High frequency water quality and flow observations of a hypereutrophic Coastal Plain millpond

Eutrophication due to runoff of N and P occurs in many impoundments in agricultural areas around the world with deleterious impacts on fisheries, drinking water, and recreational resources. Coursey Pond, a hypereutrophic, shallow, Coastal Plain mill pond located on the Murderkill River in central Delaware has seasonal algal blooms between May and October. High frequency automated water quality, meteorlogical, and flow observations initiated in June 2014 as part of the NEWRNet project provide insights into the relationships between hydrologic events, changes in water quality, and primary productivity. During blooms the pond becomes stratified, allowing for dissolved oxygen (DO) levels at the surface to exceed 150% saturation, while DO within 2 m of the surface to falls below 50% saturation. During fair weather turbidity and dissolved organic carbon (DOC) also gradually rise. Turbidity, DOC, and DO quickly decrease in response to storms and increased flow, indicating that storms are important regulators of water column stratification. Decreases in primary productivity due to decreased sunlight, dilution by addition of rain and runoff, and mixing in response to storm winds and flows abruptly end blooms, although they often return within a few days of storm events. Analysis of hourly meterological data will help determine the importance of solar insolation, winds, and rainfall intensity to the timing, rate, and magnitude of these water quality changes. Groundwater is the primary source of water to the streams that feed the pond and delivers nitrogen as nitrate. Historical grab sample nitrate concentration data from summer months (<1 mg/L) in comparison to winter months (4-8 mg/L) indicate that primary productivity consumes nearly all available nitrate during algal blooms, and perhaps improving water quality in downstream areas. There is no clear relationship between storms, flow and nitrate in the short period of high frequency observations, when nitrate concentrations rarely exceed 1 mg/L. In contrast, a negative correlation between flow and nitrate is typically observed in Coastal Plain streams where groundwater is the dominant source of both water of nitrate. It is expected that the negative correlation between nitrate and flow will re-establish when phytoplankton growth decreases in the fall.