School of Engineering and Science - Jacobs University

CHAPTER IV
the same patterns in G. dominans in a three-species treatment as was found in the first
experiment. Bottles containing only S. trochoidea and G. dominans served as controls.
We also monitored the effect of the direct presence of the larger predator on swimming
behaviour and speed of S. trochoidea and G. dominans after exposure to F. ehrenbergii
for 24 hours. Fixation for cell counts with acid Lugol’s solution took place before and
immediately after the time of incubation and filming.
Counting
Samples were enumerated under a Zeiss Axiovert 135 inverted microscope using the
Utermöhl method (Utermöhl, 1958). The samples were settled overnight and 4-6
circular transects of each settling chamber were counted at 200 or 400-fold
magnification depending on cell concentration of S. trochoidea and G. dominans. For F.
ehrenbergii the whole chamber was enumerated. Apart from the first experiment G.
dominans cells containing fresh food vacuoles (indicated by the dark colour of the
vacuoles) were recorded separately and food vacuoles were used as a proxy for
ingestion.
Calculations
Growth and grazing parameters
Growth rates k [d -1 ] of F. ehrenbergii, G. dominans and S. trochoidea in the
experiments were calculated assuming exponential growth. Grazing rates g [d -1 ],
specific filtration rates F [mL µg predator -1 d -1 ] and specific ingestion rates I [prey cells
predator -1 d -1 ] of the predators were calculated according to Frost (1972) with the
modification of Heinbokel (1978a) for the growth of predators. Predator biomass
specific ingestion rates I b [µg prey µg predator -1 d -1 ] were calculated to allow for
comparisons between the treatments by normalization of the specific ingestion rates to
mean predator and prey biomass. Growth of phytoplankton is known to be density
dependent. The S. trochoidea growth control in the general interaction experiment
significantly exceeded the initial start concentrations after 24 and 48 hours of
incubation. In contrast, the initial concentrations of S. trochoidea in the grazing
treatments at these times were close to starting concentrations of the first day. We
therefore always used the growth rate of S. trochoidea determined after the first day for
our calculations in the general interaction experiment.
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