Effect of Nitrogen Form and N:P Ratio on Phytoplankton Bloom Formation in Florida Bay, USA: A Mesocosm Study

The ongoing restoration of the Everglades has changed the hydrology in South Florida, and increasing freshwater discharge has contributed to a shift in both the concentration and composition of nutrients as well as phytoplankton abundance and composition in northern Florida Bay. To understand how the change in nutrient load and form contributes to the change in phytoplankton, five mesocosm experiments, were conducted in June 2012, Aug. 2012, May 2013, Nov. 2013, and Apr. 2014.Treatments included additions of varying concentrations of PO₄ and different N forms (NO₃, NH₄ and DON) in differing N:P ratios (4, 16, and 32) (and control). Dissolved nutrients, total biomass and algal pigment composition were monitored for 5 days. Relative to the control, chlorophyll a tripled in the PO₄ treatment, but increased 10-fold in the N plus P treatments. In the N+P treatment, the NO₃+P addition generally exhibited higher chlorophyll a than when N was supplied in other forms. Additionally, the biomass in the NO₃+P and DON+P treatment was largely diatoms, while a similar proportion of diatoms, but almost three times the proportion of cyanobacteria grew in the NH₄+P treatment. When the N:P ratio increased, the contribution of diatoms and cryptophytes to phytoplankton biomass increased, whereas the contribution of cyanobacteria and photosynthetic dinoflagellates declined. These data suggest that following full implementation of the restoration plan, the magnitude and types of phytoplankton blooms in northern Florida Bay may change, and that increased NO₃ plus PO₄ loading may result in increased diatom abundance, while N addition alone, particularly if in the form of NH₄, may contribute to sustained cyanobacteria blooms.