Amy Marcarelli

Quantity and quality: unifying food web and ecosystem perspectives on the role of resource subsidies in freshwaters

Although the study of resource subsidies has emerged as a key topic in both ecosystem and food web ecology, the dialogue over their role has been limited by separate approaches that emphasize either subsidy quantity or quality. Considering quantity and quality together may provide a simple, but previously unexplored, framework for identifying the mechanisms that govern the importance of subsidies for recipient food webs and ecosystems. Using a literature review of > 90 studies of open-water metabolism in lakes and streams, we show that high-flux, low-quality subsidies can drive freshwater ecosystem dynamics. Because most of these ecosystems are net heterotrophic, allochthonous inputs must subsidize respiration. Second, using a literature review of subsidy quality and use, we demonstrate that animals select for high-quality food resources in proportions greater than would be predicted based on food quantity, and regardless of allochthonous or autochthonous origin. This finding suggests that low-flux, high-quality subsidies may be selected for by animals, and in turn may disproportionately affect food web and ecosystem processes (e.g., animal production, trophic energy or organic matter flow, trophic cascades). We then synthesize and review approaches that evaluate the role of subsidies and explicitly merge ecosystem and food web perspectives by placing food web measurements in the context of ecosystem budgets, by comparing trophic and ecosystem production and fluxes, and by constructing flow food webs. These tools can and should be used to address future questions about subsidies, such as the relative importance of subsidies to different trophic levels and how subsidies may maintain or disrupt ecosystem stability and food web interactions.

Is in-stream N2 fixation an important N source for benthic communities and stream ecosystems?

Abstract We evaluate the current state of knowledge concerning the ecosystem- and community-level importance of N2 fixation in streams. We reviewed the literature reporting N2-fixation contributions to stream N budgets and compared in-stream N2-fixation rates to denitrification and dissolved inorganic N (DIN)-uptake rates. In-stream N2 fixation rarely contributed >5% of the annual N input in N budgets that explicitly measured N2 fixation, but could contribute higher proportions when considered over daily or seasonal time scales. N2-fixation rates were statistically indistinguishable from denitrification and DIN-uptake rates from the same stream reach. However, published N2-fixation rates compiled from a wide variety of streams were significantly lower than denitrification or DIN-uptake rates, which were indistinguishable from one another. The data set we compiled might be biased because the number of published N2-fixation measurements is small (9 studies reporting rates in 22 streams), the range of stream conditions (NO3–-N concentration, discharge, season) under which N2-fixation and other N-processing rates have been measured is limited, and all of the rate estimates have associated methodological artifacts. To broaden our understanding of how N2 fixation contributes to stream ecosystems, studies must measure all rates concurrently across a broad range of stream conditions. In addition, focusing on how N2 fixation supports food webs and contributes to benthic community dynamics will help us understand the full ecological ramifications of N2 fixation in streams, regardless of the magnitude of the N flux into streams from N2 fixation.

Salinity controls phytoplankton response to nutrient enrichment in the Great Salt Lake, Utah, USA

To examine how salinity and nutrient supply interact to control phytoplankton community composition, nutrient limitation, and dinitrogen (N2) fixation rates in the Great Salt Lake (Utah, USA), we conducted a series of bioassay experiments with plankton from both Gilbert Bay, where salinities are near 160 g·L–1, and Farmington Bay, where salinities range from 10 to 90 g·L–1. Six-day nutrient addition bioassay experiments showed that the extant phyto plankton communities in both bays were limited by nitrogen (N). However, in 28- to 30-day factorial bioassay experiments in which both salinities and nutrient supply were manipulated, phosphorus stimulated chlorophyll a as much as 500% when salinities were less than 70 g·L–1 and N2-fixing cyanobacteria were present. At salinities greater than 70 g·L–1, or with additions of combined N, N2 fixation ceased. When N2-fixing cyanobacteria were absent, the plankton community was routinely N-limited regardless of salinity. The results of these experiments suggest that ...

Predicting effects of hydrologic alteration and climate change on ecosystem metabolism in a western U.S. river

We estimated past and future hydrographs and patterns of ecosystem metabolism in a fifth-order river of the western United States, where water use and climate change are both expected to alter hydrology in the immediate future. We first reconstructed the unregulated hydrograph to estimate how the current hydrograph has been altered. Due to consumptive use, 95% as irrigation, current discharge during summer (July-September) was 70% lower than would occur if the river was unregulated. We then predicted a future hydrograph including effects of consumptive use and climate change; the magnitude of flow changes were minor under this regime relative to those already manifested by consumptive uses. We used time-series regression and a six-year continuous record of open-water metabolism to demonstrate that, under the current hydrologic regime, gross primary production (GPP) was dependent on both water temperature and flow and that ecosystem respiration (ER) was most dependent on temperature. Monte Carlo simulations under the three hydrologic regimes and three temperature scenarios (current, +2 degrees C, +4 degrees C) suggested that flow, but not temperature, may have profound effects on the magnitude of metabolism. Linking temporally detailed analyses of ecological function and hydrology may lead to better understanding and management of changes due to basin-scale water use and/or global-scale climate change.

A Non-Native Riparian Tree (Elaeagnus angustifolia) Changes Nutrient Dynamics in Streams

Russian olive (Elaeagnus angustifolia) is a non-native riparian tree that has become common and continues to rapidly spread throughout the western United States. Due to its dinitrogen (N2)-fixing ability and proximity to streams, Russian olive has the potential to subsidize stream ecosystems with nitrogen (N), which may in turn alter nutrient processing in these systems. We tested these potential effects by comparing background N concentrations; nutrient limitation of biofilms; and uptake of ammonium (NH4-N), nitrate (NO3-N), and phosphate (PO4-P) in paired upstream-reference and downstream-invaded reaches in streams in southeastern Idaho and central Wyoming. We found that stream reaches invaded by Russian olive had higher organic N concentrations and exhibited reduced N limitation of biofilms compared to reference reaches. However, at low inorganic N background concentrations, reaches invaded by Russian olive exhibited higher demand for both NH4-N and NO3-N compared to their paired reference reaches, suggesting these streams have the potential to retain the N subsidy from Russian olive N2 fixation and diminish its downstream export and effects. Our findings demonstrate the potential for a non-native riparian plant to significantly alter biogeochemical cycling in streams. Finally, we used our results to develop a conceptual model that describes predicted effects of Russian olive and other non-native riparian N2 fixers on in-stream N dynamics.

Cyanobacteria in Freshwater Benthic Environments

Cyanobacteria are widespread in freshwater benthic environments, which include wetlands, lake littoral zones, streams and rivers. This chapter outlines the major constraints on cyanobacteria in these environments. Environmental and ecological factors that determine the diversity and biomass of cyanobacteria in the freshwater benthos include physical disturbance in the form of turbulent energy and wetting/drying cycles, temperature, light, nutrients and grazing. Nutrients are particularly important, because their concentrations can control cyanobacteria within and among benthic habitats, and cyanobacteria can reciprocally influence nutrient availability via nitrogen fixation and phosphorus co-precipitation by calcareous species. Top-down control via grazing may also help to explain diverse patterns of cyanobacterial abundance, because of the interactions which occur between the cyanobacteria and their predators. Anthropogenic activities sometimes have a pronounced effect on the environmental conditions that control cyanobacterial diversity and abundance in these habitats and the resulting functional changes in the communities can result in a loss of important ecosystem services provided by these organisms.

Effects of temperature and concentration on nutrient release rates from nutrient diffusing substrates

Abstract Nutrient diffusing substrates (NDS) are an important tool for evaluating periphyton nutrient limitation. The rate at which nutrients are released from NDS depends on both the initial nutrient concentration and the length of time that NDS are in place. Whether temperature also affects nutrient release rates from NDS is unclear. However, this information is important because temperature effects on release rates could confound experimental results for NDS-based experiments testing rates of accumulation of periphyton biomass when stream water temperature is variable. We measured N and P release rates from NDS vials with 3 initial concentrations (0.05, 0.1, and 0.5 mol/L) of nutrients at 3 temperatures (4, 15, and 21°C) for 21 d. Release rates of both nutrients were greater for vials with higher nutrient concentrations and for vials at warmer temperatures. For all concentrations, release rates decreased log linearly with time, a result that might have important implications for patterns of colonization and subsequent interspecific interactions within the periphyton community. In our opinion, temperature-caused differences in release rates are not biologically important because the differences were much smaller (3%) than expected changes in periphyton maximum growth rates over similar temperature ranges (∼300%). Our results suggest that seasonal and site-related differences in temperature will not significantly affect nutrient release rates within the range of temperatures we tested, but researchers should consider nutrient concentration carefully when planning studies using NDS.

An invasive riparian tree reduces stream ecosystem efficiency via a recalcitrant organic matter subsidy

A disturbance, such as species invasion, can alter the exchange of materials and organisms between ecosystems, with potential consequences for the function of both ecosystems. Russian olive (Elaeagnus angustifolia) is an exotic tree invading riparian corridors in the western United States, and may alter stream organic matter budgets by increasing allochthonous litter and by reducing light via shading, in turn decreasing in-stream primary production. We used a before-after invasion comparison spanning 35 years to show that Russian olive invasion increased allochthonous litter nearly 25-fold to an invaded vs. a control reach of a stream, and we found that this litter decayed more slowly than native willow. Despite a mean 50% increase in canopy cover by Russian olive and associated shading, there were no significant changes in gross primary production. Benthic organic matter storage increased fourfold after Russian olive invasion compared to pre-invasion conditions, but there were no associated changes in stream ecosystem respiration or organic matter export. Thus, estimated stream ecosystem efficiency (ratio of ecosystem respiration to organic matter input) decreased 14%. These findings show that invasions of nonnative plant species in terrestrial habitats can alter resource fluxes to streams with consequences for whole-ecosystem functions.

Disruptions of stream sediment size and stability by lakes in mountain watersheds: potential effects on periphyton biomass

Abstract The location of a stream reach relative to other landforms in a watershed is an important attribute. We hypothesized that lakes disrupt the frequency of finer, more mobile sediments and thereby change sediment transport processes such that benthic substrates are more stable (i.e., less mobile) below lakes than above lakes. In turn, we hypothesized that this reduced mobility would lead to greater periphyton biomass below lakes. We tested these hypotheses in study reaches above and below lakes in 3 mountain watersheds. To expand this comparison, we analyzed the relationship between sediment attributes and periphyton biomass in one watershed with and one watershed without a lake. We hypothesized that no clear pattern or change in sediment size or chlorophyll a (chl a) would be observed over a 3-km-long study reach without a lake. In contrast, we expected a clear discontinuity in both sediment size and chl a in a 7-km-long study reach interrupted by a lake. Average median sediment size (D50) was significantly larger (p < 0.01) in lake-outlet than lake-inlet reaches (41 mm vs 10 mm). Bed sediments in lake-outlet reaches were immobile during bankfull flows, whereas sediments at lake-inlet reaches were mobile during bankfull flows. Chlorophyll a was ≥10× greater in lake-outlet reaches than in lake-inlet reaches, although this difference was not statistically significant (p = 0.17). The longitudinal analysis clearly showed geomorphic transitions in sediment size and mobility downstream of mountain lakes, and these geomorphic transitions might be associated with changes in periphyton biomass. Geomorphic transitions can alter sediment transport and should be considered in concert with other factors that are considered more commonly in benthic ecology, such as light, nutrients, and temperature.

A Critical Assessment of the Ecological Assumptions Underpinning Compensatory Mitigation of Salmon-Derived Nutrients.

We critically evaluate some of the key ecological assumptions underpinning the use of nutrient replacement as a means of recovering salmon populations and a range of other organisms thought to be linked to productive salmon runs. These assumptions include: (1) nutrient mitigation mimics the ecological roles of salmon, (2) mitigation is needed to replace salmon-derived nutrients and stimulate primary and invertebrate production in streams, and (3) food resources in rearing habitats limit populations of salmon and resident fishes. First, we call into question assumption one because an array of evidence points to the multi-faceted role played by spawning salmon, including disturbance via redd-building, nutrient recycling by live fish, and consumption by terrestrial consumers. Second, we show that assumption two may require qualification based upon a more complete understanding of nutrient cycling and productivity in streams. Third, we evaluate the empirical evidence supporting food limitation of fish populations and conclude it has been only weakly tested. On the basis of this assessment, we urge caution in the application of nutrient mitigation as a management tool. Although applications of nutrients and other materials intended to mitigate for lost or diminished runs of Pacific salmon may trigger ecological responses within treated ecosystems, contributions of these activities toward actual mitigation may be limited.