Bottom Water Stagnation and Oxygen Depletion in a Scottish Sea Loch

From March 1992 to February 1993, bimonthly hydrochemical surveys were conducted in Loch Ailort, a fjordic sea loch on the west coast of Scotland. Salinity and temperature profiles along the loch were obtained using a CTD. Water samples were also collected and subsequently analysed for dissolved nutrients and oxygen levels. The surveys revealed typical fjordic stratification, consisting of a brackish surface layer, an intermediate layer and a stagnant bottom layer. High levels of dissolved nutrients and low oxygen concentrations were observed regularly in the bottom water of the loch. Three current meters, each measuring velocity, temperature and salinity, were deployed in the basin of the loch from July 1992 to February 1993. The observations of current confirmed the weakness of the flow in the deep water, and indicated that the bottom waters of Loch Ailort were renewed, on average, once every 8 weeks. The oxygen uptake by the sediments was estimated at 36 mmol−1 m−2 day−1, indicating that the water below 30 m would become anoxic in approximately 9•5 weeks. The variability in the length of the interval between flushing events suggests that the deepest water in Loch Ailort may occasionally reach anoxia, albeit for limited periods only. The surveys described here, together with a complementary benthic survey, suggest that the sediments throughout much of the loch may contain high levels of organic carbon. A simple carbon budget suggested that fish farm food may contribute up to 50% of the sedimentary organic carbon.