Estuaries are brackish (a mix of fresh and saltwater) zones found near the mouths of certain rivers, roiled both by freshwater currents and the churning of ocean tides. Taking any number of forms – from bays and lagoons to salt marshes and big sloughs – they are some of the most outstanding ecosystems for biodiversity in the world, nourished by sediments and nutrients of both river and ocean. While some estuaries can be volatile environments, shifting from exposed mud-flats to inundated lagoons within hours, a host of animals take advantage of their plentiful resources and habitat.
Much of the estuary’s animal life takes the form of invertebrates. Some are burrowers, such as the mud shrimps and ghost shrimps of the North American West Coast, which extract nutrients from water passing through their holes during high tides. Blue crabs in the estuaries of the Atlantic-Gulf coasts release their larvae offshore; at a certain age, young crabs move into estuaries to feed with the tides, heading upriver and downriver with their advance and retreat.
Some fish, like the starry flounder of the Pacific Coast, spend their entire lives in estuary environments. Many others, however, like tarpon in the Gulf of Mexico, only visit estuaries to feed or to rear their young in the relatively hospitable, sheltered habitats. The great barracuda spends its early days in shallow-water estuaries, finding some refuge from marine predators. Such large fish as bull and tiger sharks also periodically enter estuaries to hunt; bull sharks, one of the few sharks able to tolerate prolonged exposure to freshwater, may track well upriver.
Some of the more visible and celebrated estuary visitors and residents are birds. The teeming concentrations of small animals attract large numbers of waders and waterfowl. In North America sandpipers, yellowlegs, great egrets and great blue herons patrol mudflats with the recession of the tide to snatch small creatures. Sea eagles like bald eagles in North America and fish-eagles in Africa are common estuary predators, hunting anything from fish to waterfowl.
A few species of crocodile specially favor estuaries and other brackish environments. The Indo-Pacific crocodile of Southeast Asia, Indonesia, New Guinea and Australia is an example. The massive beast, sometimes better than 20 feet long, is also called the estuarine or (less accurately) saltwater crocodile to reflect its habitat preferences. In the New World the large American crocodile occupies similar environments, including the mangrove estuaries and shallow bays of the coastal Everglades in South Florida.
Oceangoing marine mammals also utilize estuary habitats, often for specific seasonal purposes. A population of beluga whales in the East Beaufort Sea off Alaska and Canada regularly enters the fertile waters of the great Mackenzie River's estuary for summertime calving. Atlantic bottlenose dolphins cruise the brackish waters of Florida Bay – the outlet of one of the Everglades’ major drainages, Taylor Slough – to hunt small fish. Freshwater mammals like muskrats, beavers, mink and river otters (which actually readily enter coastal waters to fish) also utilize estuaries.
- “Pacific Coast”; Bayard H. McConnaughey, Evelyn McConnaughey; 1985
- National Atmospheric and Oceanic Administration: Estuaries - Adaptations of Blue Crabs
- Florida Museum of Natural History; Great Barracuda; Cathleen Bester
- BBC: Science and Nature - Estuarine Crocodile
- Canada Department of Fisheries and Oceans; Eastern Beaufort Sea Beluga Whales; 2000
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