Estuaries are places where a river meets the sea. This ever-changing ecosystem is bursting with life. Saltwater from the sea, brought in by the tides, mixes with freshwater from the river. This constant blending gathers and distributes nutrients. Estuaries are among the most abundant regions on Earth, teeming with vast blooms of plankton and algae, huge schools of fish, and countless numbers of invertebrates.

Description: a place where incoming seawater mixes with outgoing freshwater from rivers
Locations: around the world
Number in the United States: 102 estuaries along the coastlines of the U.S.
Threats: overfishing, introduction of alien species, pollution
Significance: provide nearly half of the world's commercial fish harvest

Image credits: main image, courtesy of NOAA, Mary Hollinger.