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Why does climate matter to aquatic ecosystems?

The movie above comes from data for a lake in northern Wisconsin. The thin black line is water temperature while the thick red line is the concentration of oxygen dissolved in the water. The surface of the lake is at the top of the graph. In spring, water temperature is the same from the top to the bottom of the lake. As air temperatures warm during summer, water temperatures at the surface also warm while temperatures near bottom remain cool. This creates a density gradient across the water that means that bottom waters do not mix with surface waters.

Because oxygenation of the water happens near the surface, any oxygen near the bottom that is used by microbes and other organisms is not easily replenished. As a result, you can see that oxygen near the bottom begins to decrease. In fact, it continues to decrease until cold air temperatures in fall cause the lake water temperatures to again be the same from top to bottom. The length of time during the summer that surface temperatures remain warm plays a large role in how depleted bottom oxygen will be before late fall mixing. This is one way that climate plays a role in the chemistry of lakes.

Dissolved oxygen is fundamental to aquatic ecosystems. Organisms like fish require oxygen to breathe and some, like brook trout, have high oxygen requirements. In addition, oxygen regulates many chemical reactions. This means that the concentration of dissolved oxygen has implications for things like nutrient concentrations, drinking water quality, algal blooms, toxicity of metals, and the cycling of carbon.

Many aquatic organisms have a well-defined range of thermal tolerances that they are adapted to, meaning that temperature itself is an important component of aquatic habitat. In streams and rivers, ecological communities are adapted to seasonal cycles of spring runoff, low water conditions, and rainy periods. Similarly, the timing and duration of ice cover and the progression from cool to warm surface water temperatures play an important role in the ecology of lakes.