Whales mostly feed on krill in the ocean. They swim through the ocean constantly to run seawater through their mouths to filter out the krill for digestion. As whales swim down to lower depths and then up to the surface to breathe, they churn up nutrients from the lower parts of the ocean and bring them to the surface waters. These nutrients help feed other organisms at this depth. This churning action has been called the "whale pump." One estimate places the churning action of this whale pump in the Gulf of Maine as greater than the effect of all rivers entering the gulf combined.