Buoyancy regulation byMicrocystisin Lake Okaro

Abstract

In Lake Okaro (see Vincent et al.: p. 509 this issue) for site description), a species of Microcystis produced large spherical colonies which floated rapidly when buoyant and sank rapidly when not. This organism exhibited a very clear-cut buoyancy change in response to light intensity, which would provide it with the ability to make vertical migrations over considerable depths, even when some degree of mixing occurs. The buoyancy and distribution of Microcystis colonies were investigated at Lake Okaro on 2627 February and 7-10 March 1987. Surface concentrations were estimated by counting the colonies in seven 3-litre dip samples (Walsby et al. 1983) at up to 14 stations over the lake. Individual colonies, of average diameter 3 mm, were collected one by one with a 5.5 mm ID glass tube fitted with a rubber teat. For in situ incubation experiments, 50 to 60 colonies were suspended in 2 litres of tissue-filtered lake water in a clear polycarbonate bottle closed with a screw cap and suspended at the lake surface in the south-west corner of the lake. Sinking velocities of the colonies were measured in a 100 ml graduated measuring cylinder filled to the top, about 60 mm above the top graduation mark. The cylinder was placed in a jar of water and the jar was stood in a tray of ice; this gave a temperature gradient in the jar and cylinder, which minimised convective mixing. The colony was released at the water surface and the times were recorded as the colony passed the 100, 70, 40, and 10 ml graduation marks, which were each 47 mm apart. Floating colonies were timed as they rose