What factors control the percentage of nitrogen that gets exported downstream from man-made reservoirs?

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, width, volume, and residence time play in regards to metabolism (GPP and Respiration) and nutrient retention. At the inflows and outflow of each reservoir, we measured conductivity, dissolved oxygen, total suspended solids, nitrate, phosphate, chlorophyll, and dissolved organic carbon. Using conductivity, which is conservative, and watershed area, we created a mass balance for each watershed. In most cases the conductivity mass balance indicated that water inputs and outputs were at equilibrium during sampling, allowing us to assess the alteration of non-conservative material fluxes. Dissolved oxygen and TSS were not balanced, indicating that the reservoirs acted as both a source and a sink for DO and sediments depending upon the time of day and amount of algal activity. Similar analyses will be conducted for nutrients. The net change of each variable across the reservoirs will be related to geomorphological characteristics of the reservoirs. With nutrient loading from anthropogenic sources, and increased push for small dam removal, this study provides useful information regarding the consequences of dam removal to downstream aquatic ecosystems.