relative to atmospheric N2 (15N) and VPDB (13C — Vienna Peedee belemnite).
The analytical precision for both stable isotope ratios was 0.2‰.
Carbon and nitrogen contents were calculated by the percentages of the
initial dry masses in relation to the total. Then, considering the split
part and the amount of water filtered, the carbon and nitrogen concentrations
were given in mgm−3. C/N ratios were calculated on amolar basis.
Samples were not generally acidified prior to combustion in order to remove
inorganic carbon. An experiment using 45 samples selected by
chance served as a data base to estimate the effect. The average of non
acidified samples amounted to 25.1 ± 6.9% as compared to 22.4 ± 7.1%
after acidifying them. The difference amounted to a maximum of 7.5%,
but was not significant (p N 0.05). None of the zooplankton samples contaminatedwith
phytoplanktonwere considered for isotope characterization.
In addition, biomass in terms of protein content was determined
from the samples kept for enzyme activities by using the method of
Lowry et al. (1951) modified by Rutter (1967).