Using multiple lenses to investigate trophic interactions and responses to ecological stressors in lotic ecosystems
Wood, James Lindley
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Freshwater resources around the world are increasingly impacted by human activity. Conversion of forested land into agriculture land and increasing urbanization in watersheds has been correlated with increased nutrient concentrations in surface water and alteration to the natural flow regime. These modifications can decrease resource availability to benthic organisms by decreasing standing stocks of basal resources and increasing carbon export from the system, or stimulate undesirable algal accrual. Because nutrient enrichment and flow alteration can have deleterious impacts on freshwater biodiversity and ecosystem services, we investigated how nutrient enrichment and flow affect basal resources in mid-sized streams and rivers in the eastern U.S. We investigated changes in basal resource structure and function at multiple scales (a few meters to 100s of kilometers) by using a variety of short term (24 hours) and long-term (up to 2.5 months) experiments. To test the effects of increased nutrient availability on autotrophic and heterotrophic biofilms, we deployed nutrient diffusing substrates into 15 streams in the Little Tennessee River watershed in the Southern Appalachian Mountains. We found that gross primary production was phosphorus limited on epilithic surfaces but that algal biomass was co-limited by nutrients. We also found that the effects of consumers and scour may obscure the effects of nutrients on labile carbon sources. To test the effects of flow alteration on basal resources, we setup a series of experiments to explore how water velocity and herbivory interacted to regulate biomass accrual of the widespread riverine macrophyte Podostemum ceratophyllum. We found that water velocity reduced consumer access to the plant, and that low water velocity conditions facilitate the transfer of stored biomass into the food web. Lastly, we analyzed Podostemum from rivers between Georgia and Maine for nutrient content (C, N, P), metals (Cd, Na, Zn) and isotopically enriched nitrogen (δ15N). We found that land use was reflected in the elemental composition of Podostemum, which has implications for nutrient storage and flux, and presents a link by which trace metals from urbanized landscapes can move into food webs at potentially toxic levels. Collectively, these study provide new insight into the various ways in which anthropogenic activity impact basal resource in freshwater ecosystems.