PHYSICAL LIMNOLOGICAL PROCESSES UNDER-ICE

Penetration of solar radiation through ice and snow covering northern lakes produces a gravity current between regions with varying depths. This baroclinic current is a dominant physical process in winter becau se ice cover insulates lakes from the usual turbulence sources such as breaking surface waves and near-surface shear produced by the wind. T he current forms a directed circulation from the littoral zone to the centre of the lake that is an important distribution mechanism for nut rients and other chemical and biological constituents. Heat transporte d by the current degrades the ice cover and makes surface travel hazar dous. The thinning of the ice cover is most severe at the inlet to iso lated bays with mean depths that differ significantly from the lake. A t the mouth of a bay, the gravity current takes the form of a two-laye r flow with inflow to the bay occurring near the surface. The lower la yer has the largest temperature gradients and is dominated by a succes sion of progressive internal bores which decrease in amplitude overnig ht and with increasing cloud cover. The repetition of the bores occurs very close to the period of the uninodal barotropic seiche which sugg ests that the internal bores are forced by the surface seiche.