Abstract Man-made dams influence more than just the flow of water in a river. The build up of sediments and organic matter, increased residence times, and elevated nutrient inputs from upstream can result in increased algal growth and blooms, altered DO patterns, and can also influence the flux of nutrients from watersheds. Many of the effects of dams vary in intensity based on the geomorphology of their resulting reservoirs. In this study, we examined eight reservoirs located in four different coastal watersheds in New England, USA, to analyze the role that characteristics such as depth, surface area, and flow conditions play in regards to retention of nitrate. At the inflows and outflow of each reservoir, we measured conductivity, dissolved oxygen, total suspended solids, chlorophyll, and nutrients. Using conductivity and watershed area, we created a mass balance for each reservoir. In most cases the conductivity mass balance indicated that hydrologic inputs and outputs were at equilibrium during sampling, allowing us to assess the alteration of non-conservative material fluxes. Nitrate removal was found to decrease with increasing hydrologic load, and TDN removal was similar to values described in the literature. Chlorophyll a concentrations decreased with increasing hydraulic load, suggesting that biotic assimilation plays a key role in nitrate removal at low hydraulic loads. DON production increased with nitrate removal in the smallest reservoirs, suggesting that nitrate transformation to DON is occurring. With nutrient loading from anthropogenic sources, and increased push for small dam removal, this study provides useful information regarding the benefits of low head dams, and the impact that their removal may have on nitrogen export from river networks. Water Systems Analysis Group PIE LTER NH Agricultural Experiment Station What factors control nitrate retention in man-made reservoirs? New Hampshire EPSCoR J. M. Buonpane 1 W. M. Wollheim 1,2, C.T. Whitney 1 Background Results Acknowledgements We would like to thank the following people for their help and support in making this project possible: Stanley Glidden, Dr. Gopal Mulukutla, Rob Stewart, Dr. Anne Lightbody, Christopher Cook, and Trevor Mattera. Thank you to Dr. Steve Hale for organizing this research experience. Thank you to NH EPSCoR, Plum Island Ecosystem LTER, and the NH Agricultural Experiment station for providing funding to make this research possible. Finally, thank you to the University of New Hampshire for supporting research in undergraduate education. Future Work -Analyze conductivity and nutrient retention during various seasons and flow conditions -Determine reservoir metabolism and relate it to nitrate retention -Scale estimates to entire river network to see how widely distributed dams affect nitrogen flux to coastal waters Conclusions -At low hydraulic loads, low head reservoirs help mitigate the effects of anthropogenic nutrient loading by removing nitrate from river networks. -Dam removal may results in river networks delivering greater amounts of nitrogen to coastal waters. -TDN retention similar to values measured in lakes and river reaches (Seitzinger et al. 2002) for given hydraulic load conditions. -Biotic assimilation is likely a leading contributor to overall nitrate removal in reservoirs -In some reservoirs (low hydraulic load), removed nitrate is transformed to DON, which likely explains why TDN retention is lower than nitrate retention. Central Question: What factors control the amount of nitrate that is retained in small head reservoirs, which are numerous in New England river systems? Rationale : dams in the state of New Hampshire, many more total in the rest of New England -Impacts of dam removal is currently a hot topic -Anthropogenic nutrient loading may be attenuated by reservoirs, thus insight will be gained regarding the effects of dam removal. -Coastal ecosystems are especially nutrient sensitive Approach: -Examine eight reservoirs in four coastal New England watersheds -Sample nutrients, total suspended solids, conductivity, chlorophyll a and DO at inflow(s) and outflow -Use conductivity to verify hydrologic mass balance of inputs and outputs -Estimate discharge using nearby USGS gage flow data and watershed area scaling -Analyze the relationship between nitrate retention and other variables Little Hale Pond Figure 7: Nitrate uptake velocity increases with chlorophyll a concentration, suggesting importance of biotic assimilation in nitrate removal. Figure 1: Nitrate retention decreases with increasing hydraulic load. -Within reservoirs and across systems -Nitrate production at high hydraulic loads -Some nitrate transformed to DON (Figure 5, Figure 6) Figure 3: Calculated removal of TDN is similar to TDN removal described by Seitzinger et al, Figure 4: Chlorophyll a concentrations decrease with increasing hydraulic load. Suggests that biotic assimilation may explain some of the nitrate removal. Figure 5 (left) and Figure 6 (right): Dissolved organic nitrogen (DON) production increases with increasing nitrate removal in Beards Pond (31% transformed on average) and Little Hale Pond (39% transformed on average), the two reservoirs with the lowest average hydraulic loads. Suggests that nitrogen is being transformed from inorganic to organic form in these reservoirs. Nitrate TDN TDN, Seitzinger Figure 2: Conductivity is relatively balanced across all reservoirs, suggesting hydrologic mass balance of inputs and outputs. -Retention fell between -15% and 15% 21 of 24 times H13L-1761 (1) University of New Hampshire Department of Natural Resources & the Environment, (2) Earth Systems Research Center, UNH, Durham, NH