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The Ria Formosa is a lagoon on the South coast of the Algarve, Portugal and insulates SS km of the coastline from the full impact of the Atlantic. A review of the lagoon considers its geomorphology, sedimentology, hydrography, water quality and ecology. The Ria Formosa is identified as an important regional and national resource with respect to recreation, tourism, fishing, aquaculture, shipping and salt extraction. Ma*or regional developments in the form of spreading urbanisation and intensified agriculture, together with rising numbers in both the resident population and the annual tourist influx, pose an ever increasing stress upon the lagoon system, particularly with respect to water quality. The region contiguous to the lagoon is devoid of heavy industries and the review concludes that the greatest threat to sustained development of the lagoon resources is a progressive deterioration of water quality promoted by ever increasing discharges of nutrient and organic rich sewage effluent. In consequence, this study of the water quality in the Ria Formosa focuses upon the spatial and temporal variability in the nutrient and dissolved oxygen fields. Hydrographic, nutrient and dissolved oxygen data captured during three separate sampfing programmes are presented and discussed. The first programme addressed the variability in the lagoon as a whole. From June 1987 to May 1988 hydrographic, nutrient and dissolved oxygen data were collected at monthly intervals from sixteen sampling stations distributed through the lagoon. All sampling events were coincident with high and low water on neap tides. This sequence of surveys showed that during the winter the lagoon acts as a typical estuary but for several months during the hot, dry summer the salinity of the waters within the lagoon exceeds that of the seaward boundary. No evidence of either marked or persistent stratification was found. Despite strong tidal flushing, the waters within the lagoon are not homogenous and the characteristics of the incoming coastal water are significantly modified in the inner reaches. Principal Component Analysis divided the sixteen sampling stations into four groups: (i) stations in the outer lagoon where water characteristics are little modified by the inner lagoon processes and sources; (ii) intermediate stations with water characteristics dominated by the inflowing seawater 11 but somewhat modified by inner lagoon processes and/or sources; (iii) estuarine stations where the water properties are greatly influenced by freshwater inputs; (iv) inner lagoon stations insufficiently flushed by tidal exchange and partly influenced by inner lagoon processes and sources. Stations comprising the latter group (in the vicinity of the city of Faro, the West end and the East end of the lagoon) all exhibited characteristics symptomatic of a marked reduction in water quality. These were manifest to a greater extent at the station adjacent to Faro. A comparison of the range of nutrient concentrations in the Ria Formosa with other lagoon systems identifies it as being one of the most nutrient enriched. It is concluded, however, that good water quality prevails throughout most of the lagoon at all times of the year and that the presence of, or potential for, adverse water quality is localised in particular areas. The objective of the second sampling programme was to generate a temporal record of the extent to which the properties of the coastal waters entering the lagoon were modified in the western inner reaches of the lagoon. Samples were taken weekly on Springs and Neap tides throughout 1989 coincident with high water at the coastal station and at low water in the lagoon. The offshore water showed a strong seasonal signal with an abrupt change of water type in autumn. Throughout the year, the lagoon water was consistently and significantly enriched in silicate relative to the seawater, but also with respect to ammonium and nitrite in winter. In contrast there was no evidence of an internal source of nitrate. Almost half of the morning samples showed the lagoon water to be less than 80% saturated in dissolved oxygen on most occasions during the summer months. Hypersaturation in summer afternoon samples exceeded the range recommended by European Union directives for water quality. The third sampling programme addressed short term variability of water characteristics at the western end of the lagoon. Observations every thirty minutes during the Extreme Spring Tide cycle of the autumn equinox showed that water from other parts of the lagoon (sewage contaminated water and water flushed off salt marshes) was flushed past the station during a tidal cycle. Percentage saturations of dissolved oxygen reached an early morning minimum of 54%. The nutrient and III dissolved oxygen observations indicated that the water quality situation in the western part of the lagoon was precarious. A set of circumstances which would provoke mass mortality of the biota under the conditions at the time of the study is outlined. The conditions in the Ria Formosa were compared to other lagoons and it was concluded that the Ria Formosa is relatively nutrient rich.