Water column structure and summer phytoplankton distribution in the Sound of Jura, Scotland

The distributions of water column stratification, phytoplankton and nutrients were investigated in the Sound of Jura, Scotland, during July 1982 and June-July 1983. The amount of stratification was mainly related to variation in tidal currents and bathymetry along the length of the Sound. Frontal zones separated the well-mixed northern Sound, and lateral regions, from thermally stratified water in the middle of the Sound. In the south transitionally stratified water extended into the North Channel of the Irish Sea. The phytoplankton in well-mixed regions was dominated by diatoms, whereas naked dinoflagellates and microflagellates were important in stratified regions. The distribution of phytoplankton biomass, indicated by chlorophyll concentration averaged over the water column, is explained by the effects of vertical mixing on the average illumination experienced by phytoplankton and by the supply of nutrients to the euphotic zone. Highest mean water column chlorophyll was found near Islay, perhaps due to special local input of nutrients into the euphotic zone there. Biomass was also high in the southern region of transitional stratification. The northern and lateral fronts were not clearly associated with high phytoplankton biomass, perhaps because of the small size of the features in relation to the advective field. Phytoplankton biomass was generally low in well-mixed waters, and at the surface in well-stratified regions. It is suggested that transitional stratification may enhance productivity in the southern region to two to three times that in the well stratified and mixed regions of the Sound. The variations in stratification along the Sound may have important implications for primary productivity in the lochs that exchange with it.