a1 Biologische Anstalt Helgoland, Alfred-Wegener-Institute for Polar and Marine Research, Marine Station, POB 180, 27483 Helgoland, Germany
a2 Alfred-Wegener-Institute for Polar and Marine Research, POB 12 01 61, 27515 Bremerhaven, Germany
Grazing experiments targeting the determination of in situ grazing rates are standard. In two separate experiments the effect of the frequently used siphon filling technique on the abundance of microzooplankton during the set-up of grazing experiments was investigated and compared to results from an alternative filling method. Hereby, water containing natural communities from Helgoland Roads, Germany (54°11.3′N 7°54.0′E), was transferred into incubation bottles using a funnel system (funnel-transfer technique (FTT)). The impact of pre-screening with a 200 µm net for excluding larger mesozooplankton grazers from the incubations was evaluated. Results show that the ciliate community was strongly affected by siphoning and pre-screening, leading to significant differences in abundance and Margalef diversity. The most affected ciliates were Lohmanniella oviformis and Myrionecta rubra, both important species in the North Sea. Dinoflagellates did not show any significant response to either siphoning or pre-screening with the exception of one athecate species. Such artificial bias in ciliate assemblages is very problematic for biodiversity consideration and grazing investigations. Simply changing the method of filling during the experimental set-up can ensure the measurement of accurate grazing rates of field abundances of microzooplankton. We thus recommend using conservative filling approaches like the FTT in experiments, especially when sensitive species are present, in order to avoid shifts in the overall microzooplankton community. Furthermore, we recommend introducing a control to evaluate the degree of changes in the target community due to the experimental set-up.
(Received June 25 2009)
(Accepted December 06 2009)
c1 Correspondence should be addressed to: M.G.J. Löder, Biologische Anstalt Helgoland, Alfred-Wegener-Institute for Polar and Marine Research, Marine Station, POB 180, 27483 Helgoland, Germany email: Martin.Loeder@awi.de