The Nutrient Status of the Clyde Sea in Winter

Abstract

Nitrate and silicate were measured at 47 stations in the Clyde Sea and adjacent sea-lochs during November 1990 when phytoplankton uptake of nutrients was minimal. Nutrient-salinity mixing lines were plotted between identified estuarine and marine sources. Any remineralization should show as a positive anomaly within the timescale (several months) of the Clyde Sea water circulation. Excluding data for Loch Striven, nitrate showed little anomaly while silicate showed anomalies of up to 5 μM, increasing with depth and distance towards the northern end of lower Loch Fyne. It is likely that remineralization of organic nitrogen from autumn bloom material occurred rapidly after deposition with subsequent dispersal of nitrate to coastal waters. In contrast dissolution of particulate silicate was slower, producing anomalies seen in November. Deep water in Loch Striven, isolated from late summer onwards, retained both nutrients. A comparison of the nitrate-salinity relationship for November 1990 with a hypothetical estimate of the prehistoric relationship showed that Clyde Sea waters of less than 33·2 salinity could suffer from hypernutrification.