At least 400 to 450 diatom valves were counted in random transects in each sample and
identified at 1000x magnification, using a Leica DMRBE microscope. According to Lund
et al. (1958), statistical bias in the counts does not exceed 10 % in this range. If Si
is the
number of individuals of the ith taxon and Sn is the total number of individuals counted in
the sample, the abundance of each taxon Ni
is calculated as: