Sampling designs for the estimation of phytoplankton abundance in limnetic environments

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Abstract
Several of the problems of devising efficient practical sampling schemes for estimating average phytoplankton population densities are described. The paper presents a stratified random sampling scheme with two stages of sub-sampling for estimating the concentration of chlorophyll a and several planktonic alga species in two experimental enclosures in Blelham Tarn in the English Lake District. In the example, the algae were always heterogeneously distributed. Though stratification reduced the estimator variance by as much as 50% for some species and sampling dates; for some other occasions and species there was no improvement over simple random sampling. The total variation in density was broken down into the various components of variance, which showed that generally there was little variation due to sub-sample and measurement effects compared to the natural remaining density variation between the sample units within each strata. This suggested that future sampling schemes in these areas might be more efficient if more “field” sampling units were taken within each strata, without replication of measurements and perhaps sub-samples. The problems of deriving a suitable stratification of a planktonic environment and hence the usefulness of stratified random sampling are discussed in relation to the spatially and temporally heterogeneous nature of phytoplankton populations.

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