Can multi-frequency acoustics improve the monitoring of large zooplankton in large temperate lakes?

Abstract

Large zooplankton has an important role for the functioning of the ecosystem in many lakes. Most are predatory, which makes them both competitors as well as food for many planktivorous fish species. In general, it has been elusive to measure the abundance of this group of zooplankton with traditional sampling methods, particularly in large volumes of water. In this study we tested the potential and adequacy of multi-frequency hydroacoustics as a method to map the spatial patterns in abundance of the large zooplankton in Lake Vänern, Lake Vättern and Lake Mälaren. In addition, we used a plankton net to groundtruth biovolume estimates vertically, and a Tucker trawl for the horizontal distribution. Two frequencies were used to separate the acoustic backscattering from fish, mysids and plankton. The main target species were Leptodora kindtii, Bythotrephes longimanus and Limnocalanus macrurus.

The plankton communities were very heterogeneous both within and among the lakes. Lake Vänern and Lake Mälaren were dominated by Leptodora, whereas Lake Vättern was dominated by Limnocalanus. Bottom depth at the sample site was the most important community structuring factor.

Our results indicate that the biomass of large predatory zooplankton is comparably high and in most surveyed areas even higher than the biomass of planktivorous fish. Consequently, they are more important for the dynamics of lake food webs than previously assumed. Using multi-frequency hydroacoustics show promising results and with some alterations to the sampling design this would be a valuable addition to the traditional zooplankton monitoring in large lakes.

Keywords:

• Bythotrephes
• Leptodora
• Limnocalanus
• hydroacoustics
• multi-frequency

Acknowledgments

We acknowledge the excellent working conditions on R/V Asterix and the help with counting mysids and additional sampling from Björn Kinsten. We are also grateful for valuable assistance with the hydroacoustics analysis from Professor Lars Rudstam.