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

Abstract

A model of Gymnodinium breve population dynamics modified from Liu et al. (2001; Mar Ecol Prog Ser 210:101-124) is used to investigate the influence of various nutrient conditions on the population increase of an alongshore population filament of G. breve cells as it moves onshore across a continental shelf. The environmental conditions in the model are derived from measurements or theory applicable to bloom development on the west Florida shelf. The simulations indicate that the potential nutrient input patterns here represented by nitrogen sources on the shelf, i.e., offshore, mid-shelf and coastal upwellings, a Trichodesmium-released surface nitrogen source associated 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. breve 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 stage of population growth may induce earlier bloom development, but without additional nitrogen input in coastal regions, the bloom may not fully develop. As long as the nitrogen is available continuously from offshore through coastal regions, a G. breve population can develop into a fish-killing intensity (1 to 2.5 × 105 cells 1-1) in a month or so from a background concentration of <1000 cells 1-1 with a maximum growth rate of ∼0.16 doublings d-1. An explosive growth stage is not present for the total population in the simulations in which fish-killing cell concentrations are developed in 30 d. However the illusion of explosive growth may be created by the first appearance of a high G. breve population density at the surface late in bloom development. In some cases, daily averaged surface concentration can increase by a factor of 10 in 2 d and increase from a background level of 500 cells 1-1 to bloom levels of 104 cells 1-1 in 8 d due primarily to surface accumulation resulting from appropriately directed swimming behavior. This numerical investigation further demonstrates that the vertical migration of G. breve can play a critical role not only in the efficient utilization of natural resources, but also in the population distribution.