School of Engineering and Science - Jacobs University

CHAPTER III
prior to the experiments. The 100% undiluted bottles in the dilution series served as a
control for the T. longicornis grazing experiments.
The whole set of incubation bottles (dilutions series + T. longicornis bottles = 48
bottles) was incubated for 24 hours on two plankton wheels (0.8 rpm) at the same light
and temperature conditions as the mesocosms. Sampling for plankton took place at the
beginning of the experiments and after 24 hours.
Biovolume and carbon calculation
Biovolume of each plankton species was calculated from the measurement of cell
dimensions using geometrical formula according to Hillebrand et al. (1999). The cell
volume was converted into carbon (C) according to the equations given by Menden-
Deuer and Lessard (2000) for diatoms (pgC cell -1 = 0.288 x V 0.811 ), dinoflagellates (pgC
cell -1 = 0.760 x V 0.819 ) and all other protist plankton except ciliates (pgC cell -1 = 0.216 x
V 0.939 ), whereby V refers to cell volume in µm³. Ciliate carbon was calculated using a
conversion factor of 0.19 pgC µm -3 (Putt & Stoecker, 1989). Rotifer carbon was
estimated according to McCauley (1984) and Park and Marshall (2000): After a
calculation of the biovolume by means of geometric formulas this biovolume was
converted to wet weight assuming a specific gravity of 1. Wet weight was then
converted to dry weight by a factor of 0.1 and 50% of dry weight was assumed to be
carbon. Carbon values for the copepod species T. longicornis derived from
measurements with an elemental analyser (EA 1110 CHNS-O, Thermo-Finnigan). The
mean carbon content (annual mean 2007, n = 45) of this copepod was 9.5 µg carbon
female -1 (K. L. Schoo, unpublished).
Growth and grazing calculation – Microzooplankton
Growth rates of phytoplankton species and grazing rates of the microzooplankton
community were calculated using linear regressions of apparent phytoplankton growth
(calculated for the total phytoplankton community, at a species level as well as
functional phytoplankton groups) against the dilution factor (Landry & Hassett, 1982,
Landry, 1993). Start values for the diluted samples were calculated from the 100%
undiluted samples according to their dilution factor. The growth of phytoplankton (d -1 )
was described by the exponential growth model in equation (1):
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