Ecosystem metabolism in a mediterranean shallow lake (Laguna de Santa Olalla, Donana National Park, SW Spain)

Santa Olalla is a Mediterranean permanent coastal shallow lake, with high annual solar irradiation and warm water temperature throughout the year. Ecosystem metabolism, chlorophyll a concentrations, and physical and chemical features were studied in the Santa Olalla lake from March 1998 to February 2000. Gross primary productivity (GPP) and community respiration (CR) were determined using a modification of the one-station diel oxygen change method. Chlorophyll a, Secchi depth, and total phosphorous and nitrogen reveal Santa Olalla to be a hypereutrophic system. Values of GPP and CR were very high (average 7.88 g O-2 m(-2) d(-1) and 8.52 g O-2 m(-2) d(-1), respectively). Principal components analysis and a multiple regression model showed photosynthetically active radiation and soluble reactive phosphorus to be the main factors that control primary production in Santa Olalla. Also, the annual and interannual variation of water level in Santa Olalla seems to be an important factor that influences the ecological processes of this system. Mean net daily metabolism during a hydrologic period was not statistically different from 0 (0.008 g O-2 m(-2) d(-1), p = 0.99), and the average P/R ratio was not statistically different from 1 (1.164, p = 0.343). These results suggest that the primary productivity was balanced with community respiration in Santa Olalla lake. The high values of metabolic rates and low nutrient concentrations in this aquatic ecosystem suggest a quick recycling of nutrients, which were assimilated almost instantaneously after mineralization.