Spring
When the surface heats up enough, the lake will stratify into layers based on water temperature, with the cooler water on the bottom. After the lake stratifies, the algae are no longer mixing to the bottom. The algae in the epilimnion, or top layer of water, are free to grow like gangbusters. This is called the spring bloom.

Lake stratification and the subsequent algae growth are ultimately brought about by increasing water temperatures. As the temperature rises, so does the population of grazers. Zooplankton eat the algae all winter long, but the rate of grazing increases directly with temperature. So the zooplankton grow quickly during this period. Not only do they get bigger, but they reproduce at a much greater rate. The zooplankton young are hungry and will also eat lots of algae. Both adult and juvenile zooplankton have small mouths, so they are restricted to grazing on small algae.

As organic matter settles out of the epilimnion into the hypolimnion, it decomposes. The decomposition process uses oxygen. Eventually, there will be no oxygen in the hypolimnion and phosphorus will be released from the sediments by a process called internal loading. Stratification not only keeps oxygen from reaching the hypolimnion, it also prevents the phosphorus from reaching the epilimnion (Figure B). Unless there’s a significant amount of rainfall to bring runoff to the lake, the concentration of available nutrients in the epilimnion will start to decline. Meanwhile, the population of grazers is growing rapidly. Before long, the algae will have either run out of nutrients or be eaten by zooplankton and the lake could have its clearest water of the year. This period is known as the clear-water phase.