Short term variability of chlorophyll a and phytoplankton composition in a shallow area of the Patos Lagoon estuary (Southern Brazil)

Abstract

Short-term variability of chlorophyll a concentration, abundance and composition of phytoplankton and ciliates and of abiotic parameters were analyzed at the Patos Lagoon estuary during two seasons, in the austral spring (October/November 2004) and summer (January/February 2005). Sampling was carried out at a fixed station every 2h30min for chlorophyll a, salinity, water temperature, transparency, water flow velocity and direction and, once a day, for phytoplankton and ciliates, seston and dissolved inorganic nutrients analysis. In austral spring, wind direction changed approximately every two days between northerly and southerly, and rainfall was high (212.9 mm), while in the summer period the wind direction changed less, approximately every five days, and rainfall was low (32.5 mm). The action of rainfall and wind direction fluctuation controlled temperature, salinity, water transparency and chlorophyll a, associated with shorter cycles in the spring than in the summer periods. The concentration of inorganic nutrients also differed, silicate was high in the spring (mean 108 µM) compared to the summer (mean 69 µM) period, and the main nitrogen source in spring and summer was nitrate and ammonia, respectively. The presence of freshwater and marine species in the study area was typically a wind-salinity response, with southerly wind inducing the inflow of marine diatoms and dinoflagellates, and NE wind leading to water discharge containing freshwater cyanobacteria and chlorophytes. The time lag of the wind action on the salinity-biotic response was generally short (hour scale) but may be delayed with low rainfall as it was observed in summer. It was shown that high frequency processes, in the same period of cell-division, 1-2 days, are of primary importance for the lagoon ecology. The fluctuation of freshwater outflow and wind driven processes may prevent, delay or prompt phytoplankton biomass accumulation in the estuarine area, acting as the timing mechanism of the diatom spring bloom. In summer, the increased benthic and pelagic herbivore activity apparently acted as an important chlorophyll a control typical of top-down processes.