Modeling Plankton Community Structure Under Environmental Forcing on the Southeastern United States Continental Shelf

Abstract

A system of coupled ordinary differential equations was developed to investigate the time-dependent behavior of phytoplankton, copepod, and doliolid populations associated with upwelling features on the outer southeastern U.S. continental shelf. Model equations describe the interactions of nitrate, ammonium, two phytoplankton size fractions, five copepod developmental stages, doliolids, and a detrital pool. Model dynamics are based primarily upon data obtained from field and laboratory experiments made for southeastern U.S. continental shelf plankton populations. Numerous simulations were performed to investigate the effects of environmental variability on the temporal distribution of the structure of resident plankton populations. Variations on a reference simulation, which represents average upwelling conditions without doliolids, were done to determine the effect of inclusion of doliolids, different feeding strategies, temperature and nutrient variations, and variations in ambient food concentrations on the basic plankton community structure. These simulations provide a measure of the role of environmental versus biological interactions in structuring the planktonic food web on the southeastern U.S. continental shelf. Simulations show that, when present, doliolids reach maximum concentrations 5-7 days after the onset of the phytoplankton bloom resulting from an upwelling event, which is consistent with observations from bottom intrusion upwelling events. The presence of doliolids results in a rapid decrease in copepod concentrations, with the doliolids eventually displacing the copepods. Additional simulations show that ambient temperature conditions modify the rate of increase of the doliolids and copepod populations and hence the relative abundance of these populations.