Hydrodynamics as a major factor controlling the occurrence of green macroalgal blooms in a eutrophic estuary: A case study on the influence of precipitation and river management

Data on the variation of physicochemical parameters, biomass and growth of green macroalgae (mostly Enteromorpha) collected between January 1993 and J anuary 1997 in the Mondego Estuary (western coast of Portugal) was analysed with the aim to identify the factors that control opportunistic macroalgal abundance in the system. The annual biomass of Enteromorpha spp. is strongly dependent on the amount of fresh water that enters the system during winter and spring. In turn, t he input of fresh water is regulated by precipitation and by river manageme nt practices. The optimization of the rice crops from the upstream valley d epends on their water level, which determines the number of days and hours per day during which sluice gates remain open in winter and spring. River f low has significant impacts on salinity, N:P ratios, current velocities and light extinction coefficients within the system. The interaction of all th ese factors controls macroalgal growth and biomass loss processes. In winters and springs during which sluice gates are often closed due to wa ter deficiency of the rice fields (dry winter and spring or dry winter foll owed by rainy spring), little fresh water enters the system and consequentl y, salinity remains high, N:P ratios around 20, light penetration increases , and current velocities fall. These conditions facilitate macroalgal fixat ion, enhance their growth and spring blooms occur. On the contrary, during winters and springs when fresh water is in excess of rice fields' needs (ra iny winters and springs), sluice gates remain open for long periods of time . High input of fresh water to the system causes salinity and light penetra tion to decrease, while N:P ratios and current velocities increase. These c onditions contribute both to reduced Enteromorpha growth and higher loss of macroalgal biomass from the system to the ocean. The present work shows that the inter-annual variation of macroalgal biomas s in the Mondego Estuary is controlled by hydrodynamics, which in turn depe nds on precipitation and on river management, according with the water need s of the upstream rice crop. (C) 2001 Academic Press.