Are there more large zooplankton out there?

Posted on July 20, 2015

Species of animal plankton often have very patchy distributions and this makes it difficult to assess their abundance accurately using traditional net-based sampling techniques. Recent advances in echosounding technology, in particular the use of multiple frequencies to separate the acoustic reflections of invertebrates and fish, offer the opportunity to scan sizeable volumes of water for large zooplankton quickly, thus improving sampling efficiency. To explore this technique, researchers surveyed three large Swedish lakes using two echo sounders operating simultaneously at frequencies of 38 and 120 kHz. They also used plankton nets and small trawls, both with a mesh size of 500 µm, to groundtruth the acoustic abundance estimates. Three species of large zooplankton (the water fleas Bythotrephes longimanus and Leptodora kindti and the copepod Limnocalanus macrurus) dominated the catches. Calculations based on the acoustic data gave density estimates over four times higher than the those derived from conventional plankton sampling carried out at the same time of year (August-September) during routine monitoring operations. This result suggested that the abundance of large zooplankton had previously been underestimated, with significant implications for our understanding of aquatic food webs. A sizeable biomass of large predatory zooplankton effectively constitutes an additional trophic level, the existence of which could prompt a re-evaluation of key processes such as energy transfer and trophic cascades. The researchers recommended that future acoustic surveys be carried out night, when there is a clear separation in the depths occupied by large zooplankton and other invertebrates such as mysid shrimp and chaoborid insect larvae.

Reference: Ragnarsson-Stabo, H. et al. 2014. Can multi-frequency acoustics improve the monitoring of large zooplankton in large temperate lakes? Aquatic Ecosystem Health & Management 17(4), 374–381.