Influence of environmental nutrient conditions on Gymnodinium breve (Dinophyceae) population dynamics: a numerical study

A model of Gymnodinium breve population dynamics modified from Liu et al. ( 2001; Mar Ecol Frog Ser 210:101-124) is used to investigate the influence o f various nutrient conditions on the population increase of an alongshore p opulation filament of G. breve cells as it moves onshore across a continent al shelf. The environmental conditions in the model are derived from measur ements or theory applicable to bloom development on the west Florida shelf. The simulations indicate that the potential nutrient input patterns here r epresented by nitrogen sources on the shelf, i.e., offshore, mid-shelf and coastal upwellings, a Trichodesminm-released surface nitrogen source associ ated with multi-nutrient ocean fertilization by air-borne dust input, and a coastal surface plume are all eligible to trigger and/or support a G. brev e bloom. However, the occurrence, timing, location, duration, and intensity of the bloom are determined by nitrogen concentration, input location, and temporal availability. Some nitrogen support at the offshore initiation st age of population growth may induce earlier bloom development, but without additional nitrogen input in coastal regions, the bloom may not fully devel op. As long as the nitrogen is available continuously from offshore through coastal regions, a G. breve population can develop into a fish-killing int ensity (1 to 2.5 x 10(5) cells l(-1)) in a month or so from a background co ncentration of < 1000 cells l(-1) with a maximum growth rate of similar to0 .16 doublings d(-1). An explosive growth stage is not present for the total population in the simulations in which fish-killing cell concentrations ar e developed in 30 d. However the illusion of explosive growth may be create d by the first appearance of a high G. breve population density at the surf ace late in bloom development. In some cases, daily averaged surface concen tration can increase by a factor of 10 in 2 d and increase from a backgroun d level of 500 cells l(-1) to bloom levels of 10(4) cells l(-1) in 8 d due primarily to surface accumulation resulting from appropriately directed swi mming behavior. This numerical investigation further demonstrates that the vertical migration of C. breve can play a critical role not only in the eff icient utilization of natural resources, but also in the population distrib ution.