Abstract
The influence of environmental parameters (temperature, light and nutrients) on the seasonal dynamics of Ulva spp. and Sargassum sp. in the Cabo Frio upwelling of Brazil was evaluated by empirical data analyses and simulation modeling. There is a spatial gradient in Ulva percent cover caused, in part, by differences in the influence of upwelling waters. The sites influenced by upwelling events during the spring-summer have lower water temperatures, below 18°C, and higher total inorganic nitrogen, concentrations around 10 muM, than during the non-upwelling period (inorganic nitrogen and temperature around 4 muM and 22°C respectively). There is a negative correlation between algal cover and temperature; and a positive correlation between algal cover and nutrients at sites affected by upwelling waters. A non-linear differential equation model was used to simulate algal biomass (g d. wt./m2). Data on environmental parameters were used as input to the model. Seasonal dynamics of Ulva and Sargassum can be explained by the factors included in the model (temperature, light, nutrients and intraspecific competition). Sargassum growth lags nutrient enrichment events, while Ulva growth follows more closely the nutrient pulses. The model is very sensitive to changes in the minimum nutrient internal quota for both Ulva and Sargassum biomass simulations.