SEASONAL PATTERNS AND LONG-TERM TRENDS IN AN ESTUARINE UPWELLING ECOSYSTEM (RIA DE VIGO, NW SPAIN)

Thermohaline properties, nutrient salts, chlorophyll a and meteorologi cal variables have been intensively monitored since February 1987 in t he Ria de Vigo (NW Spain), in order to examine the temporal variabilit y and the relationships between these variables over different time sc ales. In this paper, the seasonal and the long-term components of the 1987-92 time series are analysed. The seasonal changes in thermohaline properties are forced by meteorological factors, but whereas temperat ure shows a well-defined annual cycle, salinity presents a higher freq uency variation pattern due to the influence of upwelling-downwelling events and runoff. Nutrient concentrations change in a regular way thr ough the seasons, so that characteristic and well-defined cycles are o bserved, but they are different for each nutrient and, for a given nut rient, exhibit a marked contrast between surface and bottom layers. Th e seasonal changes of nutrients are not explainable by advection and w ater column processes alone; fractionation of nutrients during recycli ng and, presumably, sedimentary processes should also play an importan t role. The annual cycle of chlorophyll a shows a bimodal pattern, whi ch corresponds with the development of the spring and autumn blooms; e ven though the seasonal cycle accounts for an important amount of the observed temporal variability, variation at lower time scales is also important. Long-term trends, as a change in the mean level of the anal ysed time series, have been observed for most of the variables. Salini ty increased and temperature decreased both for surface and bottom ser ies. The largest trend, in terms of the percentage to the observed var iability it represents, was an increase in bottom salinity. In relatio n to nutrient salts, there was no evidence of increasing eutrophicatio n, although surface dissolved inorganic nitrogen, and surface and bott om phosphate increased slightly. Chlorophyll a concentration showed a decreasing trend, especially at the surface. The observed long-term tr ends could be attributed to changes of the meteorological factors that operate through an increase in the estuarine residual circulation. (C ) 1997 Academic Press Limited.