Plankton are small plants and animals present in the sea. The animal plankton feed on the plant plankton. The rate at which the plant plankton are produced depends on sea temperature and the amount of sunlight. In tropical regions there is little variation of plankton production through the year. Variations are much bigger at temperate and polar latitudes. Plant plankton production always rises before that of animal plankton. The following graphs show the production rates of plankton in the northern hemisphere: