Estuaries: Cradle of Life
What is an estuary? In simplest terms, it is a place where fresh water from rivers mixes with salt water from the sea. Under quiet conditions, “lighter” fresh water will float atop “heavier” salt water. But tides, currents and winds cause mixing, and that is the secret to the vitality of an estuary.
An astonishing miracle plays out in estuaries each spring with a cast of millions, no billions or trillions, or more.
It begins with phytoplankton. These tiny plants, such feeble swimmers they can only drift at the mercy of tide and current, are the wellspring of life in the waters of the world.
Spring sunshine penetrates shallow water. Water temperatures rise, maybe to 50 degrees.
Currents of fresh and salt water mix and layer and mix again. It’s never quite the same each year. During rainy seasons more fresh water flows from the land; during dry seasons, salt water moves further west in the Sound and up into rivers. But the miracle plays out.
Decayed remnants of past lives dissolve into nutrients that create a soup in churning waters. Phytoplankon begin to stir. They make food from these nutrients with the help of sunlight.
They divide and multiply. Or they reproduce and multiply. Or they break apart and multiply. They multiply. They multiply. They feed zooplankton and small shrimp that become food for the fish who will come.
And the fish did come, hustling in from the ocean through inlets along the barrier beach. When water temperatures rise to about 60 degrees, and plankton are on the move, fish would teem by the millions from their ocean homes through inlets into Sounds and up rivers.
In 1612, William Strachey noted “great Shoells of Herrings” (and relatives) in North Carolina. Swimming upstream in a frenzy against river currents, adult fish would seek the quiet inlets or coves where they had been born, and there they would spawn.
Unless they were eaten first. These small fish, at the bottom of the food chain, feed the world. Every imaginable fish or shellfish with a big enough mouth eats herring or shad, including man. Herring and shad are nutritious, oily, bony fish with white flesh, traditionally eaten bone-in.
Hickory shad came first, in mid- to late-February, then alewives in March. Blueback herring and American shad followed in three or four weeks, one group pacing another, each taking its turn, streaming in for the great spawning adventure.
Striped bass (or rockfish) spawn along with shad and herring (if they are not eating them or any other creature they can find) from April to mid-June. When their eggs hatch, the young depend on river currents to keep them afloat while they drift downstream to feast on a ready-made food supply of plankton and fry.
At the same time, Atlantic croaker and spot are spawning in the ocean. Their youngsters drift into the Sound through those tumultuous inlets and settle in shallow bays and coves to feed.
For centuries, this upwelling of new life, powered by plankton, this multitude of fish hurrying to spawn, was one of the grandest celebrations of springtime witnessed and welcomed by multitudes who hurried to dip their nets into the water.
The Sound is also host to resident fish who lead less tumultuous lives. They are bottom-feeders and stay here year-round. As traditional catches of herring and striped bass declined, catfish and perch became valuable resources.
Blue crabs make their homes in the Sound, too. The female can release eggs several times a season, up to three million in her short life, making the blue crab fishery one of North Carolina’s most valuable.
Crabs spawn in spring and summer. Depending on sex, molting and stages of growth they range the Sound seeking waters of varying salinity. If you are out on the Sound in summer, you can follow long lines of traps that commercial and recreational fishermen bait with shrimp heads or menhaden. Catches will go to local markets or be shipped as far away as New York City.
Crabs and fish find food and hiding places in meadows of grasses growing in soft sediments and shallow waters where sunlight can penetrate. Few of us notice these plants (unless they foul the propeller of a pleasure boat). Even fewer of us realize how much they contribute to life in the Sound.
As submerged grasses take in nutrients and produce food for themselves, they absorb carbon dioxide and release oxygen. Oxygen levels remain high for young, actively growing fish, water stays clear, and tied-up nutrients slow growth of unwanted algae.
If fish take center stage in spring, wildfowl take center stage during fall migrations. Wildlife refuges, wetlands and harvested croplands are winter destinations for thousands of migrating waterfowl: Canada and snow geese, tundra swan and ducks.
The south shore of the Sound is one of their last outposts. They once frequented wetlands along the eastern seaboard but ancestral winter homes disappeared into airfields and cities. Leigh, a friend-now-gone spoke of throngs of wildfowl darkening the skies for hours as they passed over his house each fall when he was a youngster.
As a changed landscape pushed their congregations farther south, their numbers shrank. Today, the spectacle of thousands of birds feeding, loafing, and flying at will renews faith in the survival of wildlife and the environment.
They don’t visit for long. They arrive in December for the winter party, as they have for eons, leaving a wintry tundra, commuting thousands of miles from summer homes on lakes in Alaska and Canada. They’ve begun the return trip to the Arctic by March, the great migratory flocks, ready to mate and raise young who will accompany them on their flight south next winter.
During spring and summer, the more diminutive “Kermit” frog takes over creeks and ditches throughout the coastal plain. The nasal queenk queenk queenk of mating green tree frogs rises through the night. They are acrobats, leaping with grace to catch their meals, depending on large toepads for reliable holdfasts when they land. During the day you can find these light-weights snoozing on green leaves, perfectly camouflaged.