Interannual variability of circulation under spring ice in a boreal lake

Abstract

A small range (, 1uC) of under-ice water temperature is shown to result in remarkably different circulation regimes under spring ice in a deep, oligotrophic boreal lake. With the water column at , 4uC, melting of snow led to deepening vertical convection before ice break and a final depth of convection inversely correlated with earlier deep-water temperature. We attribute that to the nonlinear dependence of water density on temperature, albeit further affected by stochastic weather factors. In four of nine study years, convection led to complete under-ice overturn of the lake, indicating that this may not be uncommon in similar lakes with steep topography. River inflow and more intensive warming of water in the littoral zone also created a horizontal density differential, convection that involved flow down the sloping bottom and a lateral intrusion of this sinking water at a depth between the vertical convection and the quiescent deep-water layers. The vertical and horizontal convection together produced a profile of temperature slightly increasing from the surface to the bottom of the convection layer. The contribution of horizontal convection to under-ice mixing was interannually variable, and in one of the study years it eventually dominated under-ice mixing. A thermal bar circulation regime developed occasionally and only in the open water between ice and shoreline. We identified five different under-ice mixing regimes that form an interannually variable continuum of behavior during the ice melting period. The dependence on a narrow temperature range likely makes the circulation regime sensitive to a warming climate.

Regular build-up of the spring phytoplankton maximum before ice-break in a boreal lake

“…In all study years, temperature increased in deep water before it was reached by vertical convection (Fig. ), indicating horizontal convection (Salonen et al ). This was most distinct in the mixing year 2012 (Fig.…”

“…The similarity of the species composition in the intrusion layer with that in the upper layer suggests that warmer water originated from shallow areas rather than from inflowing waters. Interannual variation in the amount of snow and ice and its differential melting above the deep offshore and shallow littoral areas affect the relative significance of horizontal convection currents (Salonen et al ) and hence, likely contribute to the differences in diatom biomass in the middle of the basin. The basin scale of horizontal convection tends to keep phytoplankton distribution more uniform and might also partly explain why the increase of phytoplankton biomass was not slowed during deepening convection.…”

An Investigation of Ice Surface Albedo and Its Influence on the High-Altitude Lakes of the Tibetan Plateau

“…As a result, it can significantly affect the timing of ice break-up in models [52][53][54][55]. Salonen et al [56] found that the changes in solar radiation input can significantly affect the melting date of lake ice in the last two weeks of the frozen period. Van Cleave et al [57] found that a drop in the ice duration can increase the water temperature in summer and the evaporation rates of July-August.…”

Convective mixing and high littoral production established systematic errors in the diel oxygen curves of a shallow, eutrophic lake

“…). Such convective forces may produce both horizontal and vertical penetrative currents, and are usually associated with the summer and winter months (Forrest et al ; Salonen et al ). Summertime littoral‐to‐offshore flow rates have been measured from 4 m h −1 to 20 m h −1 (James and Barko ), while littoral flushing periods have been estimated of 1–4 h, varying with macrophyte cover (Oldham and Sturman ).…”