The potential role of benthic nutrient flux in support of Karenia blooms in west Florida (USA) estuaries and the nearshore Gulf of Mexico

The project “EcoHAB: Karenia Nutrient Dynamics in the Eastern Gulf of Mexico” (NOAA EcoHAB grant NA06NOS4780246) was designed to simultaneously quantify a wide variety of nutrient sources which can support the nearly annual blooms of Karenia brevis in the estuarine and coastal waters of west central Florida. Estuarine outflows and water column stocks of nutrients have been previously demonstrated to be insufficient to support the observed cell densities (>106 cell L−1) which K. brevis can achieve. Nutrients released from sediments, however, are a nutrient source common to offshore, coastal, and estuarine environments. Prior regional measurements were generally lacking, particularly in coastal waters. Dark-only nutrient fluxes from the sediments were directly measured using large (65 L, 0.27 m2), in situ, opaque, stirred chambers deployed over 5–8 h. Stations were located in coastal waters, a lagoonal system, and in the lower portions of two major estuaries, Tampa Bay and Charlotte Harbor, and were sampled during the fall of 2007–2009. Field procedures included triplicate chambers at each station and a water column control. Fluxes were quantified for dissolved oxygen, and dissolved ammonium, nitrate-nitrite, urea, total nitrogen, orthophosphorus, total phosphorus, and silica. Fluxes were dominated by dissolved silica (900–13,800 μmol m−2 d−1), ammonium (430–3360 μmol m−2 d−1) and ortho-phosphorus (50–1200 μmol m−2 d−1) and were greater than literature values which had been previously incorporated in local ecological modeling efforts. Based on literature growth rates and on cell complement data, the measured sediment flux of nutrients could have provided up to 300% of P requirements, and up to 100% of N requirements of 50,000 cells L−1 of K. brevis, modified by water depth. Flux values, derived from dark incubation and applied to Karenia cells only, likely represented an upper boundary of sediment contributions to Karenia nutrient budgets. Density stratifications common to upwelling coastal plumes and migratory behavior of K. brevis would have increased the percentage of supply afforded by sediment flux. Despite ammonium to ortho-phosphorus ratios in sediment flux substantially less than Redfield, ammonium flux provided a larger fraction of growth needs due to low levels of water column DIN concentrations (averaging 1.0 μmol L−1). Fluxes enriched in P were consistent with bulk sediment P and the P-enriched geology of the region, and would have complemented N supplied by common nitrogen fixers such as Trichodesmium sp.