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Historic millponds are common in the mid-Atlantic Coastal Plain and these features cause multiple physical and chemical effects on both ground and surface waters. Millpond dams raise the baselevel of streams and the water table and increase hydraulic residence time in surface and sub-surface regimes. Millponds accumulate mineral and organic sediment and usually allow for increased solar insolation of the water surface, conditions that enhance biogeochemical processing of the common contaminants nitrogen and phosphorus. A detailed study of Coursey Pond, a millpond on the Murderkill River in Delaware, using automated, in-situ water quality instrument-generated data and hydrograph separation shows that seasonal, primary productivity in the pond converts far more nitrate-nitrogen to biomass than is delivered to the pond by surface and subsurface pathways. The amounts of biomass-nitrogen stored in sediment and nitrogen lost due to denitrification in the bottom sediment are as yet unknown quantities. Even in the absence of the sediment-nitrogen flux terms it is clear that Coursey Pond is providing ecosystem services to downstream locations through net removal of 20 to 40 kg of nitrate-nitrogen per day.
Mass balance calculations take advantage of concepts of whole stream metabolism where stoichiometry of photosynthesis (P) and respiration (R ) reactions, biomass composition, and streamflow measurements are the primary data inputs. P and R are calculated from rates of change in the concentration of dissolved oxygen (DO) during daylight and nighttime, respectively, and adjusted for atmospheric exchange of oxygen due to temperature and wind. Net primary production (NPP), calculated as P-R and expressed in units of milligrams liter DO-1–day-1 . The amounts of carbon and nitrogen flux associated with NPP are estimated from the flux of oxygen in streamflow, 1:1 ratio of carbon to oxygen in plants, and the Redfield ratio 106:16:1 (C:N:P).