Within Season Variability in Stream Flow and Lake Ecosystems

Recent studies have shown that the mixing and stratification of Toolik Lake may be greatly changed when one or more high-discharge spates in the inlet stream occur during the summer; this has important consequences for algal primary productivity. The incoming stream water may be differentiated from the lake water through small differences in temperature and conductivity.

One example of the impact of a spate occurred in mid-July 1999. Prior to the spate (figure 5.8A), the subbasins of Toolik Lake were strongly stratified, with the surface water at 17°C and the hypolimnion at 4°C. Much of the primary productivity occurred in the deep water as evidenced by a chlorophyll maximum at 7 m and below. Algal counts showed that the chlorophyll reflected an increase in abundance of the same algal species, small flagellates such as cryptophytes, found throughout the surface layers. When the spate occurred and flow rates exceeded 12 m3/s (figure 5.8B), the two subbasins nearest to the inflow point quickly mixed with the inflow water at 11°C. The chlorophyll maximum was dispersed, and the chlorophyll content of the upper waters increased. Primary productivity of much of the lake increased. The most likely explanation for the increase in productivity, based on detailed measurements of NH4+, of the ratio of C:N in particulate matter, and of the nutrient limitation of algal photosynthesis, is that nutrient-replete algae from the chlorophyll-maximum region of the lake became mixed throughout the upper layers and were able to obtain sufficient light to make use of excess nutrients they had stored.

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