Detrital carbon response to experimental enrichment and impacts on associated consumers in a headwater stream
Tant, Cynthia Janelle
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Nutrient enrichment of aquatic ecosystems is occurring globally as a result of mobilization of nitrogen and phosphorus from a variety of anthropogenic sources. Responses to enrichment in detritus-based systems are not as well documented as those in autotroph-based systems. This study examined the effects of enrichment on a Southern Appalachian headwater stream on detrital resources and associated consumers. Heterotrophic microbes, particularly fungi, may perform a keystone function in headwater streams by facilitating carbon and nutrient flow to consumers and in the processing of allochthonous organic matter. I assessed the effect of nutrient enrichment on the relative contribution of decomposers and detritivores. I also examined changes on different size fractions of organic matter resources with enrichment and relationships between microbial biomass and nutrient content. Application of the Hieber-Gessner model suggests that nutrient enrichment may cause shifts toward increasing fungal and shredder contribution to leaf litter breakdown, intensifying losses of carbon through both respiration and downstream transport. The response of CPOM (both maple and rhododendron substrates) to enrichment was primarily positive, but the response of FBOM to enrichment was unexpectedly low. Nutrient enrichment increased fungal biomass and microbial respiration and decreased C:N and C:P on CPOM substrates, suggesting a general increase in resource quality. Estimates of TERs and comparison to associated organic matter resources in each stream suggest that enrichment may reduce P limitation for shredders while potentially increasing C limitation for collectors. In a laboratory setting, CPOM mass loss and FPOM production were highest in treatments with both fungi and elevated nutrients. Both the presence of fungi and exogenous nutrients affected the N content of CPOM, but only nutrients had an effect on P content. Although fungi increased the availability of FPOM, the combined effects of fungi and nutrients did not appear to trickle down to FPOM nutrient content during organic matter transformations. These results highlight potential mechanisms for food web shifts and pathways of carbon processing that may result from nutrient enrichment of detritus-based systems.