Freshwater resources are vital to environmental sustainability and human health; yet, they are inundated by multiple stressors, leaving aquatic communities to face unknown consequences. Headwater streams are highly reliant on allochthonous sources of energy. Riparian trees shade the stream, limiting primary production, causing macroinvertebrates to consume an alternative food source. Traditionally, leaf litter fallen from riparian trees is the primary allochthonous resource, but other sources, such as salmon carrion associated with annual salmon runs, may also be important. An alteration in the quantity or quality of these sources may have far reaching effects not only on the organisms that directly consume the allochthonous resource (shredders), but also on other functional feeding groups. Allochthonous resources directly and indirectly change stream microbial communities, which are used by consumers with potential changes to their life histories and behavior traits. The objective of my research was to determine the influence allochthonous resources have on stream communities of macroinvertebrates and microbes using two systems: salmon carrion decomposition and emerald ash borer (EAB), Agrilus planipennis Fairmaire (Coleoptera: Buprestidae) invasion. It was hypothesized that with an alteration in allochthonous resource quantity and/or quality, the aquatic community would be altered.When salmon carcasses, a heterotrophic allochthonous resource, are introduced to a stream, the macroinvertebrate and microbial (bacteria and microeukaryotes) communities changed compared to a control stream reach without salmon carcasses over time. Specifically, Heptagenia (Heptageniidae: grazer) density was five times higher in the salmon reach compared to the control. In the salmon reach during year one, Stramenopiles (i.e., eukaryotic microbes) decreased in biofilm communities after two weeks of decomposition. Although unique microbial taxa, introduced to the naive stream via salmon carrion, persisted in biofilms on benthic substrate and internal to insects during both years, those taxa represented < 2% of the relative abundance in microbial communities. These results highlight the importance of allochthonous carrion resources in the microbial ecology of lotic biofilms and macroinvertebrates.Mortality of ash trees along stream corridors as a result of EAB invasion can result in canopy light gaps, which potentially alter subsequent organic matter subsidies into streams. We characterized the coarse woody debris, leaf litter and their associated bacterial communities (terrestrial and aquatic), and macroinvertebrate communities upstream, downstream, and at the center of one EAB-related canopy gap in six headwater streams of Michigan. Downstream locations had significantly lower dissolved oxygen and macroinvertebrate diversity, but we did not detect watershed and gap location effects on aquatic leaf litter. These findings reveal EAB invasion impacts stream ecosystems through indirect routes downstream of canopy gaps, yet leaf litter subsidies are resilient. Decomposition rates and macroinvertebrate colonization in leaf packs of four species (ash, oak, buckthorn, and cotton control) upstream, downstream, and at the center of one EAB-related canopy gap were assessed. There was no gap effect on decomposition rates. Shredder genera were more abundant on ash compared to buckthorn leaves, and macroinvertebrate diversity was significantly higher in the gap, compared to upstream and downstream. Our findings suggest a shift in macroinvertebrate communities in response to EAB by indirect routes of leaf litter subsidies and light availability. From these results, it can be concluded that allochthonous resources were a significant contributor to stream biodiversity patterns, and my dissertation research represents a significant contribution of knowledge on community assembly in streams.
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