Changes in buoyancy of a planktonic blue-green alga in response to light intensity

Abstract A comparison was made of three different methods for determining the proportion of filaments in a culture of Anabaena flos-aquae which sank or floated. The most satisfactory method was to leave an aliquot of the culture standing on a haemocytometer slide and then to count the number of filaments which floated up in contact with the coverslip and the number which sank on to the platform of the slide. The position of the filaments could be distinguished by the plane of focus. Only a short time was needed for filaments to float or sink to the top or bottom of the haemocytometer chamber and this minimized any buoyancy change during the assay. Roughly equal numbers of floating and sinking filaments occurred in cultures left at light intensities of 13 to 22 μE m-2 s-1. At higher intensities more of the filaments sank, and at lower intensities more floated. This buoyancy response was found to permit stratification of the alga in a experimental stabilized water column with a vertical light intensity gradient. The algal population formed a peak at a depth where the intensity was 1·0 klx, equivalent to 17 μE m-2 s-1.