Nutrient dynamics in a coastal lagoon (Ria Formosa, Portugal): The importance of lagoon-sea water exchanges on the biological productivity

Silicates, nitrates, phosphates, chlorophyll a and primary productivity were monitored at low and high tide between September 1985 and September 1986, at four stations in Ria Formosa, a meso-tidal coastal lagoon located in the south of Portugal. The water sampling and the in situ observations were done alternatively in neap and spring tide. While silicates at low tide exceeded values at high tide thorughout the year and a weak seasonal trend was discerned, nitrates and phosphates exhibited a clear-cut seasonal variation at high spring tide. Pronounced maxima in the period of low temperatures (early spring), 7 muM of nitrates and 3 muM of phosphates, indicate their import from the adjacent coastal zone. Lower values at low tide and chlorophyll a increases (up to 2 mug g(-1)) point to a rapid biological consumption of the imported nutrients. Differences of temperature between coastal waters and those of the lagoon in early spring account for the more efficient consumption by the primary producers of the lagoon. The primary productivity varied from 2 mg C m(-3)h(-1) (winter) to 30 mg C m(-3)h(-1) (summer), reflecting the intensive C-14 incorporation during the warmer period. High rates of nitrates and phosphates consumption could be observed during spring, when their concentrations decreased 50% within a narrow period of 6 hours from high to low tide.