Absorption coefficient of particulate matter on the south-west coast of Europe: a contribution to the validation of the MERIS-Medium Resolution Imaging Spectrometer sensor

Dissolved oxygen (DO) is one of the most important environmental variables of water quality, especially for marine life. Consequently, oxygen is one of the Chemical Quality Elements required for the implementation of European Union Water Framework Directive. This study uses the example of the Ria Formosa, a meso-tidal lagoon on the south coast of Portugal to demonstrate how monitoring of water quality for coastal waters must be well designed to identify symptoms of episodic hypoxia. New data from the western end of the Ria Formosa were compared to values in a database of historical data and in the published literature to identify long-term trends. The dissolved oxygen concentration values in the database and in the literature were generally higher than those found in this study, where episodic hypoxia was observed during the summer. Analysis of the database showed that the discrepancy was probably related with the time and the sites where the samples had been collected, rather than a long-term trend. The most problematic situations were within the inner lagoon near the city of Faro, where episodic hypoxia (<2 mg dm3 DO) occurred regularly in the early morning. These results emphasise the need for a balanced sampling strategy for oxygen monitoring which includes all periods of the day and night, as well as a representative range of sites throughout the lagoon. Such a strategy would provide adequate data to apply management measures to reduce the risk of more persistent hypoxia that would impact on the ecological, important natural resource. economic and leisure uses of this important natural resource.

Understanding the importance of sediments to water ouality in coastal shallow lagoons

The undesirable enrichment of water by nutrients may be a problem, especially in areas with restricted exchange with the sea. The tidal regime flushes the system and contributes for the removal of phytoplankton, favouring phytobenthos as the target of enhanced nutrients. Water samples were collected during the years of 2006 and 2007-08 for nutrients, chlorophyll a and dissolved oxygen. Sediment sample s were also collected for pore water nutrients and benthic chlorophyll a. From comparison with previous work, a decrease in the nitrogen concentration in the water column can be pointed out, which may indicate an improvement of the water quality. Pore water DAIN represents approximately 75% of the total DAIN of the whole lagoon. Benthic chlorophyll a concentrations were much larger than in the water column, representing around 99% of the total chlorophyll existent in the lagoon. Benthic microalgae play a relevant role in this system and therefore standard monitoring programs of the WFD, which do not consider this component, may fail to track nutrient-driven changes in primary producers. Dissolved oxygen concentration could be near critical levels during the summer (early in the morning), especially in the inner channels.