Co-occurrence of dinoflagellate and cyanobacterial harmful algal blooms in southwest Florida coastal waters: dual nutrient (N and P) input controls

During July 2006, 2 distinctly different harmful algal blooms (HABs), one dominated by the pelagic red tide dinoflagellatesKareniaspp. and the other by the benthic cyanobacteriumLyngbya majuscula, occurred simultaneously in the coastal embayments surrounding Sanibel and Captiva Islands, Florida, USA. The co-occurring HABs were investigated usingin situbioassays with additions of nitrogen (N) and phosphorus (P) alone and in combination to assess nutrient controls of these ‘dueling’ toxin-producing species. Photosynthetic, biomass (chlorophylla), and (inL. majuscula) nitrogen fixation responses to nutrient enrichment were examined over 4 d. Primary productivity inKareniaspp. was consistently stimulated by N additions, while P additions failed to show stimulation. When added in combination with N, P did not lead to additional stimulation above N alone. Similar patterns of chlorophyllastimulation were observed. These patterns were observed at 2 d, after which the cells fell out of suspension. Nutrient stimulation ofL. majusculametabolic activities as well as biomass production was smaller and much slower, relative to controls, than responses observed inKareniaspp. After the demise ofKareniaspp.,L. majusculawas able to continue utilizing subsequent nutrient additions, and it responded most strongly to the N+P additions after 4 d. This study confirms previous estuarine and coastal studies that indicated that when non-N₂-fixing HABs co-occur with N₂-fixing cyanobacterial HAB species, both N and P inputs need to be carefully considered and, in all likelihood, controlled.