THE INFLUENCE OF NITROGEN FORMS ON THE GROWTH AND TOXIN LEVELS OF MICROCYSTIN-PRODUCING CYANOBACTERIA IN
Wisniewski, Nicole Leigh
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Development in coastal South Carolina (SC) includes the construction of shallow (< 3 m) and poorly-flushed stormwater detention ponds to mitigate flooding. Ponds accumulate nutrient (nitrogen-N; phosphorous-P) runoff, providing ideal environments for harmful algal blooms (HABs), including those caused by cyanobacteria. Cyanobacteria HABs present public health concerns because many species produce the hepatotoxin, microcystin. This study examined responses of phytoplankton biomass (chlorophyll <i>a</i>), assemblage composition, and microcystin levels in response to N and P using <i>in situ</i> nutrient addition bioassays (June and August, 2013) as well as and water quality (temperature, salinity, DO, and pH) from monitoring (June 2013-May 2014) in two stormwater detention ponds with a history of HABs. Monitoring results showed that ponds were rich in orthophosphate, much of the total N (TN) was composed of dissolved organic N, and increases in TN were associated with several phytoplankton blooms, including cyanobacteria blooms. Bioassay results showed that within urea treatments, mean chlorophyll <i>a</i> was significantly greater than the control (no additions) (ANOVA; p < 0.05), growth rates were typically faster, and phytoplankton had greater cell densities than other treatments with assemblages composed of chlorophytes, dinoflagellates, and cyanobacteria. Cell densities of diazotrophs were typically greater than non-diazotrophs across all treatments, suggesting that diazotrophy may be a favored N-strategy. Microcystin levels were minimal (< 1.00 ppb) during bioassays. Since organic N-rich fertilizers are frequently applied to areas surrounding stormwater detention ponds, these findings suggest that managers should consider limiting their use to reduce the risk of cyanobacteria HAB proliferation.