Modeling Ulva spp. dynamics in a tropical upwelling region

The development and application of a model to represent the seasonal dynamics of Ulva spp. biomass at the Cabo Frio tropical upwelling region is here described. In addition to biomass, the model simulates nitrogen and phosphorus internal concentrations in the algae. The factors considered to affect algal growth at Cabo Frio are sunlight, water temperature, nitrate, ammonium and phosphate, and the alga carrying capacity. Peaks in algal abundance (up to 1000 g d.wt./m2) were observed to occur and were simulated by the model during the upwelling season. The inclusion of a variable optimum temperature was found to be necessary to avoid a phase lag between model and observations. Calibration of some of the model parameters with a simplex method has further improved the model results. The simulations shown here suggest that the initial growth of Ulva, after a downwelling period, is stimulated by the increase in nutrient concentrations in the water column during upwelling.