Oceanographers have identified many sources of ocean pollution. Although oil spills are damaging to the ocean environment, they do not pose the largest threat to the ocean in context to other pollutants. Land runoff and river discharge make up the largest source of ocean pollution, which can include heavy metals, synthetic organic compounds, and eutrophication-causing nutrients. Other types of pollution include oil, radioactive waste, sewage, and thermal and sound pollution. No matter the type, pollution can adversely affect the environment and the organisms that live there. In turn, it can affect all organisms up the food chain, starting with the smallest organism that relies on the ocean.