The Keystone Role of Heterotrophic Microbes in Driving Ecosystem-level Effects of Nutrient Enrichment.

We have been continuously enriching a detritus-based headwater stream for 5 years. We have observed remarkable changes in the production of heterotrophic microbes and invertebrates, and consequent effects on ecosystem function in terms of decomposition rates and carbon balance. As a result of our long-term nutrient enrichment, fungal production and activity dramatically increased, decay rates of leaf litter increased, benthic organic matter standing crop has been reduced and export of fine particulate organic matter has increased. Long-term data from these streams indicate that benthic invertebrate production is highly correlated with benthic organic matter. Thus, despite increased invertebrate biomass and production we observed after 2 years of enrichment, we predict that over longer time scales (> 4 yr.), annual invertebrate production will return to a lower stable mean in the treatment stream, based on carbon limitation. The first objective of this study was to continue the nutrient enrichment of this headwater stream for one year to examine longer-term trends in system changes. The effects we have observed in this system have been largely driven by fungal rather than bacterial response, suggesting a keystone role of fungi in driving the response of a detritus-based system to nutrient enrichment. The second objective of the study, therefore, is to determine the role of fungi in controlling the system-wide response to nutrient addition. We are currently testing the keystone role of fungi by reducing fungal response to enrichment (by continuous addition of a fungicide) and measuring changes in the response of other variables. Critical to our predictions is whether there is a synergistic, facilitative relationship between fungi and bacteria, or whether these groups of microbes compete (e.g., for labile carbon or nutrients). Whether bacteria respond positively or negatively to reductions in fungi will test these alternative hypotheses. To the extent that microbial diversity and production is important to invertebrate production and ecosystem function will also be shown via differences in production and litter processing following reductions in fungi. This study will add to our understanding of mechanisms driving detritus-based system response to nutrient enrichment and the role of microbial diversity in ecosystem functioning.