A stratified sampling approach to compensating for non-random sedimentation of phytoplankton cells in inverted microscope settling chambers

Abstract

A pronounced “edge effect” resulting in non-random cell sedimentation is documented for standard-design inverted microscope counting chambers. Comparison of mean cell densities of two green algae from random fields in central and peripheral regions of chambers substantiate a marked peripheral settling bias. The non-random settling pattern is independent of phytoplankton cell size and population density over the ranges encountered in this analysis. If this problem is not compensated for, the magnitude of the bias is such that large errors in estimates of mean cell density/chamber can result. A stratified sampling approach is developed to minimize the error resulting from non-random settling without increasing the amount of time spent enumerating cells. Counting of random fields rather than patterned transects is strongly recommended as the standard procedure in phytoplankton studies.