Modeling the effects of doliolids on the plankton community structure of the southeastern US continental shelf

A model of the lower trophic levels that consists of a system of coupled or dinary differential equations was developed to investigate the time-depende nt behavior of doliolid populations associated with upwelling features on t he outer southeastern US continental shelf. Model equations describe the in teractions of doliolids with two phytoplankton size fractions, five copepod developmental stages and a detrital pool. Additional equations describe ni trate and ammonia. Model dynamics are based primarily upon data obtained fr om field and laboratory experiments for southeastern US continental shelf p lankton populations. Variations on a reference simulation, which represents average upwelling conditions without doliolids, were carried out to determ ine the effect of inclusion of doliolids, temperature and nutrient variatio ns, and variations in ambient food concentrations on the basic plankton com munity structure. These simulations provide a measure of the role of enviro nmental versus biological interactions in structuring the planktonic food w eb on the southeastern US continental shelf. Simulations show that the cope pod population is significantly reduced when doliolids are present. This ha ppens primarily as a result of direct predation of the doliolids on copepod eggs and juveniles as opposed to an increase in competition for phytoplank ton, the primary food source. Additional simulations show that the cooler t emperatures associated with the newly upwelled water temporarily decrease t he growth rates of the doliolids and copepods, bestowing an even greater ad vantage on the rapidly reproducing doliolids.