A simplified method of phytoplankton counting

Abstract

The problems connected with the study of phytoplankton for species composition and volumetric determination of the biomass are discussed. A simplified method of the Utermöhl technique is reported, where a limited number of species is counted, covering about 90% or more of the total volume of phytoplankton. If the number of individuals of each species is restricted, for example to 60, the estimated error will be ±26%. The volumes of individual species are calculated and the mean sizes and mathematical formulae used in the calculations are given. The median volume is preferred to the arithmetical mean. A statistical evaluation was performed on material from Lake Mälaren, central Sweden, in May and August 1972 to estimate the error of phytoplankton counting where a limited number of species and at least 60 individuals of each species were counted. The results show that the smallest variations between the samples were found in Asterionella formosa and Chrysococcus minutus in May and in Aphanizomenon flos-aquae and Microcystis spp. in August. The largest variations between the samples were in the counts of Melosira spp. However, the previously calculated error of ±26% was valid for most of the species and all the total volumes. General conclusions are made on the utility of the counting method. The importance of information on individual species and on the composition of the phytoplankton flora is emphasized.