Monitoring the yearly evolution of temperature stratification is a critical component in many lake management and research programs due to its pronounced effects on aquatic chemistry and biology. Fisheries management, hydroelectric plants, selective withdrawal dams, and numerous aspects of aquatic and sediment research often depend on having temperature profile data for a water body.
Lake stratification develops seasonally as increasing solar radiation in the spring and summer heats up surface waters. Heat is quickly dissipated in the upper layers of water, and deeper waters remain cool. Because warmer water is significantly less dense than colder water, temperature-dependent density differences develop that prevent mixing and result in the formation of isolated layers of water. Due to the absence of water circulation between the layers, each strata of water develops its own distinct chemistry.
Among the more common concerns is the depletion of oxygen in the deeper layers of stratified lakes. In a typical summer stratification pattern, dissolved oxygen declines sharply at the thermocline, or region of greatest temperature change. Below 7 m depth, dissolved oxygen is insufficient to support most aquatic life.
In the fall, the temperature and density gradient weakens as surface waters cool and sink. Mixing by wind and the sinking of cooler waters will eventually destroy the gradient, a process known as fall turnover. The resulting isothermal conditions restore water circulation and oxygen to the deeper layers of the lake.
Selective Withdrawal Dams
The release of overly warm surface water, or oxygen-depleted bottom water, at selective withdrawal dams could devastate downstream fish populations. At a number of reservoirs across Pennsylvania, the US Army Corps of Engineers Pittsburgh District obtains real-time, on-line temperature stratification data using NexSens TS110 temperature strings suspended from buoys. Every hour, temperature profile data is transmitted via radio telemetry to a host computer, which automatically posts the data to an online, password-protected website.
With the easy, on-line access provided by the NexSens system, dam operators and reservoir managers are able to selectively release water from different depths in order to maintain acceptable water quality conditions downstream of the reservoirs. Additionally, tracking stratification conditions throughout the year provides long-term data sets that aid in research efforts and the development of best management practices.
For more information on obtaining temperature profile data, please contact Fondriest Environmental at 888-426-2151 or visit www.Fondriest.com.
Air Quality