Where rivers meet oceans, the young of many species find ideal nurseries. These transition zones between fresh and salt water are called estuaries. They are rich in river-deposited and recycled nutrients, and sea grasses provide good hiding places for the young. Many commercially useful fish feed in estuaries when young. Shrimps, crabs, oysters, cockles, and mussels are caught in estuaries.
Oceans offer other energy sources.
In tropical areas, the difference in temperature between sun-warmed surface water and cold, deep water can be converted into electricity. Tidal and wave energy can also be used to generate electricity. Water, carbon, nitrogen, phosphorus, and other compounds reside in oceans for periods ranging from minutes to millions of years. Because it holds and transports heat, water profoundly affects climate. And, because the water cycle is the largest movement of any substance on earth, it has great control over life on our planet.