In summer, the temperature of the water in a lake is not the same. The top part of the water gets more of the sun's heat and is about 19-25ºC, the middle layer is cooler because it gets less of the sunlight about 8-19ºC , and the bottom layer is pretty cold usually about 4-8ºC. In the winter their is something called winterkill which is when the top part of lakes aquire ice on the water and block out the sunlight and some of the oxygen so alot of species die. On the contrary summerkill is consisted of algae mostly single-celled, make their food through photosynthesis which then the algae release oxygen into the water. Photosynthesis uses sunlight as a source of energy to combine carbon dioxide, water and salts to form chlorophyll . This means algae is numerous in the summer. When there is too much algae, the decaying algae reduce the oxygen levels in the water, and this can kill plants and animals.