Long-term and seasonal trends in phytoplankton production and biomass In Tampa Bay, Florida.

Phytoplankton production is a basic process in aquatic ecosystems that converts inorganic carbon into organic matter and provides an important indicator of trophic state. The City of Tampa Bay Study Group maintains a 32 year long monthly record of phytoplankton production rates and biomass (chlorophyll-a) in Hillsborough Bay (HB) and Middle Tampa Bay (MTB), and a recent record during the last nine years for Old Tampa Bay (OTB). Production is measured using the classic in situ 14C method with samples incubated vertically in the water column. Annual production rates during the most recent decade are about 410gCm-2 for HB, 350gCm-2 for MTB, and 390gCm-2 for OTB. The current rates for the two former bay segments are near half the rates measured during 1980-1985. Reductions in biomass have been greater; current HB and MTB concentrations are near 70% lower than those measured during 1980-1985. Water column averaged chlorophyll-a concentrations during the most recent decade are about 10mgm-3 for HB, 5.8mgm-3 for MTB, and 6.3mgm-3 for OTB. The decreases in production and biomass, and also reductions in phytoplankton abundance, are reflected in a large reduction in anthropogenic nitrogen loading to the bay that primarily occurred during the late 1970s and early 1980s. This is strong evidence the long-term trend in Tampa Bay phytoplankton production and biomass has been regulated by the supply of nitrogen from external sources. However, similar to other productive estuarine and coastal systems, in-bay recycling provides a substantial fraction of the nitrogen needed to sustain the observed daily production rates. The vertical distribution of phytoplankton production in HB and MTB has shifted during the study period, most noticeable during the wet summer seasons, from a large proportion of total water column production occurring in the upper meters to a more even distribution with depth. Further, seasonal water column production generally reaches maximum during the summer months and follows variations in water temperature. Finally, a comparison of current phytoplankton and seagrass carbon production in the bay segments studied indicates that pelagic phytoplankton dominates production and will most likely continue to do so in the future