Consumer-driven nutrient cycling has been shown to be an important process in supplying inorganic nutrients to autotrophic and heterotrophic organisms in aquatic ecosystems. Theory indicates that consumer nutrient excretion is influenced primarily by an organism's nutrient composition; however, an organism's diet should also play an important role in nutrient excretion, especially if the consumer is nutrient limited. This study asks the question, how does diet influence nutrient excretion of consumers at different trophic levels? Macroinvertebrates and fish were collected from six streams and nitrogen (N) and phosphorus (P) excretion were quantified. Epilithon, leaf detritus, and seston (fine particulate organic matter in transport) were collected and analyzed for carbon (C), nitrogen (N), and phosphorus (P) content in an attempt to qualitatively assess the nutritional status of the diet of primary consumers. Macroinvertebrates were also analyzed for C, N, and P content to assess their nutritional composition in relation to their excretion and also to assess the nutritional composition of the diet of predatory insects and fish. Fish were also analyzed for C, N, and P. Similar to theoretical predictions, fish and macroinvertebrate P excretion was negatively related to P content and the N:P excretion ratio was negatively related to the body N:P ratio. However, this relationship was driven primarily by two phosphorus rich species, mottled sculpin in the fish and crayfish in the macroinvertebrates. Some relationships did emerge between consumer excretion and diet. For example, hydropsychid caddisflies had the highest macroinvertebrate P excretion, possibly explained by the low N:P of seston. However, shredders, eating on a very low N and P diet of leaf detritus, had very low N and P excretion. The relationship between consumers, their food, and nutrient excretion is a matter of mass balance. If the food N:P ratio is higher than that of the consumer, then the N:P excretion should be higher than the consumer N:P and the food N:P, especially if organisms are P-limited. However, N:P excretion by macroinvertebrates and fish were very similar despite large differences in diet. The majority of macroinvertebrates and fish had a lower N:P excretion ratio than the predicted N:P of their food, possibly indicating that 1) consumers were either selectively consuming more P-rich foods than the diets that I assigned them or 2) consumers are generally not N or P limited or influenced by the N or P in their diet. Mottled sculpin and crayfish were the only organisms with a higher N:P excretion than their resources and both had a higher %P than the other fish and macroinvertebrates, respectively. High N:P excretion along with high phosphorus content is indicative of P-limitation. Macroinvertebrates and fish, excluding mottled sculpin and crayfish, had a lower N:P excretion and the N:P ratio of the water column. If consumers do play a role in nutrient dynamics, then consumers could alter the relative abundance of nitrogen and phosphorus by supplying more phosphorus. However, the presence of a P-limited organism, such as mottled sclupin or crayfish, could alter the relative abundance of nitrogen and phosphorus by supplying less phosphorus. / Master of Science

As one of the largest and most productive estuaries in the United States, the Chesapeake Bay is a great economic, ecological, and cultural asset to the Mid-Atlantic region. Excess nitrogen and phosphorus discharge, however, has contributed to reduced levels of dissolved oxygen in various locations throughout the Bay. In 2010, the EPA developed a Total Maximum Daily Load (TMDL) for the entire watershed that established nutrient reduction targets to achieve Bay water quality objectives. The TMDL required states in the Chesapeake Bay watershed to create implementation plans to meet nutrient reductions. Maryland and Virginia specifically established stringent point source regulatory policies designed to meet point source reduction targets. Point source control programs created financial incentives for reducing nutrient discharge beyond regulatory requirements. This thesis will examine the extent to which Maryland and Virginia wastewater treatment plants undertake operational improvements to increase nutrient removal in response to state program incentives. Through quantitative and qualitative analysis, this thesis found evidence of lowered nitrogen discharges in response to financial incentives presented by each states point source control programs at municipal wastewater treatment plants. Maryland achieves modest improvements at a subset of advanced treatment WWTPs as a result of financial incentives presented by the state's public subsidy program. Although Virginia advanced treatment plants operating within a nutrient trading program have little incentive to over-comply, there is some evidence of operational improvements at less advanced nitrogen removal plants / Master of Science

A 2-yr field study was conducted to reduce nutrient losses from Virginia dairy farms through nutritional and herd management practices. Ten collaborator herds were identified, all at state DHIA average or better for milk yield and days open. Baseline feed samples and ration information were collected for 2 mo and analyzed for phosphorus (P) and nitrogen (N). Feeds were analyzed monthly, and monthly DHIA milk yield, milk composition, milk urea N (MUN), and reproductive data were recorded. Blood and fecal samples were collected from 25 cows/herd every 3 mo to monitor P excretion and blood urea N. Nutrient balances were developed for each farm for N and P at the start of the study and following ration and management changes. Collaborator herds imported, on average, 290% more N and 320% more P onto the farm than was removed through milk, culled animals, crop sales, or manure sales. By following NRC (1989) recommendations, collaborator farms could reduce N inputs by 21% and P inputs by 45%. Minimizing P in purchased feed, purchased feeds/cow, purchased feeds/ha, and total P input could cause significant reductions in P balance for participating collaborator herds. None of the N variables tested (purchased feed, purchased feed/cow, purchased feed/ha, and total N input) provided significant reductions. / Master of Science

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