Nitrogen, phosphorus and silica on the West Florida Shelf: Patterns and relationships with Karenia spp. occurrence

A large scale investigation was conducted in major estuaries and the oligotrophic coastal waters of the West Florida Shelf during the fall of 2007 through 2010 to identify statistically significant and persistent associations of the nearly annual blooms of the harmful algae, Karenia brevis, with dissolved and particulate nutrients, and with other taxonomic groups of the coastal phytoplankton community. Conflicting interannual patterns of Karenia densities and riverine flows indicated that links between Karenia blooms and terrestrial contribution of nutrients were neither direct nor certain. There was no evidence in these data that Karenia blooms were enhanced by estuarine outflow. There was no characteristic nutrient ensemble with which Karenia presence was associated, which implied a wide range of nutrient strategies for the organism. Drawdowns of urea and DIN were not a universal feature of the blooms sampled. The only persistent relationship with individual parameters were a correlation of Karenia cell density with DON and an inverse correlation with DIN:Si(OH)4. Temporal and spatial patterns of DON and δ15N provided evidence of regional processes controlling nitrogen supply. Correlation of δ15N values with cyanophyte percentage of biomass demonstrated the significance of N2-fixation to the region. Karenia was correlated with haptophyte and cyanophyte biomass and inversely correlated with diatoms. The phytoplankton community was not significantly different with Karenia present and indicated that blooms replaced differing components of the phytoplankton community at different times. Principal component analysis of phytoplankton community composition identified a modest association of Karenia with nutrient-poor conditions. The results emphasized that strong interannual variability existed in nutrient regimes, phytoplankton community, and nutrient-mediated Karenia responses.