General Lake Chemistry-Part V

Biological activity peaks during the spring and summer when photosynthetic activity is driven by high solar radiation. The combination of thermal stratification and biological activity causes characteristic patterns in water chemistry. In the early spring and late fall, both oligotrophic and eutrophic lakes tend to have uniform, well-mixed conditions throughout the water column because conditions in each laryer diverge. The dissolved oxygen (DO) concentration in the epilimnion remains high throughout the summer because of photosynthesis and diffusion from the atmosphere. However, conditions in the hypolimnion vary with trophic status. In eutrophic (more productive) lakes, hypolimnetic DO declines during the summer because it is cut-off from all sources of oxygen, while organisms continue to respire and consume oxygen. The bottom layer of the lake and even the entire hypolimnion may eventually become anoxic, or devoid of oxygen.

Oligotrophic lakes are clear, deep and nutrient poor, and have sandy/gravel bottoms with non rooted vegetation. They are cold, plankton is scarce, and support cold water types of fish such as trout.

Eutrophic lakes are shallow and nutrient rich; may be murky, cloudy or turbid with clay bottoms; plankton growth present with rooted vegetation; water is warm and support sport fish such as crappie and bass.

In oligotrophic lakes, low algal biomass allows deeper light penetration and less decomposition. Algae are able to grow relatively deeper in the water colmun and less oxygen is consumed by decomposition. The DO concentrations may increase with depth below the thermocline where colder water is carring higher DO leftover from spring mixing. Remember that oxygen is more soluble in colder water. In extremely deep, unproductive lakes DO may persist at high concentrations near 100% throughout the water all year. These differences between eutrophic and oligotrophic lakes tend to disappear with fall turnover.

In the winter, oligotrophic lakes generall have uniform conditions. Ice-covered eutrophic lakes, however, may develop a winter stratification of dissolved oxgyen. If there is litle or now snow cover to block sunlight, phytoplankton and some macrophytes may continue to photosynthesize, resulting ina small increase in DO just below the ice. But as microorganisms continue to decompose material in the lower water column and in the sediments, they consume oxygen, and the DO is depleted. No oxygen input from the air occurs because of the ice cover, and, if snow covers the ice, it becomes too dark for photosynthesis. This condition can cause high fish mortality during the winter, known as the "winter kill." Low DO in the water overlying the sediments can exacerbate water quality deterioration, because when the DO level drops below 1 mg O2/L chemical processes at the sediment-water interface frequently cause release of phosphorus from the sediments into the water. When a lake mixes in the spring, this new phosphorus and ammonium that has built up in the bottom water fuels increased algal growth.