Identification of lacustrine dissolved oxygen depletion by acoustic remote sensing

Bathymetric surveys of the Lake of Menteith, Scotland, have repeatedly revealed the presence of a second echo from within the water column of its deepest basin. The intra-water column echo, which had its upper surface at a depth of 15 m, was detected only during summer thermal stratification using a 200 kHz echo-sounder. A 150 kHz acoustic source did not, however, detect a reflection at this depth. The upper surface of this zone of reflectivity has been found to correlate with the level at which the dissolved oxygen saturation falls to less than 1% and it is believed to be due to the release of gases into the water column from the bottom sediments by anaerobic decay of organic matter. It is suggested that acoustic remote sensing using a high frequency source may have wider application as a means of indirectly assessing the areal distribution of dissolved oxygen depletion in lentic environments.