Relationship between N:P ratios and phytoplankton biomass given by Chlorophyll-a concentration in coastal and estuarine waters of South Florida

The discovery of Alfred Redfield that the elemental composition of plankton was very similar to that of the major dissolved nutrients, nitrogen and phosphorous in the deep ocean has been used for about 80 years as a paradigm, despite the recognition of settings where phytoplankton stoichiometry diverges from the Redfield ratio. Coastal and estuarine waters are some of those elusive environments where significant departures from the ratio are observed. We have explored the variability of phytoplankton biomass, expressed as chlorophyll-a concentration in the water column, as a function of the N:P ratios for South Florida forty-two coastal and estuarine water types, spanning Biscayne Bay, Florida Keys, Florida Bay, Ten-Thousand Islands and Southwest Florida coast. Data comes from Florida International University water quality monitoring which was regularly performed in this region at 350 stations from early 1990s to 2008. Only the Florida Keys are still monitored. Lower ratios (average= 17) occur along the SW Florida coast, where coastal sediments are P-rich. Mangrove forest of the Ten Thousand Island average is 61; Florida Bay average 156; Florida Keys average 73; the Gulf Shelf 44; and Biscayne Bay 116. Except for Biscayne Bay, highly impacted by the City Of Miami, most waters show a decline in CHLa as the ratio increases, highlighting the P-limited nature of these oligotrophic water bodies. We transformed the CHL-a data into cumulative sum (cusum) data series to determine TN:TP thresholds separating highly productive (high CHLa content) from less productive water columns. Results underscore the span of TN:TP values where high productivity occurs away from Redfield ratios.