THE ROLE OF TRACE NUTRIENT LIMITATION IN THE CENTRAL BASIN OF LAKE ERIE CYANOBACTERIAL BLOOMS

Periodic summer blooms ofthe diazotrophic cyanobacterium Dolichospermum occur in the central basin of Lake Erie. High concentrations of nitrates and low phosphorus (P) make the presence of Dolichospermum in the central basin enigmatic. Trace nutrients molybdenum (Mo) and iron (Fe) are needed for nitrate-nitrogen (N) assimilation and low concentrations of these nutrients may create N limitation even with nitrate present. Six nutrient enrichment bottle bioassays were conducted using offshore central basin water from June 2 to August 26 in 2016 to determine which nutrient(s) limited algal growth and we hypothesized that more growth would occur when P(1pM) and trace nutrients, Fe(0.5 pM), Mo(0.1 pM), were enriched together compared with P-only enrichment. Chlorophyll a (Chla) (proxy for phytoplankton biomass) and ambient nitrate concentration were measured before and after incubation. In all experiments, P-only treatments demonstrated significantly (P 0.05) between P-only and P with trace nutrients in any experiments, indicating that Fe and Mo did not limit nitrate assimilation. Therefore, the trace nutrient colimitation in the central basin may favor Dolichospermum typically associated with high P and low N.