Response of Benthic Microalgae to Phosphorus Inputs in Grand Bay National Estuarine Research Reserve

Benthic microalgae are an important, but often understudied component of shallow, photic estuaries in the Gulf of Mexico. Grand Bay National Estuarine Research Reserve (GBNERR) is located in a small and relatively pristine estuary in the northern Gulf of Mexico. Freshwater input into the estuary is primarily local runoff from bayous and tidal creeks, including Bayou Cumbest, Bayou Heron, and Bangs Lake. Nutrient loading to Grand Bay is relatively small, with ambient nutrient concentrations often below detection. However, several events in 2005, 2012, 2013, and 2014 due to breaches in a containment levee from a gypsum stack have led to high phosphate levels near Bangs Lake. GBNERR staff assembled a phosphate working group to investigate scientific questions related to these phosphate loadings. This working group includes members from GBNERR, regional universities, marine labs, and Mississippi Department of Environmental Quality.

In marine ecosystems, nitrogen availability normally limits growth of phytoplankton and previous research has shown this to be the case in Grand Bay. However, little is known about benthic microalgae in Grand Bay and what their response is to these phosphorus inputs. Between 2013 and 2015, summer concentrations of water column and benthic chlorophyll a were positively correlated, with the highest concentrations occurring in Bangs Lake. Benthic chlorophyll was also positively correlated with the percent surface irradiance reaching the bottom. Bottom light levels range from 3 to 36% surface irradiance. This along with experiments that showed no enhancement of growth of benthic microalgae following addition of nutrients (ammonium, phosphate or both) suggest that benthic microalgae are predominantly light limited rather than nutrient limited. Preliminary nitrogen fixation measurements suggest that nitrogen fixation was positively correlated with extractable phosphate concentrations. Thus, enhanced sediment nitrogen fixation and excess phosphate from the fertilizer plant runoff in this high light environment may enhance benthic microalgal production. The results of this research are part of the larger effort by the phosphate working group to understand the impact of repeated phosphate impacts. These results will provide information needed to help manage the reserve.