Sinking movements of phytoplankton indicated by a simple trapping method

Abstract

Small traps designed to intercept movements of freshwater phytoplankton in situ were suspended at different levels in a stratified lake. Recoveries of a single non-motile species, Fragilaria crotonensis, are analysed to assess trap-performance. Significant correlations are demonstrated between the numbers of alga trapped and the size of the standing population, and between the fraction of the population trapped and estimates of epilimnetic eddy diffusion coefficients, from which it is deduced that the traps performed most efficiently under conditions of stable stratification. It is also shown that Fragilaria probably sinks throughout most of the season at rates not dissimilar from those obtained in vitro.