Yearto Year Variability in Lake Heating and Fish Habitat

The course of changes in temperature and stratification has been described many times for temperate lakes in the spring. The water temperature immediately beneath the ice is 0°C and warms to 3-4°C in the depths of the lake. When the ice leaves the lake, wind action circulates the entire water column before the surface waters warm and stratification begins again. During the circulation period, the lake inflow inflow

Subsurface Chlorophyll Maximum Layers

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Figure 5.8 Schematic diagrams of the flow path of stream inflows into Toolik Lake based on unpublished data of S. MacIntyre and G. Kling. (Top) In early July, there are low inflow rates of water less dense than upper waters of the lake; the water flows in as a subsurface intrusion. The stability of the stratification allows a deepwater layer of algae to develop (chlorophyll maximum). (Bottom) In mid-July, the first days of a spate show strong mixing of the stream waters with the lake water; the mixing is vigorous enough to destroy the chlorophyll-maximum layers in the basins close to the inflow point.

0 200 400 600 800 1000 1200 1400 1600 Cross Section Distance (m)

Figure 5.8 Schematic diagrams of the flow path of stream inflows into Toolik Lake based on unpublished data of S. MacIntyre and G. Kling. (Top) In early July, there are low inflow rates of water less dense than upper waters of the lake; the water flows in as a subsurface intrusion. The stability of the stratification allows a deepwater layer of algae to develop (chlorophyll maximum). (Bottom) In mid-July, the first days of a spate show strong mixing of the stream waters with the lake water; the mixing is vigorous enough to destroy the chlorophyll-maximum layers in the basins close to the inflow point.

waters absorb oxygen to replace the amount lost during winter respiration. The situation is often different in Toolik Lake. During some years there may be no circulation period after the ice leaves the lake; instead, the summer stratification begins immediately as a result of calm conditions and high amounts of solar radiation in the crucial few days after the ice leaves the lake. The ecological result is that the deep waters of the stratified lake begin the summer with reduced concentrations of oxygen, and the deepwater oxygen can be further reduced by the breakdown of plankton settling from the surface layers. Although the oxygen is eventually restored during the long-lasting fall circulation of the water column, the impact of lowered levels of oxygen can eventually reduce the habitat for fish. For example, lake trout, the dominant predator in lakes of the arctic foothills, require 3 mg O2

L-1, a level approached during a several-year experimental addition of low-level of nutrients to a lake next to Toolik Lake. In Hobbie et al. (1999), a model of lake circulation and temperature predicts that too low oxygen concentrations and too warm temperatures in the surface waters will drastically reduce lake trout habitat if a 4°C change in the annual air temperature occurs.

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