The Importance of Measuring Dissolved Oxygen and Temperature in Lakes
Oxygen is essential to nearly all organisms on earth including life in lakes. Given that fish breathe oxygen dissolved in water (referred to as dissolved oxygen or DO), the sport fisheries and healthy aquatic ecosystems that many of us enjoy, are absolutely dependent on water having an adequate supply of DO. Its concentration also has a great effect on the biochemical nature of a lake; long-term changes in oxygen concentration, especially at the lake bottom, can markedly affect nutrient availability and alter the productivity of the entire lake. As such, oxygen is one of the most important resources influencing lake management and its measurement should be part of any lake sampling program.
What is Dissolved Oxygen?
Dissolved Oxygen (DO) is the amount of O2 molecules that are dissolved into the water. The amount of O2 that is dissolved in a lake depends on several factors including the mixing that occurs during the overturn of a lake, the amount of wind action a lake experiences, the temperature of the lake, the number of plants that photosynthesize in the lake, and daily fluctuation patterns.
The transfer of oxygen from the air to the lake is facilitated by increasing the surface area exposed to the atmosphere. The surface area of a water body in contact with the atmosphere is increased by wind-driven waves and ripples, as well as by forcing water into droplets by splashing over obstacles or forcing through a fountain.
While O2 transfer to a water body is primarily through the atmosphere, photosynthesis by aquatic plants also plays a role in the amount of DO in a lake. The net effect on DO concentration is usually very small or neutral since comparable amounts of DO are consumed by these same organisms at night through respiration, when photosynthesis is not actively occurring. In addition to the respiration needs of photosynthetic organisms during darkness, oxygen within the system is also consumed through aerobic respiration by other organisms including: aquatic vertebrates and invertebrates, and bacterial and fungal communities involved in degrading dead plants and animals.
How do DO and temperature change throughout a lake?
Oxygen concentrations and cycling are mainly dependent on the lake thermal stratification patterns which are seasonally variable. Thermal stratification is a physical response to the effect of solar heating to water’s properties of temperature and density. Generally, water density decreases as temperature increases. Warm water is less dense than cold water so it will be found towards the surface and cold water will be found near the bottom of the lake. If this held true for the winter, ice would form on the bottom of lakes. Ice, frozen water, floats due to the expanded volume of water molecules when in the solid, crystal phase. Because water expands as it freezes, ice is slightly less dense and a bit lighter than water. Therefore, water is most dense at 4oC – not at 0oC when it freezes.
During the spring and fall, lakes become completely mixed as wind energy overcomes the reduced temperature and density differences between surface and bottom waters. Water temperature and density are similar throughout the lake depth and a complete mixing of water, oxygen, and nutrients can occur. This is called turnover.
With the onset of summer, the lake’s vertical oxygen profile will begin to change as the lake separates into layers controlled by temperature and thus water density (Figure 1). The upper layer of water will have contact with atmospheric oxygen and light allowing for diffusion of oxygen into the water and oxygen production by photosynthetic action of rooted aquatic plants and algae. This oxygen will not, however, be available to underlying waters. Waters of the lower layers in the lake have no contact with oxygen during the summer and are too dark to support most photosynthetic plants. The thermocline is found in the middle of the lake and is defined by a temperature change of at least 1°C per vertical meter and marks the transition between the top and bottom layers. This temperature/density gradient can be very sharp and can effectively prevent the mixing of the top and bottom layers.
Figure 1. Lake stratification in the summer.
The depth of the thermocline and overall lake stratification depends on factors including the local climate and the size and depth of a lake. In order to capture changes in the water column, DO and temperature should be measured every 1 meter from the surface to the lake bottom at regular intervals throughout the ice-free portion of the year.
Why take dissolved oxygen and temperature profiles?
A profile is done by measuring DO and T every meter from the lake surface to 1 m above the lake bottom. By developing an understanding of what DO and temperature look like from the surface to the bottom of the lake, more insight can be gained concerning the health of the lake. With climate warming, it is becoming increasingly important to document DO and T profiles for lakes in order to better predict locations and time periods that may experience increased or decreased DO concentrations in relation to changing temperatures.
How to take DO and temperature profiles?
The best way to measure DO and temperature in a water column is to use an oxygen meter which measures the amount of dissolved oxygen in an aqueous solution and has a built-in thermometer so that temperature can be recorded with every DO reading. There are other methods available to measure DO such as the Winkler method, however, these methods are more cumbersome, time consuming, and more prone to user error, making it challenging to obtain an accurate profile of a lake.
Conclusion
When assessing water quality and the health of a lake, dissolved oxygen is one of the most important parameters as it is essential for the survival of aquatic organisms. When oxygen becomes too low, fish and other aquatic organisms become stressed and may die. Nutrient availability and productivity of the entire lake rely on the concentration of DO in a lake, so it is important to monitor for changes in order to implement best management practices.
By taking profiles of a lake, it may be possible to detect changes to the depth of the thermocline, or changes in the time of lake turnover, which can have marked effects on DO and nutrient cycling. Due to the sensitivity of lakes and lake ecosystems to climate change, it is important that proactive measures be taken to better predict how lakes throughout BC will respond. The more lakes that provide temperature and DO profiles, the more data points there will be for different regions and lake characteristics so that steps can be taken to mitigate impacts or provide information in advance of an extreme shift.
If you would like to learn more about monitoring dissolved oxygen and temperature changes in your lake, contact the BCLSS for information.
References
British Columbia Lake Stewardship Society (2021). Chapter Three: An Introduction to Lakes, LakeKeepers Manual 2021 Edition, accessed May 2021.
Wilson, P. C. (2010). Water quality notes: Dissolved oxygen. EDIS, 2010(2), retrieved May 2021 from https://bit.ly/3yfBW1E
YSI Inc. (2021). Dissolved Oxygen Measurement in Water with Oxygen Meters, retrieved May 2021 from https://bit.ly/3uQ5i4i
Article by Marie McCallum
