Seasonal variability in nutrient and phytoplankton distributions on the southwest Florida inner shelf

Surface nutrient concentrations and phytoplankton biomass were monitored on the southwest Florida inner shelf during 2 yrs of contrasting rainfall (1999–2000) to assess potential linkages between freshwater discharge and surface phytoplankton communities. A diatom bloom began between April and June each year near Cape Sable during the period of peak freshwater discharge and maximum annual nutrient flux from the Shark River watershed. The bloom began as netplankton biomass (> 5 μm size-fraction Chl a) and increased following an increase in discharge from the Shark River. Maximum phytoplankton biomass occurred in October when Rhizolosenia spp. dominated the netplankton community, and the annual maximum occurred in biogenic silica (BSiO₂) concentrations, at 10 and 20 μmol L⁻¹ in 1999 and 2000, respectively. Results suggest riverine discharge onto the SW Florida inner shelf is an important nutrient source that influences the timing and distribution of annual diatom blooms. Although particulate matter composition suggests nitrogen potentially limits phytoplankton biomass during most of the year, the diatom community may be limited by silicon availability at bloom termination.