Biogeochemical responses of a Coastal Plain pond to hydrologic and nutrient loading variability - implications for long-term management and mitigation

Summer blooms of cyanobacteria are common in many small millponds in the mid-Atlantic Coastal Plain, and are a legacy of decades of nutrient inputs from groundwater and streams. Because blooms sometimes generate toxins and frequently impact habitat and recreational use, it is becoming important to forecast bloom onset. Data from commercially available, high frequency, continuous, and automated biogeochemical sensors at Coursey Pond, Delaware, have identified the hydrophysical and hydrochemical conditions that favor the onset, evolution, persistence, and dissipation of these blooms and, options for managing them.

Cyanobacterial blooms occur in Coursey Pond during summer months when high solar insolation, greater air and water temperatures, and larger rates of evapotranspiration and irrigation in the watershed cause low discharge and increased water residence time in the pond. These conditions lead to uptake and depletion of bioavailable nitrogen in the pond surface waters, and provide a competitive advantage for temperature-tolerant and nitrogen-fixing cyanobacteria. Once initiated, blooms typically continue with variable intensity until mid-October to mid-November when temperatures cool, day-length decreases, and discharge and nitrate concentrations and loads recover. Intense summer storms sufficient to increase flow and decrease residence times to less than two to three days also temporarily disrupt blooms. Blooms re-establish in a few days after residence times and water temperatures increase back to pre-storm values.

Changes in one or more of these easily monitored physical or chemical parameters such as flow, temperature, dissolved oxygen, and nitrate can, therefore serve to anticipate the onset, intensity, persistence, and the eventual dissipation of cyanobacterial blooms at Coursey Pond and similar ponds elsewhere. With groundwater as the dominant source of nitrogen, and residence times of decades, managing future nutrient inputs will not have measureable affects for years. Instead, it may be possible to mitigate blooms by managing flow through changes to irrigation practices.