Water Quality Data
Looking at secchi measurements, temperature variation and the dissolved oxygen concentration in real time time can help to explain water quality in your lake. SECCHI DEPTH (Meters): is the distance that light penetrates through lake water. This measurement of water clarity is taken at the deepest part of the lake between the hours of 9 am and 3 pm from the downward, shaded side of the boat. Water clarity is determined from true color (materials dissolved in the water) and turbidity (materials suspended in the water). Secchi disc values vary throughout the summer as algal populations increase and decrease, and are used to assess water quality, productivity and long term trends. A pristine lake will have a reading of ten meters. A secchi reading of 2 meters or below is defined as an algae bloom.
Click here to try out a Secchi Simulator courtesy of the Maine Volunteer Lake Monitoring Program.
TEMPERATURE vs DEPTH: Water quality is affected by the extent to which the water mixes. The depth, size, and shape are the most important factors. Climate, shore topography and orientation to prevailing winds, in/outflow and vegetation also play a big role. The variation in water density caused by different temperature (water density is greatest at 39F) can create stratification. Warmer water remains near the surface while colder water remains near the bottom. The mixing of the water by the wind determines the depth of surface layer (epilimnion), usually about 6-10 meters. The transition layer (metalimnion) between the warm and cold bottom layer (hypolimnion) is referred to as the thermocline. The uniform water temperature in spring allows the lake to mix completely and will recharge the bottom water with oxygen and bring nutrients (particularly phosphate and nitrogen) up to the surface water. A spring algae bloom often appears when nutrients mix and rise to the surface. During summer temperatures, deeper lakes will develop a thermocline and trap oxygen and nutrients released from the sediment in the hypolimnion. As fall temperatures cool the surface water, the loss of temperature variation will cause the lake water to mix. A fall algae bloom often appears when nutrients mix and rise to the surface. Winter stratification, with usually only a temperature difference of about 7F will remain constant because the ice cover will block the wind from mixing the water.
DISSOLVED OXYGEN (%RDO) vs DEPTH: The solubility of oxygen depends on water temperature. Cold water can hold more oxygen than warm water. Continuous mixing allows oxygen exchange between water and the atmosphere. Oxygen is also produced by green plant growth by day (photosynthesis), but they use oxygen and produce carbon dioxide (respiration) by night. The balance between these reactions largely determines the amount of oxygen and carbon dioxide presnt at different times of the day and at different depths. Surface waters commonly remain saturated with oxygen, but deeper, colder water resulting from stratification often experience oxygen depletion. Following, if the lake produces too much algae which falls to the bottom layer to decay, oxygen becomes depleted.
Phosphate is the limiting nutrient in lakes; limiting algae growth because it is less available than other lake nutrients. Algae blooms reduce water transparency. Bloom material dies, sinks to the bottom layer of the lake and decomposes (by bacteria and fungi) using up much of the deep water oxygen. Lake stratification prevents surface oxygen from mixing into the bottom layer. This ultimately leads to anoxic (lack of oxygen) conditions. Anoxic conditions (oxygen levels below approx. 20%) allow more phosphate to be released from the sediment. This internal source of phosphate creates a feedback cycle as the phosphate mixes with the upper lake water as a result of spring and fall mixing often resulting in seasonal algae growth. Phosphate has many natural and human sources including fertilizer, animal waste and plant decay. Even small additions of phosphate from external sources can have a dramatic effect leading to algal growth. Sedimentary phosphate accumulated over decades.