School of Engineering and Science - Jacobs University

DISCUSSION
to know who they are. Therefore, investigations on species composition and seasonality
as presented in this study are imperative for a basic understanding of processes within
the planktonic food web and should be extended especially on locations providing
excellent background data as the Helgoland time series.
Methods for grazing experiments with microzooplankton grazers
Grazing plays a major role in the fate of biomass in the oceans (Moline et al., 2008).
The measurement of grazing rates is thus crucial for the understanding of the flow of
matter in the marine pelagic food web. Since microzooplankton was recognized to be
one driving factor in the consumption of phytoplankton diverse techniques to measure
the grazing impact of microzooplankton have been developed and a multitude of
techniques for the investigation of protozoan feeding is available (Kivi & Setälä, 1995).
Direct methods measure food uptake in the predators, indirect methods measure the
disappearance of food from the environment (see introduction for a more detailed
description). Both types of methods mostly involve an artificial alteration of natural
food web relationships. One aim of this thesis was to find the most appropriate method
for the measurement of the grazing under such “natural” conditions.
Laboratory grazing experiments are restricted to simple model systems containing only
a few different organisms and are biased towards culturable species (Heinbokel, 1978a,
Jonsson, 1986, Hansen, 1992, Jeong et al., 2004). Alternative techniques dealing with
natural microzooplankton assemblages mostly make use of artificial food particles
(Heinbokel, 1978b, Kivi & Setälä, 1995) or extrapolate from laboratory-determined
feeding relationships to field situations (Heinbokel & Beers, 1979). Although important
for the understanding of basic mechanisms, e.g. functional and numerical responses of
predator species, all these different techniques manipulate natural conditions or neglect
food web interactions.
Microzooplankton dilution experiments provide us with an alternative to determine
grazing rates of microzooplankton by indirect, labour-intensive, and taxonomically
selective techniques (Landry & Hassett, 1982, Calbet & Landry, 2004). The dilution
technique also includes the simultaneous measurement of specific growth rates of the
phytoplankton along with the specific grazing rate of the microzooplankton. There are,
however, methodological restrictions which have to be taken into account (Gallegos,
1989, Landry et al., 1995, Dolan, 2000, Moigis, 2006, Teixeira & Figueiras, 2009). The
most crucial restrictions are related to the theoretical assumptions the dilution method is
based on (see introduction for detailed information). To overcome for, e.g., nutrient
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