Nutrient enrichment of a detritus-based ecosystem
Cross, Wyatt Firmin
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Nutrient enrichment of aquatic ecosystems is occurring globally as a result of anthropogenic changes to the availability and mobility of elements such as nitrogen (N) and phosphorus (P). Currently, very little is known about the effects of nutrient enrichment on detritus-based ecosystems despite the prevalence and global significance of detritus in most energetic budgets. This study was aimed at quantifying effects of an experimental nitrogen and phosphorus enrichment on invertebrate consumers and basal resources in a detritus-based headwater stream. Efforts were focused on examining nutrient-induced changes in a) the quantity and quality of detrital food resources and b) invertebrate growth rates, secondary production, food web dynamics, and stoichiometry. Enrichment had a large positive effect on the quality of basal resources (i.e., leaf litter, fine benthic organic matter, and epilithon) though increased N and P content. In contrast, the quantity of basal resources declined overall due to a significant reduction in benthic leaf litter. Both primary and secondary invertebrate consumers exhibited a positive response to enrichment through increased biomass and secondary production. However, there were large differences in response among individual taxa, which was related to an interaction between two dominant life history characteristics – larval lifespan and feeding behavior. Long-lived taxa (>1 year) showed little or no response to enrichment, whereas many short-lived taxa (<1 year) showed large positive responses to enrichment. Individual growth rates of chironomid larvae increased during enrichment, while those of Tallaperla spp. were not affected. Enrichment had no effect on the diets of consumers, but there were large effects on total organic matter flows to all functional feeding groups. Patterns of consumer stoichiometry indicated distinctly lower relative P content of detritus-based consumers in comparison to previously published data from plant- or algal-based communities. In addition, consumer P content was elevated in the treatment stream during enrichment, demonstrating that some insect consumers are much less homeostatic than other well-studied crustaceans. These data suggest potential evolutionary adaptations of detritus-based consumers to nutrient poor food resources. This dissertation should provide a basis for predicting how landscape-scale nutrient enrichment may affect community structure, production, and material cycling in forested headwater streams.