In the five years to 1978 hydrographic surveys were conducted in Sullom Voe, Shetland and these have been supplemented since 1976 by current measurements. Conditions varied from year to year mainly due to climatic factors, the most marked changes occurring at the head of the voe where varying degrees of stagnation were encountered.

In summer the voe is weakly stratified. In winter longitudinal gradients are encountered mainly because of a slight reduction in salinity towards the head. Because of this the surface waters of the voe are more readily flushed in summer than in winter when the flushing time, calculated from the freshwater balance, is at least 15 days. The current measurements show that rapid renewal of the surface waters occurs in the summer during storms. Because the voe is open to the north, southerly winds, blowing surface waters out of the voe, drive a much stronger circulation than winds from the north and this is the main reason for the intermittent breakdown of stagnation in the upper basin. Tidal currents are very weak (<7 cm/s) but occasionally seiches, generated by sudden changes in the wind direction, with a period of about 1 hour and current amplitude of 15 cm/s are encountered. Nutrients from the adjacent tidally mixed Yell Sound are intermittently supplied to the voe by these wind-driven processes and as a result, high levels of productivity are encountered at intervals throughout the summer.