Abstract
As coastal watersheds are developed for commercial and residential purposes, nutrient pollution to coastal embayments frequently follows with negative ecological consequences. In areas affected by anthropogenic nutrient loading, environmental managers and researchers have been working together to reduce the nitrogen and phosphorus load to impaired estuaries to restore these ecosystems. Previous data suggest that watershed nitrogen loading can be reduced by natural processes in freshwater ponds along the transport pathway, with removals of as much as 50%, or more, of the nitrogen load passing through them. However, the pathways of nitrogen attenuation are not well documented. In the present study, nitrogen inputs and outputs were measured in two-century old man-made freshwater ponds to evaluate the mechanisms of removal in these systems. Flow, salinity, and nitrogen was measured to construct water, salt and nitrogen budgets for each pond. The water and salt budgets, balanced to within 6% for both ponds, verify that all sources and sinks were identified and increase the confidence of the derived nitrogen budget for quantifying nitrogen attenuation. The nitrogen budgets of each pond also balanced but indicated nitrogen attenuation of 62% and 7% of the total nitrogen entering annually, due to the combination of attenuation processes: sediment burial (17% and 35% of the attenuated nitrogen) and denitrification (75% and 63% of the attenuated nitrogen). However, the primary difference in total attenuation between the ponds was due to differences in sites of denitrification affected by periodic anoxia, nitrate + nitrite availability, and macrophyte coverage. The combination of macrophyte over-abundance, low bottom water oxygen, and low nitrate-nitrite availability, shifted denitrification from the sediments into the upper water column, associated with biofilms on macrophytes. Measurements indicated that dense epiphyte layers (biofilms) on macrophytes spanning the water column had associated denitrification as determined by dinitrogen gas production. Together these data provide insight to environmental managers on the use of man-made ponds for permanent removal of watershed nitrogen load in coastal watersheds for restoration of downgradient estuaries.