Due to their ever-changing conditions, estuaries support the life cycles of many different species. Striped bass, for example, hatch in freshwater and live in the ocean as adults.

Whether in the Bay of Bengal, Puget Sound or the Niger Delta, estuarine waters continuously blend seawater rushing toward the shore and river water running from the land. A single estuary can be salty at one time and fresh at another. Surprisingly, some species can handle this wide range of conditions-and thrive there. Estuaries around the world provide some of the biggest harvests of fish, mollusks and crustaceans.

Why are estuaries so full of life?

Mixing and blending
Rising and falling with the ocean tides, estuarine waters are constantly shifting between salty and fresh. The mixing often forms a layered gradient, which determines what can live there.

Like a butter churn, tides and currents cycle salt and fresh water in an estuary, filtering sediment and circulating nutrients. In most cases, heavy salt water enters along the bottom, eventually mixing with river runoff flowing out along the surface. When there's not enough fresh water due to drought, even the surface water becomes salty. But estuary water-carrying nutrients-is replaced on a regular basis, whether by the tides, the rains or the rivers.

A plant or animal does well if it can survive in changing conditions. In the Hudson River, for example, abundant white perch can handle nearly any salinity and temperatures ranging from -1.2 to 25 degrees Celsius (30 to 80 degrees Fahrenheit).

Meals in the Mud
In estuaries, one species' waste is another's dinner. Bottom dwellers munching in the mud survive on sinking debris.

The bottom-dwelling clamworm (Neanthes virens) eats vegetation and detritus off of the New England coast. In other regions it scavenges dead animals and may prey on other worms and crustaceans. Naturally occurring in Europe and North America, the clamworm is also grown commercially for fishing bait. © Robert DeGoursey / Visuals Unlimited

While many species of algae and other plankton live at the sunlit surface, others dwell along the darker bottom. Some of the most nutritious sources of food are scattered over an estuary floor in the form of detritus, or decaying plants, animals and other matter, such as feces.

Most of the benthos, or species on the bottom, are filter feeders. Clams, numbering in the millions, bury themselves in the mud, and filter nutrients that wash by. They actually clean the water as they filter. Worms, crabs, lobsters and snails are some of the other species scouring the sediment for supper. The bottom dwellers, in turn, become prey for fish and crustaceans-and even humans.

Mass Transit
Some fishes commute through estuaries to spawn in nutrient-rich waters. Others live a full life in fresh water and then return to salt water to breed.

Often called cradles of the sea, estuaries serve as nurseries for many fish species. Atlantic sturgeon and striped bass are anadromous fishes, which live their adult lives at sea but head to fresh water to spawn. As the young mature, they migrate downstream to mildly salty, or brackish, water where they can adjust to a higher salt level. Many estuaries also have shallow spots away from currents that are filled with plants and plankton. These areas provide food and protection for young fish until they go to sea.

Catadromous species do the reverse commute. American eels, for example, live out their lives in fresh water and return to the sea to spawn.