Dissolved Oxygen in Lake Conroe
This study was the result of many days spent on Lake Conroe watching my dad try to find fish to catch. We always wondered were the fish were in the lake. There are many factors that one has to look at, but it seems to come down to the oxygen since many organisms require that to exist and the cover that is available. My study was to determine what impacts the levels of oxygen in the lake and how a seasonal change would affect the level of oxygen. The change that I was trying to identify was the “mixing” of the three different layers of the lake. I began testing on the last hot day of the season.
Dissolved Oxygen in water is the level of free, non-compounded oxygen available in the water. It is measured in milligrams per liter of water. Non-compounded oxygen is oxygen not combined with any other molecules. Is as important to health of the lake as the water itself. The level of dissolved oxygen determines the health of the lake and if organisms can survive or thrive in the water. Fish require more oxygen than bottom feeders, crabs and oysters and worm. Plants will even require dissolved oxygen when there isn’t enough light for photosynthesis.
Dissolved oxygen comes from two main sources – the air and as a by-product from plants. The air will slowly diffuse oxygen into the water or rapidly by aeration – natural or man-made. Natural aeration of the water can be wind causing waves to form on the lake, rapids in a river or waterfalls. Manmade serration could be from a dam, boats, or aeration pumps. Dissolved oxygen is the by-product of photosynthesis from algae, seaweed, grass and other aquatic plants is. This reaction reaches its peak during the daylight hours because it requires sunlight for the reaction.
The levels of dissolved oxygen in lakes will vary depending on the time of day, the weather, the temperature, depth and pollution. Since photosynthesis requires sunlight and organisms use oxygen 24 hours a day, the lowest level of dissolved oxygen on any lake will be just before sunrise. Rain in the area and in the watershed of the lake will increase the level of dissolved oxygen through natural aeration. A windy day will create waves and increase the amount of dissolved oxygen. The water temperature will directly impact the amount of dissolved oxygen. Colder water can hold more oxygen than warm water. The level of dissolved oxygen will also vary depending on the water depth. Pollution can also cause a decrease in dissolved oxygen levels. Organic material (sewage, lawn clippings, soils from upstream and lake erosion) is introduced into the lake, it will require oxygen to help breakdown the material. If it grew in the lake naturally, some oxygen is produced and helps to balance the demand.
There is a separation of levels of dissolved oxygen called Stratification. This happens when the top section of the lake becomes warmer from the sunlight. This section is called the epilimnion. There can be the most amount of dissolved oxygen levels in this layer because most of the aeration occurs as well as algae and phytoplankton producing oxygen. The next layer is the metalimnion. This layer can vary in thickness and temperature. The change between these two layers is called the thermocline. This is the area that water temperature begins to fall. If this layer is within the sunlight’s reach, it will be where the most oxygen is in the water column. The bottom layer is called the hypolimnion. This layer is separated by a chemocline. Thais is the area where chemically the levels are significantly different. Lake Conroe happens to be a holomictic lake or a “mixing” lake meaning the water will “turnover” at least once a year, distributing the oxygen through all the depths of the water columns and starting the process of stratification over again.
To measure the dissolved oxygen level, you will need either a dissolved oxygen meter or a method to extract the water at varying depths without aerating the sample. For my study, I used an electronic meter from Milwaukee. It measures temperature and dissolved oxygen levels with a twelve foot cord. The testing site(s) should also be consistent because all the environmental factors will affect the levels as well and that needs to be a constant in this experiment.
It is important to measure and monitor the level of dissolved oxygen in the lake. It will provide a window into the overall health of the lake. For example, if the level is too low, many of the organism cannot survive and the would be large “fish kill” on the lake. Too high of oxygen levels can also cause disease in fish and invertebrates. The health of the lake is important to many industries and people. For example, Lake Conroe is part of the city of Houston’s drinking water. Texas Parks and Wildlife also has an interest in managing and maintain the fish population because that brings in millions of dollars. Tourism would be impacted with an unhealthy lake and values of the homes on and near the lake would be reduced as well.
1. Dissolved Oxygen. (n.d.). Retrieved from https://www.fondriest.com/environmental-measurements/parameters/water-quality/dissolved-oxygen/#14
2. Flugrath, M. W., Andrews, F. L., ; McPherson, E. (1994, January 01). Water-quality of Lake Conroe on the west fork San Jacinto River, southeastern Texas. Retrieved from https://pubs.er.usgs.gov/publication/wri854301
3. Home. (n.d.). Retrieved from http://www.miseagrant.umich.edu/lessons/lessons/by-broad-concept/physical-science/dissolved-oxygen-and-lake-stratification/
4. Perlman, H., ; USGS. (n.d.). Water properties: Dissolved oxygen. Retrieved from https://water.usgs.gov/edu/dissolvedoxygen.html
5. I. SALMONID WATERS. (n.d.). Retrieved from http://www.lakeaccess.org/russ/oxygen.htm
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