Seasonality of phytoplankton in relation to silicon cycling and interstitial water circulation in large, shallow lakes of central Canada
The main basins of Lake Winnipeg (52°N 97°E) and Southern Indian Lake (57°N 99°W) had similar phytoplankton cycles during their open water seasons. A brief spring algal maximum was followed by an early summer minimum and, subsequently, an extended autumnal increase when highest biomasses were observed. The maxima were dominated by Melosira spp. The seasonal cycle of Melosira followed closely the seasonal cycle of dissolved Si. These basins exhibited a typical phytoplankton cycle for dimictic lakes even though they did not form a significant thermocline (1°C per meter).
The lakes were well-mixed because they were shallow and had large wind fetches. Although thermal stability of the water column was always low, it was positive until maximum heat content was achieved at which time it became nil or negative. These lakes heated and cooled rapidly, and sediment heat storage was a substantial fraction of the total heat budget. Because heating and cooling of water and of sidements were out of phase, heat exchange at the sidement surface could control vertical circulation of interstitial water, nutrient exchange across the sidement-water interface and the seasonality of phutoplankton. Thermal gradients in the sediments during the heating season would be quite pronounced (4°C per meter).
It is proposed that positive stability in interstitial waters during the heating season would impose molecular diffusive transport on the sidement column. When the lakes begin to cool, the upper interstitial water column would become thermally unstable and circulation would occur within the sidements. This would result in the observed net flux of disolved Si. and other nutrients, out of the sidements into the overlying waters. As a consequence, in Lake Winnipeg and Southern Indian Lake the highest phytoplankton biomasses and productivity occured in the late summer and autumn.
KeywordsWater Column Seasonal Cycle Shallow Lake Overlie Water Interstitial Water
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