Seasonal Patterns and Long-term Trends in an Estuarine Upwelling Ecosystem (Ría de Vigo, NW Spain)

Thermohaline properties, nutrient salts, chlorophyllaand meteorological variables have been intensively monitored since February 1987 in the Ría de Vigo (NW Spain), in order to examine the temporal variability and the relationships between these variables over different time scales. 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 temperature shows a well-defined annual cycle, salinity presents a higher frequency variation pattern due to the influence of upwelling–downwelling events and runoff. Nutrient concentrations change in a regular way through the seasons, so that characteristic and well-defined cycles are observed, but they are different for each nutrient and, for a given nutrient, exhibit a marked contrast between surface and bottom layers. The seasonal changes of nutrients are not explainable by advection and water column processes alone; fractionation of nutrients during recycling and, presumably, sedimentary processes should also play an important role. The annual cycle of chlorophyllashows a bimodal pattern, which corresponds with the development of the spring and autumn blooms; even 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 analysed time series, have been observed for most of the variables. Salinity increased and temperature decreased both for surface and bottom series. The largest trend, in terms of the percentage to the observed variability it represents, was an increase in bottom salinity. In relation to nutrient salts, there was no evidence of increasing eutrophication, although surface dissolved inorganic nitrogen, and surface and bottom phosphate increased slightly. Chlorophyllaconcentration showed a decreasing trend, especially at the surface. The observed long-term trends could be attributed to changes of the meteorological factors that operate through an increase in the estuarine residual circulation.