Synthesis of stream ecosystem responses to nutrient enrichment at multiple trophic levels

The goals of this LTER Synthesis Working Group are (1) to initiate and coordinate the integration of data from multiple sites on stream ecosystem responses to nutrient amendment, (2) to synthesize these data with a quantitative evaluation of functional responses at the microbial, primary producer, consumer and whole-stream levels and (3) to promote interaction between LTER and the NEON experiment (Stream Experimental and Observatory Network, STREON): of 10 STREON sites, 6 are located at or near LTER sites. Streams are hotspots of nutrient processing on the landscape, receiving, transforming and cycling inorganic nutrients (nitrogen (N) and phosphorus (P)) that are delivered from the watershed. The cycling of inorganic nutrients is a result of the interactions between geochemical settings and the integrated activity of biota that inhabit streams, including microbial life (bacteria and fungi), primary producers (plants, algae and cyanobacteria) and secondary producers (invertebrates and fish). As such, stream water nutrient concentration is a key driver of stream function at these three trophic levels, and of integrated ecosystem processes like leaf litter decomposition and whole-stream metabolism. Increases in stream nutrient concentrations are occurring globally, as a result of increased nutrient input and runoff from intensifying agricultural and urban land use and atmospheric deposition related to the use of fossil fuels. Nutrient increases may negatively affect stream ecosystem goods and services and exacerbate efforts to conserve stream biota. Aptly, research on the effects of nutrient enrichment in streams (some of it from LTER sites) has expanded. However, consideration of these effects across multiple trophic levels and ecosystem response metrics is limited, and an integrated framework to forecast the long-term nutrient enrichment effects on stream ecosystem health is still lacking. The proposed synthesis effort will address these gaps. We predict that regardless of geochemical setting nutrient enrichment will increase activity and/or biomass at all (microbial, primary producer, secondary producer and integrated ecosystem) levels of stream function. However, a lack of response to nutrient enrichment may occur in some systems due to several possible mechanisms. Stream function in waters with high ambient nutrient concentrations may be insensitive to increased nutrient inputs. If biotic activity is primarily limited by factors other than nutrient concentration, such as temperature, pH, conductivity, energy availability (light and/or organic matter) or even the presence of water, the response to increased nutrient concentrations may be muted. Stream physical factors may reduce the bioavailability of added nutrients. Nutrient effects on stream function may also be masked or counteracted by organismal interactions, reflecting a stronger effect of biotic interactions than abiotic conditions on ecosystem function. With an integrated multi-site synthetic dataset of nutrient enrichment effects on stream ecosystem function, we can test these mechanistic hypotheses in a systematic manner. As data from numerous individual studies have now been published, we can also consider the effect of level of enrichment, duration and stoichiometry of nutrient addition on the response ratio of stream ecosystem function. Participants in the LTER and STREON synergies and opportunities: Next generation questions in stream ecological research workshop at the 2012 LTER All Scientists Meeting expressed strong interest in the proposed data integration and synthesis goals. At this ASM workshop, we agreed that a key step in integrating the soon-to-be widely available STREON data with ongoing LTER stream ecology research activities would be the synthesis of existing data to inform hypotheses on multi-site and long-term nutrient enrichment effects on stream ecosystems. The current and continued participation of stream ecologists with research interests encompassing microbial, primary producer, secondary producer and whole-ecosystem dynamics drives the need to coordinate and synthesize information on multiple trophic levels of stream ecosystem response to nutrient enrichment. This proposed synthesis group will also work synergistically with researchers using modeling approaches to integrate data and create quantitative predictive frameworks for future research in stream ecology. This synthesis effort will address the LTER Network synthesis themes of inland climate change and future scenarios of change, and link three LTER Network core research areas (movement of inorganic matter, movement of organic matter, primary (and secondary) productivity). Members of the proposed working group will include organizers of the LTER ASM workshop plus stream ecologists encompassing a balanced representation of junior and senior investigators, research interests at different trophic levels, and field research based at widespread LTER sites.