School of Engineering and Science - Jacobs University

CHAPTER III
therefore, instead of µ 0 , values of microzooplankton growth rate k obtained from the
100% undiluted seawater incubation bottles were taken to calculate P p (equation 7).
Sometimes negative P i and P p values were found in our grazing experiments. These
resulted from either negative g (for P i ) or µ 0 /k (mortality in the control without grazer)
(for P p ) and were set to zero. The same was done for positive P p values resulting from
negative g and µ 0 .
Selectivity and Electivity
Prey selectivity α of the microzooplankton community and T. longicornis was
calculated for each prey type according to Chesson (1978, 1983):
i
i
m
r / n
j1
r
j
i
/ n
j
(10)
Whereby r i is the frequency of prey i in the diet and n i is the frequency of prey in the
environment, divided by the sum of all relationships between the frequency of prey in
the diet and in the environment. Negative T. longicornis ingestion rates were set to zero
for the calculation of the frequency of prey in the diet according to Nejstgaard (2001).
We chose Chesson’s case 1 equation (n i assumed to be constant) (Chesson, 1983)
because our values of ingestion and percentage of prey in the environment were
obtained by averaged prey concentrations and phytoplankton initial stocks were high, so
that a strong depletion of food was unlikely.
Values of α were used to calculate the electivity index E* according to Vanderploeg and
Scavia (1979a, 1979b).
1
i
E * n
1
i
n
(11)
(n = total number of prey types)
Values of E* cover a range from -1 to 1. E* values of 0 indicate non selective feeding,
values > 0 indicate preference, values < 0 indicate discrimination against a prey type.
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