In this study, all available phytoplankton (chlorophyll and taxa blooms) and nutrient data from a period of more than 50 years were gathered. During the periods of closure, these lagoons have favoring conditions for phytoplankton growth, which can reach high levels of chlorophyll a (chl a, 290?¦Ìg?L?1), as observed in ¨®bidos lagoon during the 80's. At that time, during the open periods, the highest chl a concentration registered was about one fifth lower (64?¦Ìg?L?1). At present, this lagoon is artificially maintained open during almost all the time and the highest observed chl a concentration was 7?¦Ìg?L?1. Comparatively, the lagoon of Albufeira still presents high phytoplankton biomass levels during the closed periods (e.g. 55?¦Ìg?L?1, during winter 2004). In these semi-enclosed systems, phytoplankton blooms are recurrent and HAB species are frequently observed.
Significant linear relationships were found between nutrient and chlorophyll concentration, during the open and closed periods, for the lagoon of ¨®bidos. This assessment was performed to validate the use of phytoplankton as an indicator of anthropogenic pressures. A tool for the classification, under the WFD, of the ecological quality of water bodies within these coastal lagoons is also described based on two different situations: open vs closed lagoon. A set of reference conditions and boundaries are also proposed. This tool was tested and validated against a scale of risk. In addition, the usefulness of specific species as ecological quality indicators is discussed, highlighting the occurrence of frequent Prorocentrum minimum blooms and its association with nutrient enrichment events worldwide.