Algal blooms, circulators, waterfowl, and eutrophic Greenfield Lake, North Carolina.

Mallin MA, McIver MR, Wambach EJ, Robuck AR. 2016. Algal blooms, circulators, waterfowl, and eutrophic Greenfield Lake, North Carolina. Lake Reserv Manage. 32:168–181. Lake rehabilitation measures can sometimes have unintended consequences, including increasing certain eutrophication symptoms. An example involves Greenfield Lake, an urban blackwater impoundment within the City of Wilmington, North Carolina. This popular recreational impoundment has experienced noxious phytoplankton and macrophyte blooms, anoxia and hypoxia, and fish kills for many years. Chlorophyllaand biological oxygen demand are strongly correlated within the lake, indicating that decaying algal blooms are an important source of oxygen demand resulting in low dissolved oxygen. Nutrient addition bioassays demonstrated that the phytoplankton strongly responded to nitrogen (N) inputs. Spatially, ammonium, nitrate, and N/P ratios were higher in the upper lake compared with the lower lake, indicating tributary input of inorganic N, whereas phosphorus (P) was higher in the lower lake. A year-long survey indicated that waterfowl, especially cormorants, contributed somewhat to the lake's total N load but a considerable amount of P, particularly in winter. The lake sediments apparently function as P reservoirs, supplying P to the water column during summer, setting the stage for runoff-induced nitrate pulses and subsequent algal blooms. Lake restoration measures initiated in 2005 included installation of solar-powered circulators, introduction of grass carp, and herbicide treatments. These measures resulted in loss of surface macrophyte and algal mats and reduced dissolved oxygen violations but led to a significant increase in phytoplankton chlorophyllaand a tripling of chlorophyllastandard violations in comparison with pre-restoration years. The increased chlorophyllaviolations have led to inclusion of the lake on the North Carolina 303(d) list of impaired waters. [ABSTRACT FROM PUBLISHER]