Nutrient dynamics in a eutrophic blackwater urban lake

Abstract

Iraola ND, Mallin MA, Cahoon LB, Gamble DW, Zamora PB. 2022. Nutrient dynamics in a eutrophic blackwater urban lake. Lake Reserv Manage. 38:28–46. Greenfield Lake is a eutrophic blackwater urban lake in Wilmington, North Carolina, and in 2014 was declared by the state as impaired waters due to excessive chlorophyll a concentrations. Phytoplankton production is strongly nitrogen (N) limited, and the lake supports filamentous green algal blooms in spring and N-fixing cyanobacteria blooms in summer. To inform future lake restoration efforts, we quantified nutrient loads from 5 perennial streams that drain the highly impervious Greenfield Lake watershed to determine their impact on the lake’s eutrophic state. The 5 streams were sampled monthly from 2016 to 2017 during periods of dry weather and again after rain events of at least 1.2 cm to compare stormwater runoff to baseflow conditions. Two streams alone accounted for 76% of inorganic N and 63% of inorganic P surface inflow, whereas 2 other streams, despite high (53–74%) impervious coverage, had low loading. A lake-wide sediment phosphorus survey also revealed P accumulations mirrored streams of high nutrient load, while low P accumulations were in areas draining streams with large wetland and stormwater treatment areas. Drainage area was positively correlated with nitrate concentration and nitrate and ammonium loading. Unconventionally, percent impervious coverage was negatively correlated with nutrient concentrations and loads; we suspect that the presence of a large golf course in the watershed with the lowest impervious cover was a key driving factor. Our work also demonstrates the significance of catchment features: Drainages with large stormwater retention structures and significant natural wetlands in the lower reaches yielded low nutrient loadings.