INVESTIGATING THE SOURCES AND PROCESSING OF NITRATE IN FRESHWATER PONDS OF THE MURDERKILL WATERSHED

Nitrogen is one of the most abundant elements on earth, yet it is often the limiting nutrient in ecosystem productivity. However, human production of reactive nitrogen during the past century has increased over ten-fold with the overwhelming majority of the nitrogen coming from fertilizer production. This has led to an enormous increase in the nitrogen load transported from the landscape to marine ecosystems, resulting in severe environmental problems such as groundwater contamination and coastal eutrophication. High nutrient loads from agriculture and residential developments have been identified as major sources of pollution in the Murderkill watershed of Delaware and are having significant adverse effects on the water quality in the watershed and the abutting estuary. This study documented the patterns of nitrogen, phosphorus, and carbon concentrations laterally, vertically, and temporarily to determine the impact that man-made ponds have on the reduction of nutrient loads to the estuary. Water and particulate samples were collected from four sites between the stream inflow and the outflow dam at Killens Pond, a small man-made freshwater pond located within the Murderkill Watershed. Four sets of samples for nitrate, ammonium, phosphate, particulate phosphorus, and chlorophyll a, along with key field parameters (dissolved oxygen, specific conductivity, and temperature), were collected at the surface for all 4 sites and at varying depths for 3 sites between June and July 2013. Phytoplankton species abundance was also determined for one sample site. Significant reductions in nitrate and phosphate concentrations were observed between the stream inflow and outflow of the pond. Reductions in phosphate concentrations were consistent with phytoplankton uptake, but reductions in nitrate concentrations were far greater than could be accounted for by uptake alone for three of the four sample dates. Although the conditions observed in the water column were not appropriate for denitrification, the excess nitrate reduction could be the result of denitrification occurring within or near the bottom sediments of the pond. Understanding the mechanisms of potential nitrogen removal in Killens Pond can aid in the development of better management practices for the Murderkill Estuary and the watershed as a whole.