Measuring the distribution, abundance, and biovolume of zooplankton in an oligotrophic freshwater lake with a 710 kHz scientific echosounder

Acoustic surveys of the distribution and abundance of freshwater zooplankton were conducted in Lake Giles, an oligotrophic freshwater lake. Volume backscatter data from a 710 kHz scientific echosounder were converted to high‐resolution spatial and temporal numerical density estimates of small zooplankton. Vertical net tows of a 153 μm mesh closing bongo net at multiple depth intervals provided both identification of the types and sizes (0.5–1.5 mm length) of crustacean zooplankton present in the lake as well as an independent measurement of zooplankton numerical density. Net and acoustic estimates of zooplankton abundance, biovolume, and distribution were very similar. The improved resolution of the high‐frequency acoustic sampling provides insight into several aspects of freshwater zooplankton ecology including: separation of migrating and non‐migrating zooplankton, high resolution measurements of in situ zooplankton biovolume, calculation of in situ vertical velocities of migrating zooplankton, and fine‐scale (sub‐meter) horizontal and vertical zooplankton distribution during daytime, nighttime, and vertical migration events. These methods allow for more detailed and accurate estimates of zooplankton distribution than traditional net sampling methods can provide, including determining the total abundance of organisms within a specific habitat. They also provide higher resolution data in both space and time of smaller zooplankton taxa than have been measured previously in freshwater ecosystems.