomitant negative and positive effects were common even within a given stressor-function pair. Some part of this variability in effect direction could be explained by the structural heterogeneity of the landscape and latitudinal position of the streams. Ranking human stressors by their absolute effects on ecosystem multifunctionality revealed significant effects for all studied stressors, with wastewater effluents (194%), agriculture (148%), and urban land use (137%) having the strongest effects. Our results demonstrate that we are at risk of losing the functional backbone of streams and rivers if human stressors persist in contemporary intensity, and that freshwaters are losing critical ecosystem services that humans rely on. We advocate for more studies on the effects of multiple stressors on ecosystem multifunctionality to improve the functional understanding of human impacts. Finally, freshwater management must shift its focus toward an ecological function-based approach and needs to develop strategies for maintaining or restoring ecosystem functioning of streams and rivers., (© 2022 The Authors. Global Change Biology published by John Wiley & Sons Ltd.)

Tracking carbon (C) flow through ecosystems requires quantification of myriad biophysical processes, including C routing through microbial and metazoan food webs. Yet detailed organic matter budgets are rarely combined with simultaneous measurement of C flows supporting microbial and animal production. Here, we synthesize concurrent data sets on organic matter, microbes, and macroinvertebrates from two detritus-based stream ecosystems, one of which was subject to experimental nitrogen (N) and phosphorus (P) enrichment. Our synthesis provides new insights into C flow through forest stream ecosystems. Over 3 yr, the reference stream showed a striking balance of inputs and outputs, with a mean surplus of only 7 g C·m -2 ·yr -1 (~1% of annual inputs), presumably stored in sediments as fine particulate organic matter (FPOM). In contrast, N and P enrichment over 2 yr resulted in severe deficits of C (-576 g C·m -2 ·yr -1 or ~170% of annual inputs), a shortfall presumably met by stored C. Our data set provides an ecosystem-based estimate of the fate of forest litter C at ambient nutrient concentrations: 6.2% was leached as dissolved organic C, 40.6% and 8.5% flowed to litter-associated fungi and bacteria, respectively, 7.5% was consumed by macroinvertebrates, 1.8% was exported as coarse particles, and the remainder (35.4%) was presumably fragmented by biophysical processes. Our calculations also allowed an estimate of inputs into the heterogeneous FPOM pool, which is otherwise difficult to obtain. At naturally low nutrient concentrations, 50.7% was derived from fragmented litter, 39.1% from microbial biomass (mostly fungal), and 10.2% from macroinvertebrate egesta. Nutrient addition drove large changes in C fluxes in the experimental stream, especially in flows of leaf litter to fungi (×1.7 pretreatment) and macroinvertebrates (×2.7), and of FPOM to hydrologic export (×2.6). Our results underscore the key roles of both microbes and metazoans in controlling C flow through detritus-based ecosystems, as well as how release from persistent nutrient limitation may perturb steady-state conditions of C inputs vs. outputs. Our analysis also suggests areas for future research, including assessing the relative importance of stored vs. recycled C in fueling detrital food webs subject to altered nutrient regimes and other global-change drivers., (© 2020 by the Ecological Society of America.)

Climate warming is predicted to alter routing and flows of energy through food webs because of the critical and varied effects of temperature on physiological rates, community structure, and trophic dynamics. Few studies, however, have experimentally assessed the net effect of warming on energy flux and food web dynamics in natural intact communities. Here, we test how warming affects energy flux and the trophic basis of production in a natural invertebrate food web by experimentally heating a stream reach in southwest Iceland by ~4°C for 2 yr and comparing its response to an unheated reference stream. Previous results from this experiment showed that warming led to shifts in the structure of the invertebrate assemblage, with estimated increases in total metabolic demand but no change in annual secondary production. We hypothesized that elevated metabolic demand and invariant secondary production would combine to increase total consumption of organic matter in the food web, if diet composition did not change appreciably with warming. Dietary composition of primary consumers indeed varied little between streams and among years, with gut contents primarily consisting of diatoms (72.9%) and amorphous detritus (19.5%). Diatoms dominated the trophic basis of production of primary consumers in both study streams, contributing 79-86% to secondary production. Although warming increased the flux of filamentous algae within the food web, total resource consumption did not increase as predicted. The neutral net effect of warming on total energy flow through the food web was a result of taxon-level variation in responses to warming, a neutral effect on total invertebrate production, and strong trophic redundancy within the invertebrate assemblage. Thus, food webs characterized by a high degree of trophic redundancy may be more resistant to the effects of climate warming than those with more diverse and specialized consumers., (© 2019 by the Ecological Society of America.)

River ecosystems receive and process vast quantities of terrestrial organic carbon, the fate of which depends strongly on microbial activity. Variation in and controls of processing rates, however, are poorly characterized at the global scale. In response, we used a peer-sourced research network and a highly standardized carbon processing assay to conduct a global-scale field experiment in greater than 1000 river and riparian sites. We found that Earth's biomes have distinct carbon processing signatures. Slow processing is evident across latitudes, whereas rapid rates are restricted to lower latitudes. Both the mean rate and variability decline with latitude, suggesting temperature constraints toward the poles and greater roles for other environmental drivers (e.g., nutrient loading) toward the equator. These results and data set the stage for unprecedented "next-generation biomonitoring" by establishing baselines to help quantify environmental impacts to the functioning of ecosystems at a global scale.

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 N 2 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., (© 2017 John Wiley & Sons Ltd.)

A central question at the interface of food-web and climate change research is how secondary production, or the formation of heterotroph biomass over time, will respond to rising temperatures. The metabolic theory of ecology (MTE) hypothesizes the temperature-invariance of secondary production, driven by matched and opposed forces that reduce biomass of heterotrophs while increasing their biomass turnover rate (production : biomass, or P:B) with warming. To test this prediction at the whole community level, we used a geothermal heat exchanger to experimentally warm a stream in southwest Iceland by 3.8°C for two years. We quantified invertebrate community biomass, production, and P : B in the experimental stream and a reference stream for one year prior to warming and two years during warming. As predicted, warming had a neutral effect on community production, but this result was not driven by opposing effects on community biomass and P:B. Instead, warming had a positive effect on both the biomass and production of larger-bodied, slower-growing taxa (e.g., larval black flies, dipteran predators, snails) and a negative effect on small-bodied taxa with relatively high growth rates (e.g., ostracods, larval chironomids). We attribute these divergent responses to differences in thermal preference between small- vs. large-bodied taxa. Although metabolic demand vs. resource supply must ultimately constrain community production, our results highlight the potential for idiosyncratic community responses to warming, driven by variation in thermal preference and body size within regional species pools., (© 2017 by the Ecological Society of America.)

How ecological communities respond to predicted increases in temperature will determine the extent to which Earth's biodiversity and ecosystem functioning can be maintained into a warmer future. Warming is predicted to alter the structure of natural communities, but robust tests of such predictions require appropriate large-scale manipulations of intact, natural habitat that is open to dispersal processes via exchange with regional species pools. Here, we report results of a two-year whole-stream warming experiment that shifted invertebrate assemblage structure via unanticipated mechanisms, while still conforming to community-level metabolic theory. While warming by 3.8 °C decreased invertebrate abundance in the experimental stream by 60% relative to a reference stream, total invertebrate biomass was unchanged. Associated shifts in invertebrate assemblage structure were driven by the arrival of new taxa and a higher proportion of large, warm-adapted species (i.e., snails and predatory dipterans) relative to small-bodied, cold-adapted taxa (e.g., chironomids and oligochaetes). Experimental warming consequently shifted assemblage size spectra in ways that were unexpected, but consistent with thermal optima of taxa in the regional species pool. Higher temperatures increased community-level energy demand, which was presumably satisfied by higher primary production after warming. Our experiment demonstrates how warming reassembles communities within the constraints of energy supply via regional exchange of species that differ in thermal physiological traits. Similar responses will likely mediate impacts of anthropogenic warming on biodiversity and ecosystem function across all ecological communities., (© 2016 John Wiley & Sons Ltd.)

Although much effort has been devoted to quantifying how warming alters carbon cycling across diverse ecosystems, less is known about how these changes are linked to the cycling of bioavailable nitrogen and phosphorus. In freshwater ecosystems, benthic biofilms (i.e. thin films of algae, bacteria, fungi, and detrital matter) act as biogeochemical hotspots by controlling important fluxes of energy and material. Understanding how biofilms respond to warming is thus critical for predicting responses of coupled elemental cycles in freshwater systems. We developed biofilm communities in experimental streamside channels along a gradient of mean water temperatures (7.5-23.6 °C), while closely maintaining natural diel and seasonal temperature variation with a common water and propagule source. Both structural (i.e. biomass, stoichiometry, assemblage structure) and functional (i.e. metabolism, N2 -fixation, nutrient uptake) attributes of biofilms were measured on multiple dates to link changes in carbon flow explicitly to the dynamics of nitrogen and phosphorus. Temperature had strong positive effects on biofilm biomass (2.8- to 24-fold variation) and net ecosystem productivity (44- to 317-fold variation), despite extremely low concentrations of limiting dissolved nitrogen. Temperature had surprisingly minimal effects on biofilm stoichiometry: carbon:nitrogen (C:N) ratios were temperature-invariant, while carbon:phosphorus (C:P) ratios declined slightly with increasing temperature. Biofilm communities were dominated by cyanobacteria at all temperatures (>91% of total biovolume) and N2 -fixation rates increased up to 120-fold between the coldest and warmest treatments. Although ammonium-N uptake increased with temperature (2.8- to 6.8-fold variation), the much higher N2 -fixation rates supplied the majority of N to the ecosystem at higher temperatures. Our results demonstrate that temperature can alter how carbon is cycled and coupled to nitrogen and phosphorus. The uncoupling of C fixation from dissolved inorganic nitrogen supply produced large unexpected changes in biofilm development, elemental cycling, and likely downstream exports of nutrients and organic matter., (© 2015 John Wiley & Sons Ltd.)

Temperature and nutrient availability play key roles in controlling the pathways and rates at which energy and materials move through ecosystems. These factors have also changed dramatically on Earth over the past century as human activities have intensified. Although significant effort has been devoted to understanding the role of temperature and nutrients in isolation, less is known about how these two factors interact to influence ecological processes. Recent advances in ecological stoichiometry and metabolic ecology provide a useful framework for making progress in this area, but conceptual synthesis and review are needed to help catalyze additional research. Here, we examine known and potential interactions between temperature and nutrients from a variety of physiological, community, and ecosystem perspectives. We first review patterns at the level of the individual, focusing on four traits--growth, respiration, body size, and elemental content--that should theoretically govern how temperature and nutrients interact to influence higher levels of biological organization. We next explore the interactive effects of temperature and nutrients on populations, communities, and food webs by synthesizing information related to community size spectra, biomass distributions, and elemental composition. We use metabolic theory to make predictions about how population-level secondary production should respond to interactions between temperature and resource supply, setting up qualitative predictions about the flows of energy and materials through metazoan food webs. Last, we examine how temperature-nutrient interactions influence processes at the whole-ecosystem level, focusing on apparent vs. intrinsic activation energies of ecosystem processes, how to represent temperature-nutrient interactions in ecosystem models, and patterns with respect to nutrient uptake and organic matter decomposition. We conclude that a better understanding of interactions between temperature and nutrients will be critical for developing realistic predictions about ecological responses to multiple, simultaneous drivers of global change, including climate warming and elevated nutrient supply., (© 2014 John Wiley & Sons Ltd.)

Understanding and predicting how global warming affects the structure and functioning of natural ecosystems is a key challenge of the 21st century. Isolated laboratory and field experiments testing global change hypotheses have been criticized for being too small-scale and overly simplistic, whereas surveys are inferential and often confound temperature with other drivers. Research that utilizes natural thermal gradients offers a more promising approach and geothermal ecosystems in particular, which span a range of temperatures within a single biogeographic area, allow us to take the laboratory into nature rather than vice versa. By isolating temperature from other drivers, its ecological effects can be quantified without any loss of realism, and transient and equilibrial responses can be measured in the same system across scales that are not feasible using other empirical methods. Embedding manipulative experiments within geothermal gradients is an especially powerful approach, informing us to what extent small-scale experiments can predict the future behaviour of real ecosystems. Geothermal areas also act as sentinel systems by tracking responses of ecological networks to warming and helping to maintain ecosystem functioning in a changing landscape by providing sources of organisms that are preadapted to different climatic conditions. Here, we highlight the emerging use of geothermal systems in climate change research, identify novel research avenues, and assess their roles for catalysing our understanding of ecological and evolutionary responses to global warming., (© 2014 John Wiley & Sons Ltd.)

Responses of detrital pathways to nutrients may differ fundamentally from pathways involving living plants: basal carbon resources can potentially decrease rather than increase with nutrient enrichment. Despite the potential for nutrients to accelerate heterotrophic processes and fluxes of detritus, few studies have examined detritus-nutrient dynamics at whole-ecosystem scales. We quantified organic matter (OM) budgets over three consecutive years in two detritus-based Appalachian (U.S.A.) streams. After the first year, we began enriching one stream with low-level nitrogen and phosphorus inputs. Subsequent effects of nutrients on outputs of different OM compartments were determined using randomized intervention analysis. Nutrient addition did not affect dissolved or coarse particulate OM export but had dramatic effects on fine particulate OM (FPOM) export at all discharges relative to the reference stream. After two years of enrichment, FPOM export was 340% higher in the treatment stream but had decreased by 36% in the reference stream relative to pretreatment export. Ecosystem respiration, the dominant carbon output in these systems, also increased in the treatment stream relative to the reference, but these changes were smaller in magnitude than those in FPOM export. Nutrient enrichment accelerated rates of OM processing, transformation, and export, potentially altering food-web dynamics and ecosystem stability in the long term. The results of our large-scale manipulation of a detrital ecosystem parallel those from analogous studies of soils, in which net loss of organic carbon has often been shown to result from experimental nutrient addition at the plot scale. Streams are useful model systems in which to test the effects of nutrients on ecosystem-scale detrital dynamics because they allow both the tracking of OM conversion along longitudinal continua and the integrated measurement of fluxes of transformed material through downstream sites.

Most aquatic and terrestrial ecosystems are experiencing increased nutrient availability, which is affecting their structure and function. By altering community composition and productivity of consumers, enrichment can indirectly cause changes in the pathways and magnitude of material flows in food webs. These changes, in turn, have major consequences for material storage and cycling in the ecosystem. Understanding mechanisms and predicting consequences of nutrient-induced changes in material flows requires a quantitative food web approach that combines information on consumer energetics and consumer-resource stoichiometry. We examined effects of a whole-system experimental nutrient enrichment on the trophic basis of production and the magnitude and pathways of carbon (C), nitrogen (N), and phosphorus (P) flows in a detritus-based stream food web. We compared the response of the treated stream to an adjacent reference stream throughout the study. Dietary composition and elemental flows varied considerably among invertebrate functional feeding groups. During nutrient enrichment, increased flows of leaf litter and amorphous detritus to shredders and gatherers accounted for most of the altered flows of C from basal resources to consumers. Nutrient enrichment had little effect on patterns of material flows but had large positive effects on the magnitude of C, N, and P flows to consumers (mean increase of 97% for all elements). Nutrient-specific food webs revealed similar flows of N and P to multiple functional groups despite an order of magnitude difference among groups in consumption of C. Secondary production was more strongly related to consumption of nutrients than C, and increased material flows were positively related to the degree of consumer-resource C:P and C:N imbalances. Nutrient enrichment resulted in an increased proportion of detrital C inputs consumed by primary consumers (from -15% to 35%) and a decreased proportion of invertebrate prey consumed by predators (from -80% to 55%). Our results demonstrate that nutrient enrichment of detritus-based systems may reduce stoichiometric constraints on material flows, increase the contribution of consumers to C, N, and P cycling, alter the proportion of C inputs metabolized by consumers, and potentially lead to reduced ecosystem-level storage of C.

Most nutrient enrichment studies in aquatic systems have focused on autotrophic food webs in systems where primary producers dominate the resource base. We tested the heterotrophic response to long-term nutrient enrichment in a forested, headwater stream. Our study design consisted of 2 years of pretreatment data in a reference and treatment stream and 2 years of continuous nitrogen (N) + phosphorus addition to the treatment stream. Studies were conducted with two leaf species that differed in initial C:N, Rhododendron maximum (rhododendron) and Acer rubrum (red maple). We determined the effects of nutrient addition on detrital resources (leaf breakdown rates, litter C:N and microbial activity) and tested whether nutrient enrichment affected macroinvertebrate consumers via increased biomass. Leaf breakdown rates were ca. 1.5 and 3x faster during the first and second years of enrichment, respectively, in the treatment stream for both leaf types. Microbial respiration rates of both leaf types were 3x higher with enrichment, and macroinvertebrate biomass associated with leaves increased ca. 2-3x with enrichment. The mass of N in macroinvertebrate biomass relative to leaves tended to increase with enrichment up to 6x for red maple and up to 44x for rhododendron leaves. Lower quality (higher C:N) rhododendron leaves exhibited greater changes in leaf nutrient content and macroinvertebrate response to nutrient enrichment than red maple leaves, suggesting a unique response by different leaf species to nutrient enrichment. Nutrient concentrations used in this study were moderate and equivalent to those in streams draining watersheds with altered land use. Thus, our results suggest that similarly moderate levels of enrichment may affect detrital resource quality and subsequently lead to altered energy and nutrient flow in detrital food webs.