Echoes received from the depth of the thermocline in Windermere, the North Sea, the Barents Sea and off Spitsbergen have been associated with aggregations of plankton organisms: zooplankton at a density of 25/litre in Windermere and phytoplankton at a density of 100,000 cells/litre (Thalassiosira gravida and Chaetoceros sp.) in the Barents Sea.
Cushing, D. H., A. J. Lee, and I. D. Richardson. 1956. "Echo traces associated with thermoclines." Journal of Marine Research 15, (1). https://elischolar.library.yale.edu/journal_of_marine_research/860