The Ria Formosa is a coastal lagoon in south eastern Portugal, covering an area of, approximately, 110 Km2 and it is a major centre of bivalve production and salt extraction. There is little information about the planktonic communities in the Ria. The current work aimed to study the distribution of the planktonic community in the western section of the lagoon. The Ria Formosa is mesotidal hence the study was aimed to focus on the tidal exchange within the Ria Formosa.
Initially monthly sampling of temperature, salinity, dissolved oxygen concentration, pH and chlorophyll a concentration was undertaken, simultaneously with quantitative samples of the phytoplankton and zooplankton. Samples were collected, on both flood and ebb tides, between July 2001 and July 2002, at three stations corresponding to a potential gradient from the sea to the inner lagoon. In the winter and spring of 2004, a more lagrangian approach was adopted by sampling, as above, twelve times while
drifting into and out of the Ria.
The annual sampling revealed seasonal patterns in the plankton communities similar to those found in warm temperate waters. Maximal phytoplankton densities were recorded in the summer and maximal zooplankton densities in the autumn. The winter was characteristically poorer in plankton abundance and diversity. The communities were largely dominated by diatoms and copepods throughout the year. Acartia clausi was the most abundant copepod and was omnipresent. The two outer stations showed higher densities of characteristically neritic forms, such as chain-forming diatoms and
holoplanktonic animals, such as the Cladocera, the Appendicularia or the Chaetognatha. The inner Ria showed significantly fewer neritic species, with an increase in the abundance of benthic diatoms in keeping with the shallowness found in the inner lagoon, and also in the abundance of organisms with a weaker marine character (such as the copepod Acartia grani).
The data from the abiotic parameters and the composition of the planktonic
assemblages suggested a higher water residence time at the western end of the Ria Formosa, than that estimated for the whole lagoon by previous studies.
The drift samplings suggested that the spatial distribution of the zooplankton in the Ria Formosa may result from a combination of tidal movement and behavioural patterns. Different sorts of behaviour were exhibited by different organisms. Mollusc larvae, e.g. , appeared to ensure retention inside the lagoon, neritic holoplankton, e.g., hydrozoans, appeared to avoid transport into the inner reaches of the lagoon, while decapod zoea adopted a stratagem increasing the probability of export from the lagoon.