A three-dimensional biophysical model of light, nutrient, and grazing controls on phytoplankton competition affecting red tide maintenance on the west Florida shelf

A coupled, three-dimensional, time-dependent numerical model of water circulation, spectral light, plankton dynamics, nutrient/CDOM loadings, and zooplankton grazing provided an assessment of the factors affecting the growth and maintenance of red tides on the west Florida shelf (WFS). The coupled biophysical model consisted of state variable quantities for temperature, salinity, horizontal/vertical velocity components, turbulent diffusion, spectral light, colored dissolved organic matter (CDOM), dissolved organic and inorganic carbon, particulate silica, four dissolved inorganic nutrient pools (nitrate, ammonium, phosphate, and silicate), and four phytoplankton groups (diatoms, microflagellates, non-toxic dinoflagellates, and the red tide organism Karenia brevis). The model also included a complex grazing scheme that utilized thirteen different zooplankton groups to explore the effects of selective herbivory, feeding periodicity, diel vertical migration, fecal pellet egestion, and ammonium/phosphate excretion within a diverse zooplankton community. Over the shelf and slope of the eastern Gulf of Mexico, from the Mississippi River delta to the Florida Keys, four cases of the model were run during August – November to explore the dynamics of red tide maintenance with respect to: (1) no refuge from grazing for K. brevis; (2) grazer avoidance of K. brevis during CDOM shading; (3) grazer avoidance of K. brevis in Case II waters; and (4) increased grazing stress on K. brevis competitors. NEGOM and ECOHAB data sets during July – November 1999 were used to establish the initial/boundary conditions and provided validation data for the coupled model as well. Model results indicate that the red tide of 5.9 x 10 6 cells L-1 witnessed offshore Sarasota, Florida on 07 October 1999 was initiated by an inoculum of K. brevis observed in near-bottom waters above the 30 m isobath offshore Sarasota on 31 August 1999. Flowfields measured at moored ADCPs, observations from AVHRR satellite imagery, and west Florida shelf circulation models indicate that conditions of coastal upwelling existed during the period of bloom development, such that the K. brevis inoculum was delivered to the coast in the bottom Ekman layer. As a shade-adapted species capable of vertical migration, K. brevis cells aggregated near the bottom in order to escape photo-inhibitive light intensities in the overlying water column during the day and harvested the recycled nitrogen excreted by zooplankton grazers. This concomitant relaxation of light inhibition and nitrogen-limitation ultimately led to the growth and maintenance of the red tide, constrained in near-bottom waters during much of the day and preferentially advected inshore as a result of coastal upwelling. As K. brevis was advected inshore, self-shading, CDOM, and suspended inorganic particulates all contributed to the prevention of photo-inhibitive light intensities that, in combination with the excretion of recycled ammonium, ultimately led to the maintenance of a significant red tide at the coast.