Trophic state of Foz de Almargem coastal lagoon (Algarve, South Portugal) based on the water quality and the phytoplankton community

Seasonal variation of water quality and phytoplankton community was studied in Foz de Almargem coastal lagoon in order to evaluate the trophic state of the wetland. This small coastal lagoon has temporary connections to the sea, when the sand barrier is naturally or artificially opened, but for most of the year is isolated receiving just the freshwater input from small rivers. Sampling took place in three stations along a gradient of marine influence from June 2001 to July 2002. During summer and autumn, the water in the lagoon was low, influencing positively water temperature, salinity, total solids in suspension (TSS), orthophosphates, total phosphorus (TP) and pH. In winter and spring, there was an increase of nitrates, nitrites, dissolved inorganic nitrogen (DIN) and N:P ratio with the raise of water level in the lagoon. Chlorophyll a and phaeo-pigments concentrations were positively related to the previous parameters. Phytoplankton density was low and switching in dominant taxa was observed along time but the community was mainly dominated by Dinophyceae and Bacillariophyceae. Chlorophyceae and Euglenophyceae were related to higher TSS and lower DIN, salinity and TP; Cyanophyceae were stimulated by the opposite conditions. The decrease of water level jointly with the raise of salinity and TP influenced positively pico-nano flagellate algae. Cryptophyceae were positively correlated with the same factors and also TSS. Several trophic state indexes and water quality indicators have been applied and an overall analysis pointed out to a coastal lagoon with mesotrophic characteristics. During the studied period no serious eutrophication events occurred, however there were some situations of nutrient enrichment due to human activities, such as agriculture and non-treated sewage discharges, which might have favoured the development of potentially toxic phytoplankton species, namely Prorocentrum minimum.