Convection in ice-covered lakes: effects on algal suspension

Convection occurs in ice-covered lakes if solar radiation warms near-surface water from the freezing point towards the temperature of maximal density. One effect of convective mixing may be to suspend non-motile phytoplankton in the upper water column, providing cells with enough light for growth during ice-covered periods. Observations of the diatom Aulacoseira baicalensis under the ice cover of Lake Baikal, Siberia, support the hypothesis that convective mixing causes net suspension of cells. This paper presents a theoretical examination of the conditions under which convective how fields can suspend algae in the photic zone of the upper water column. It is shown that the efficiency of algal suspension depends on the ratio of the still-water algal sinking rate, W-p, to convective updraft speed, W-u. The suspension efficiency is also shown to be affected by asymmetries in the flow field and night-time cessation of convection, but only if W-p and W-u are comparable in value. It is concluded that convection in Lake Baikal should be vigorous enough to increase the mixed-layer residence time of A.baicalensis from a few days to over a month, at least during years with thin snow cover.