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Other publication, 2012

Kvantifiering av stora djurplankton i de stora sjöarna

Ragnarsson Stabo, Henrik; Axenrot, Thomas; Sandström, Alfred; Vrede, Tobias


Large zooplankton (>2 mm) are important for the functioning of the ecosystem in many large lakes. In several cases they have severely altered the food web where they have invaded. Most are predatory, which makes them both direct competitors to, and the dominant food of, many planktivorous fish species. In addition, juveniles of several predatory species, such as zander (Sander lucioperca) have been reported to utilize them extensively. In this case study we used different methods to map the spatial patterns in abundance of the large zooplankton in Lake Vänern, Lake Vättern and Lake Mälaren. The cladocerans Leptodora kindtii and Bythotrephes longimanus are predators whereas the copepod Limnocalanus macrurus is an omnivore. Generally, quantitative measurements of large zooplankton in large volumes of water are difficult to obtain with traditional sampling methods. In this case study we sampled plankton with a large plankton net - to obtain biovolume estimates horizontally, and with a tucker trawl for the vertical distribution[TA1]. A total of 58 samples were collected. In addition, multi-frequency hydroacoustics was tested as a monitoring method, with the tucker trawls as ground truth. Two frequencies were used to separate the acoustic backscattering from fish, mysids and plankton. The plankton communities were very heterogenous both within and among the lakes. Both Lake Vänern and Lake Mälaren were dominated by Leptodora, whereas Lake Vättern was dominated by Limnocalanus. The depth at the sample site was most important for the structure of the community, with Limnocalanus predominantly present below the thermocline. The hydroacoustics showed some potential for sampling the species that do not share habitat with mysids. The high densities of predatory zooplankton found in all lakes indicate that they have the potential of being important prey species for fish as well as competitors, and thus any ecosystem models in these lakes should include them. Our results indicate that large predatory zooplankton are found in the same amount (biomass) as planktivorous fish. Consequently, they are important in the food web of all three lakes. Hydroacoustic monitoring of some of the species show promising results and with some alterations in the sampling methods this could be a useful addition to the regular zooplankton monitoring of the large lakes.

Published in

Rapport - Vänerns vattenvårdsförbund
2012, number: 115/70, pages: 89-106
Book title: Glacialrelikter och makrozooplankton i Vänern och Vättern 2011
Publisher: Vätternvårdsförbundet/Vänerns vattenvårdsförbund