The Roles of Freshwater Discharge, Advective Processes and Silicon Cycling in the Development of Diatom Blooms in Coastal Waters of the Southwestern Florida Shelf and Northwestern Florida Bay (1999-2001)
Jennifer L. Jurado
Broward County DPEP, Ft. Lauderdale, Florida
Gary L. Hitchcock
University of Miami, Miami, Florida
Peter B. Ortner
National Oceanographic and Atmospheric Administration, Miami, Florida
Our research investigated the seasonal development of diatom blooms in coastal waters of the southwestern Florida shelf and northwestern Florida Bay during two years of contrasting rainfall (1999-2000). Spatial and temporal variability in the surface distributions of phytoplankton biomass were analyzed. Biogenic silica (BSiO2) and netplankton chlorophyll a (>5m size-fraction) served as indices of diatom biomass. We investigated the relationship between coastal freshwater discharge and phytoplankton biomass, and the role of advective processes in the transport of phytoplankton biomass and nutrients. The dynamics of diatom growth were analyzed and nutrients limiting the phytoplankton biomass were identified. External sources and internal supplies of silicic acid (Si(OH)4) were compared and quantified. Silicon (Si) budgets were constructed to identify trends in the silicon cycle during three phases of a diatom bloom in northwestern Florida Bay and a conceptual model was developed to describe the evolution of the annual diatom blooms.
Initiation of the annual diatom bloom occurred between April and June as defined by a 2-fold increase in phytoplankton biomass above average background concentrations <1 g l-1 chlorophyll a (Chl a) measured in nearshore waters of the southwestern Florida Shelf between Cape Romano and Cape Sable. The greatest increase in Chl a occurred near Middle Cape Sable, which was identified as the region of bloom initiation. Netplankton biomass near Middle Cape Sable responded to seasonal variability in Shark River discharge, with a rise in biomass that paralleled the increase in river discharge. This response suggests that the Shark River is an important nutrient source that influences the timing and location of the annual diatom bloom. However, interannual variability in the amount of freshwater discharge from the Shark River had less of an influence on the annual development of diatom blooms than seasonal variability in freshwater discharge. Despite a 3-fold difference measured between the maximum rates of Shark River discharge between 1999 and 2000, the maximum concentration of Chl a varied by <10% during the bloom’s peak in October. Furthermore, during 2000 the shorter duration of enhanced discharge during the wet season did not result in a corresponding reduction in the duration of the annual diatom bloom.
Seasonal trends in alongshore advection, and longer retention times in nutrient-rich coastal waters were hypothesized to contribute to the annual development of diatom blooms in late spring. Between April and June, the period of bloom initiation, salinity distributions showed greater retention of low salinity water near the shore of Cape Sable, compared to winter and early spring. Currents measured at an ADCP located near the region of bloom initiation and development switched from net-southward to net-northward flow between April and June. During that time, current speeds were reduced. The trajectories of surface drifters released at the mouth of the Shark River also showed slower alongshore transport during late-spring and early-summer, compared to winter.
Nutrient fluxes out of the Shark River varied seasonally, with maximum fluxes measured during the fall when discharge was greatest. While the Shark River flux of nutrients was locally significant, the alongshore flux of dissolved inorganic nutrients frequently exceeded that supplied by the Shark River. However, combined riverine and alongshore fluxes of inorganic nutrients were less than estimated phytoplankton nutrient quotas, and were deemed to be incapable of satisfying the daily phytoplankton demand for nutrients. It was concluded that continued phytoplankton growth depended upon regenerated nutrients.
Regeneration of Si(OH)4 was hypothesized to provide an important mechanism for maintaining diatom biomass in northwestern Florida Bay, where there was an inverse relationship between diatom biomass (BSiO2) and Si(OH)4 during the bloom peak in 1999. There was no clear seasonal trend in the benthic flux of Si(OH)4 although there was a general pattern of lower fluxes of Si(OH)4 with lower water temperature and, perhaps, salinity. Assuming an average depth of 2 meters for northwestern Florida Bay, the benthos provided 2.76 mol Si l-1 day-1 to the overlying water column. In comparison, regeneration of Si(OH)4 in the water column produced an average of 1.94 mol Si l-1 day-1. The flux of Si(OH)4 across the northwestern boundary of Florida Bay could have supplied an additional 1 mol Si l-1 day-1. During the biomass maximum in summer and fall, the demand by diatoms for silicon (ca. 7 to 12 mol Si l-1 day-1) exceeded the rate of Si(OH)4 supply (ca. 6 mol Si l-1 day-1).
While (SiOH)4 was not identified as a primary nutrient limiting the production of netplankton biomass, the supply and availability of Si(OH)4 were likely important factors influencing the dynamics of diatom blooms in northwestern Florida Bay. Silicon budgets were developed to constrain rates of silicon turnover during the evolution of an annual diatom bloom. During bloom initiation in spring, the pool of Si(OH)4 was 2.5-times greater than that of BSiO2 and the turnover time for each was ca. 2 days. During the period of bloom maintenance, summer and fall, the pool of BSiO2 had doubled compared to spring, and the pool of Si(OH)4 had begun to decline. The production of BSiO2 was 4-times faster than the rate at which Si(OH)4 was supplied to northwestern Florida Bay. During the fall diatom bloom, the pool and supply of Si(OH)4 were balanced by the standing stock and production of BSiO2, suggesting the system was in a steady state. At bloom termination in winter, the pool of BSiO2 was ca. 5-times greater than the standing stock of Si(OH)4 and concentrations of Si(OH)4 had declined by ca. 85% compared to spring. At bloom termination, the rate of silica dissolution was 4-times greater than the rate of BSiO2 production.
Termination of the diatom bloom in December is hypothesized, in part, to be the result of increased advection of phytoplankton cells out of coastal waters and northwestern Florida Bay. Isohalines in surface waters show a broad distribution of low salinity water along the inner shelf and into western Florida Bay between December and February. The trajectories of surface drifters released at the mouth of the Shark River generally show south and southeastward transport of coastal water during winter and spring. Analysis of coastal currents (T. Lee et al.) and alongshore fluxes of nutrients support this interpretation; the seasonal transition from north- to southward alongshore velocities occurred between October and December. During periods of strong southward advection, the flux of phytoplankton biomass out of the north was incapable of replacing biomass lost to the south. Reduced retention in nutrient-rich waters, and replacement of bloom concentrations with low-biomass waters from the north, are believed to have contributed to bloom termination.
Jurado, Jennifer, Broward County Department of Planning and Environmental Protection, 218 SW 1st Street, Ft. Lauderdale, Florida, 33301, Phone: 954-519-1464, Fax: 954-519-1496, , Question 3