Algal blooms are episodes during which large quantities of algae feed on nutrients present in the water, namely phosphorus and nitrogen. These nutrients enter Lake Erie and its tributaries primarily from agricultural fertilizer and manure runoff, wastewater discharges, failing septic systems, dredge materials, and lawn fertilizer runoff. The nutrients are also present in various degrees in existing lake sediment. When there is a high concentration of these nutrients and the lake warms up in the summer, harmful algal blooms can be the result.