By August, in the deepwater zone, oxygen is depleted, which changes the water sediment zone’s chemistry at the bottom of the lake. The chemical changes in iron in an area without oxygen (anoxic) causes the phosphorus bound to the iron to be released, creating phosphorus-rich water. This phosphorus water will stimulate green algae and cyanobacteria when the lake mixes in the fall.
In the winter, when ice covers a lake, deep layers of water without access to air may become devoid of oxygen and may lead to winter fish kills. If there is an anoxic zone at the bottom of the lake, it will cause phosphorus release. Oxygen depletion in the winter is a slower process due to the low-temperature water. That photosynthesis continues under the ice when the ice is clear with limited snow cover. Photosynthesis will produce oxygen. Under clear ice, Eurasian watermilfoil is seen topped out.
Spring and fall mixing are driven by wind, and prevailing wind direction changes by seasons, with more turbulent conditions occurring in the spring and fall. A typical dimictic pattern of temperature changes.
Before the ice melt, the lake’s oxygen levels are depleted. Springtime arrives, and the ice slowly melts with warmer temperatures exposing the lake water once again. Stratification happens once again. The wind begins to mix the lake water, and the surface water begins exchanging gases with the air, pushing the surface water deeper and infusing it with oxygen. Next time you are swimming in a lake, notice the temperature differences the deeper you dive down. If you have a swimming face mask on, you may see a layer of trapped debris and pollen at the zone of temperature changes.
When the lake turns over in the spring and fall, there is internal recycling of phosphorus from the deep water. The fall turnover recycles more phosphorus than the spring turnover. The spring ice melt also contributes phosphorus since most phosphorus is in the dust and dirt accumulating on the ice. At the same time, there is increased runoff, so there is a slug of phosphorus and nitrogen compounds.
The green color is seen in the lake early spring, and late fall is due to green algae; if there is a white color, you have diatom blooms. If it like an oily sheen of green or a thick green soup, it may be a harmful algae bloom (HAB). Below is a link to the New York State Department of Environmental Conservation HAB identification website. https://www.dec.ny.gov/chemical/81962.html
Algae blooms are nothing new to Saratoga Lake. There are many types of algae, and most are harmless. Algae generate oxygen through photosynthesis, providing oxygen in our atmosphere and forming the aquatic food chain that supports all the animals associated with lakes.