02 - Natural Communities of the Chesapeake Bay



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The Chesapeake Bay contains multiple distinct natural communities. These communities are defined by the interplay of light, nutirients and the mixing of salt from the oceans and fresh water from the rivers.




from The Longstreet Highroad Guide to the Chesapeake Bay By Deane Winegar


Tidal Marsh

Tidal marshes are wetlands in the tidal zone characterized by grass-like vegetation. Marshes form a vital transition area between shallow water and uplands. Depending on their location, water in tidal marshes can be fresh, salty, or brackish, with daily, seasonal, and annual variations. The saltiness of a marsh greatly affects the type of life the marsh supports. Generally speaking, the fresher the marsh, the greater the variety of organisms that live there. Salt water poses problems for plants and animals that require adaptations for survival.

Complex ecosystems have evolved to thrive in the tidal fluctuations of marshes in the Chesapeake Bay watershed. Incoming tides bring nutrients needed by marsh plants for optimal growth. Larvae that depend on the marsh for sustenance are also brought in by waves and currents. Outgoing tides flush the marsh of decaying organic matter called detritus, carrying it to other environments where it serves as food for microorganisms and minute aquatic invertebrates. Species higher on the food chain such as aquatic insects, fish, birds, and mammals feed on the microorganisms, in turn.

In addition, marshes provide shelter and food for a wide variety of birds, mammals, reptiles, amphibians, fish, and shellfish. Many North American bird species use wetlands, according to the Chesapeake Bay Foundation. More than two-thirds of Atlantic and Gulf Coast fish and shellfish that are significant commercially rely on wetlands for at least part of their lives, say biologists at the National Marine Fisheries Service. These include striped bass, menhaden, bluefish, flounder, spot, blue crabs, oysters, and clams.

Before humans gained an understanding of their importance, wetlands were viewed as an obstacle to development and transportation. Farming the soggy ground was a challenge, although some people figured ways to do it. The marshes and swamps also challenged the ingenuity of developers and road builders. Consequently, thousands of acres were lost to ditching and draining or to filling operations. Now, in the light of research and media coverage, marshes are seen for the critical part they play as filters for pollutants, buffers that protect the shoreline and soil from waves and wind, and as nesting grounds, nurseries, food providers, and hiding places for fish, birds, crustaceans, and other life forms.

Tidal wetlands can be divided into freshwater, brackish, and saltwater habitats.

Tidal Freshwater Marsh

Any angler who has sought out lily pads to find bass or pike, then silently cursed the plants for snagging his hook, has come face to face with some of the vegetation that is key to the health of tidal freshwater marshes. These marshes occur at the head of the Chesapeake Bay and along the upper reaches of the tidal portions of tributary rivers.

Besides the yellow pond lily or spatterdock (Nuphar luteum), plants associated with tidal freshwater marshes include broad-leaved plants called emergents extending above the water surface such as arrow arum (Pedtandra virginica) and pickerelweed (Pontederia cordata). Growing closer to water’s edge are narrow-leaved cattails (Typha augustifolia), marsh hibiscus (Hibiscus moscheutos), American three-square (Scirpus pungens), and various rushes and sedges. Large expanses of wild rice (Zizanis aquatica) grow in the mud of the shallows.

Insects such as dragonflies, damselflies, honeybees, and butterflies flying above a freshwater marsh allude to the rich variety of life sustained here. Of course, there are also the ubiquitous biting flies, midges, and mosquitoes to contend with. A closer inspection reveals grasshoppers, crickets, and a variety of bugs—all of which attract insect-eaters such as swallows, flycatchers, and even spiders. The freshwater marsh is also home to reptiles and amphibians not found in the salt marsh such as the green frog (Rana clamitans melanota), the Eastern painted turtle (Chrysemys picta picta), and the Eastern ribbon snake (Thamnophis sauritus sauritus). Great blue herons, great egrets, a variety of marsh ducks, rails, sandpipers, and songbirds find protection and food in the marshes.

Beneath the surface are crustaceans such as the transparent freshwater grass shrimp (Palaemonetes paludosus), and mollusks such as the common river snail (Goniobasis virginica). Buried in the mud are even more critters, including freshwater mussels (Anodonta).

Freshwater habitats ( are found in the ) Head of the Chesapeake Bay, Elk Neck State Park, Idylwild Wildlife Management Area, and Tuckahoe State Park.

Brackish Marsh

The farther down river from the fall line of tributary rivers and the closer to the mouth of the bay you go, the more brackish the tidal marshes are along the shoreline. Freshwater and saltwater plant species may mix here or dominant species may take over. One notable example is the nearly total domination in some area by that kudzu of tidal marshes, reed grass (Phragmites australis). Reed grass or phragmites can tolerate a wide range of salinity and grows prolifically in brackish marshes, especially where the soil has been disturbed, often crowding out more beneficial species. Olney three-square (Scirpus americanus) is a common plant in brackish marshes, along with big cordgrass (Spartina cynosuroides) and narrow-leaved cattail.

Brackish marshes ( are found in the ) Ragged Island Wildlife Management Area, Talbot County, Eastern Neck National Wildlife Refuge, Kings Creek Preserve, Blackwater National Wildlife Refuge, Robinson Neck/Frank M. Ewing Preserve, and Smith Island.

Salt Marsh
Salt Marsh Hay (Spartina patens) Perrennial marsh grass that grows from rhizomes. Recognized by its tousled appearance.

As the salinity of the lower bay exceeds 25 grams of dissolved salts per thousand grams of water (25 parts per thousand or 25 ppt) and approaches that of pure seawater (35 ppt), salt marshes predominate. Along most of Virginia’s bay shoreline, at the bay mouth, and on the backside of the islands of the Virginia Coast Reserve on the Eastern Shore, smooth cordgrass (Spartina alterniflora), also called saltmarsh cordgrass, spreads along the lower edges of higher marshes or in pure stands as far as the high can see.

A tall form of the smooth cordgrass grows to 10 feet high in areas regularly and deeply flooded by incoming tides. Where the water is somewhat less salty, other species such as black needlerush (Juncus roemerianus) and saltmarsh bulrush (Scirpus robustus) are able to compete with the cordgrass for space. A short form of the cordgrass grows in higher areas near the high-tide line, along with salt hay cordgrass, spikegrass, saltmarsh aster, and glasswort. At the level of spring tides and storm tides (a level seldom flooded), just below the uplands, switchgrass and high-tide bush grow.

Despite the harshness of the environment, salt marshes are among the most productive ecosystems on earth. They serve as hiding places and food factories for many fish, crustaceans, and mollusks. Marsh crabs, marsh fiddler crabs, and the Atlantic ribbed mussel are at home here. Periwinkles and saltmarsh snails ascend and descend the stalks of cordgrass with the tides, feeding on detritus. The saltmarsh snail (Melampus lineatus) is strictly an air breather, and must time its climb to stay above the incoming tide.

Salt marshes are more productive than farmland. In general, East Coast wetlands can produce 5 to 10 tons of organic matter per acre annually compared to 0.3 to 5 tons for agricultural fields, according to the Virginia Institute of Marine Science. Young blue crabs as well as 14 species of fish have been shown to be more abundant in salt marsh wetlands than in areas without vegetation. Mummichogs (minnows), fiddler crabs, snails, and other species live out their entire lives in these wetlands. Wetlands serve as nurseries for the young of spot, menhaden, mullet, and many other coastal fish. In fact, 30 percent of a menhaden’s diet comes from marsh detritus, while 70 percent is derived from plankton. Shorebirds such as Forster’s terns, clapper rails, willets, and laughing gulls make their nests in coastal salt marshes.

Very few reptiles and amphibians can tolerate the conditions of a salt marsh. Most birds that feed in the marshes, such as the great blue heron (Ardea herodius), glossy ibis (Plegadis falcinellus), and great egret (Casmerodius albus) are not full-time residents. Exceptions are the clapper rail (Rallus longirostris) and willet (Catoptrophorus semipalmatus).

Salt marsh habitats ( are found in the ) Horsehead Wetlands Center, Chincoteague National Wildlife Refuge, Mockhorn Island Wildlife Management Area, Virginia Coast Reserve, and Eastern Shore of Virginia and Fisherman Island National Wildlife Refuges.

Intertidal Zone

The area of land around the edges of the bay that is exposed at low tide and covered at high tide is called the intertidal zone. This area forms a transition zone between uplands and shallow water. Depending on such factors as the salinity of the water and whether the bottom is composed of sand, mud, or a combination of these, the intertidal zone can host a variety of plant and animal communities. Where the zone is composed mostly of sand, beaches occur. On flat land composed of mud, an entirely different realm exists called mud flats.

Beach

Beaches occupy a unique niche where they occur around the edges of the bay. To have the kind of wide, sandy beach that vacationers look for, certain conditions must be present, such as wave action, a gently sloping shoreline, and currents sufficient to gather up sand and deposit it on the shore. These conditions are more prevalent in the lower bay. In the upper bay, beaches are generally flatter and made up of a combination of sand and mud deposited in river deltas.

The fiddler crab digs its burrows in the sand of the upper beach at or above the high tide mark. A creature that is a survivor from prehistoric times, the Atlantic horseshoe crab (Limulus polyphemus), comes ashore to deposit its egg cases in the sand. Empty egg cases of whelks and skates are commonly found on beaches in the lower bay and along the Atlantic coastline.

Many of the life forms supported on the intertidal beach between the high and low tide mark in the Chesapeake Bay are not readily apparent. However, the flocks of tiny peeps and sandpipers that run back and forth with the edge of the waves, stopping to probe here and there, is evidence that there is life beneath the sands. Oystercatchers, gulls, and even grackles are attracted to beaches for bits of marine life washed ashore or tiny crustaceans such as sand diggers and beach fleas that burrow into the sand.

Children often learn how to scoop up handfuls of hard-shelled mole crabs (Emerita talpoida), which burrow immediately back into the sand when deposited back on the beach. Although abundant along beaches on the Atlantic, mole crabs are pretty much restricted in the bay to the beaches near the mouth of the bay. They move higher and lower on the beach with the tides, always staying within the zone of breaking waves.

Also hidden within the sands is a microscopic community of copepods, protozoa, and other minute organisms moving upward between the grains of sand to greet each rising tide. Copepods are members of the zooplankton that float freely at the whim of currents. These tiny, uncelebrated crustaceans are the most abundant animals in the Chesapeake Bay, making up as much as 95 percent of the biomass.

Mud Flat

If the land is flat enough, low tide may expose wide expanses of mud flats made up of silt, clay, and organic material. The flats may look deceptively like a wasteland of ooze where no plant or animal could survive. A closer look reveals the truth. Just beneath the surface is a variety of burrowing worms, snails, and clams. If you can view it just before the receding tide uncovers it, you may also see soft-bodied worms and other critters before they retreat to the mud. After the tide withdraws, fiddler crabs and snails appear. In fact, these flats are teeming with life and are a good place to take inquisitive children. Just like snow that varies in consistency, so do mud flats. Some will support your weight, but in others you may sink into the ooze a few inches.

A small chimney of mud reveals the home of the nocturnal burrowing crayfish (Cambarus diogenes). Various breathing holes in the ooze may give away the burrows of the common clamworm, the soft-shelled clam, or the common jackknife clam. Less obvious are mud shrimp and snapping shrimp, which rarely leave their sometimes-deep burrows.

Opportunistic gulls, terns, and shorebirds such as the American Oystercatcher (Haematopus palliatus), semipalmated plover (Charadrius semipalmatus), and dunlin (Calidris alpina) may also appear at low tide. Gulls and terns scavenge on the surface, while shorebirds have beaks adapted for probing into the mud.

Shallow Water and Submerged Aquatic Vegetation (SAV)

People swimming or playing in the bay or ocean at beach areas may have come to realize that what one day may seem an empty body of shallow water will another day be full of schools of fish bumping the legs, crabs underfoot, or even worse, stinging jellyfish or stinging nettles. Shallow waters of a few feet deep harbor a fluid mix of marine life, including organisms buried in the bottom, migrating fish and the predators that follow them, and a whole microscopic community of microscopic plankton that float with the currents.

Burrowing worms, clams, snails and other benthic organisms inhabit the bottom. Life forms vary according to depth, salinity, substrate composition, and vegetation, if there is any. Sunlight can usually penetrate to the bottom in shallow waters, so aquatic grasses may take hold, creating an entirely new habitat for marine biologists to study.

Grass beds in the bay’s shallow waters are known as submerged aquatic vegetation or SAV. The Chesapeake Bay has more than a dozen native species, including wild celery (Vallisneria americana), common waterweed (Elodea canadensis) and redhead grass (Potamogeton perfoliatus). Species vary according to salinity, among other factors. The wild celery is a freshwater species, while wigeon grass (Ruppia maritima) tolerates very brackish water and eelgrass (Zostera marina) can live in pure seawater.

SAV, perhaps more than any other plant community, is vital to the bay’s health, producing much-needed oxygen underwater just as trees do above ground. They also provide hiding places and food for marine life, as well as for migrating, wintering, and nesting waterfowl. The grass beds act as a filter, absorbing nutrients such as nitrogen and phosphorous, and their thick stands trap sediments that might otherwise settle on oyster beds.

Just as wild turkey or deer inhabit above ground forests, SAV beds harbor aquatic animals such as minnows and blue crabs. In saltier areas, barnacles and scallop larvae attach themselves to the leaves and stems of eelgrass. In fresher water in the bay tributaries, anglers know to cast to underwater grass beds for bluegill and largemouth bass. Tiny zooplankton that feed on the grasses are themselves food for larger organisms.

Sedimentation and a decline in water quality have taken a heavy toll on the Chesapeake’s grass beds over the last several decades. Nutrients from car exhausts and power plant emissions can be carried for miles in air currents to settle in the bay. Governments have made good strides in curbing this point-source pollution—pollution that comes from an obvious source such as a smokestack or underground pipe. Finger pointing is easy when the pollution is so visible. Harder to get a handle on are the nonpoint sources such as fertilized farms and lawns, animal and human waste, and construction sites. Pollution from these sources runs off into streams or seeps into groundwater, often traveling many miles from the point of origin to reach the bay.

In smaller amounts, nutrients are a good thing. A healthy estuary is remarkably resilient and can even deal with excessive nutrients up to a point. Uncontrolled nutrient loading, however, causes algae bloom that clouds the water and blocks out sunlight necessary for the growth of SAV.

Only 11 percent of the bay’s historic underwater grasses remain. The loss of vegetation is one more factor in the decline or disappearance of many species that depend on the bay. An estimated 80,000 migratory redhead ducks, for instance, once visited the bay annually to feed on seeds, roots, and tubers in the grass beds. Only a few thousand redheads now stop each year.

An overall increase in grasses has encouraged biologists over the past few years, but it’s too early to tell if the trend will be long term. A reduction in nutrients is considered to be the primary reason for the resurgence in grasses. It is tempting to point to restrictions that have been put on industry and other point-source polluters as a factor in the nutrient reduction. However, the present increase in aquatic grasses has occurred during years of drought. Because the streams and rivers have been so low, they transported a much lighter load of nutrients. These nutrients build in the soils of the watershed, only to be flushed out in years of heavy rains.

For unknown reasons, the recent resurgence of underwater grasses did not benefit Tangier Sound on Maryland’s lower Eastern Shore. Grass beds there, which have played an important historic role in the health of the ecosystem, continue a serious decline. More than 60 percent of the sound’s underwater grasses have disappeared in just seven years.

Oyster Bar

The reason humans have survived as a species, or so it has been postulated, is because they taste bad. Oysters, on the other hand, have been decimated because of their flavor.

The delectable oyster (Crassostrea virginica)—so abundant at the time European settlers arrived—has been largely wiped out. Much like the forests of North America that stretched as far as the eye could see when Lewis and Clark crossed the continent, oysters were once so abundant in the Chesapeake Bay they were imagined to be as eternal as the seas themselves. Reefs of oysters stacked upon oysters were so massive they were navigational hazards and appeared on maps.

These reefs, with live oysters living upon the heaped up empty shells of dead oysters, built into huge masses over thousands of years and were quite remarkable in their complexity. Examination of an oyster shell reveals the many bumps and ridges that greatly increase the surface area where other sessile creatures such as sea squirts, mussels, barnacles, anemones, and a variety of bryozoans can attach. These, in turn, provide food, hiding places, and nurseries for a variety of worms, crustaceans, fish, and the like.

Just as the American forests turned out to be finite after all when logging and mining operations of the nineteenth and early twentieth centuries left nothing but denuded, eroding hillsides, so too did oyster reefs turn out to have limitations. The world’s insatiable taste for the Virginia oyster invited non-restricted harvesting. Sediment-filled runoff from development within the bay watershed and pollution have taken their toll. Dredging the bottom to create deeper water for navigation has destroyed oyster bars or covered them with sediment.

In recent years, the oyster’s weakened immune system proved insufficient to battle two ravaging diseases caused by microscopic parasites known as MSX (Haplosporidium nelsoni) and Dermo (Perkinsus marinus), which are not harmful to humans. Although pathologists in the past five years have gained a much greater knowledge about the mechanisms the parasites employ in overcoming the oysters’ defenses, no progress has been made toward curbing their devastating effects on the oyster population.

Loss of habitat, sedimentation, dredging, disease, oxygen-robbing nutrients, and over-harvesting have reduced this important bivalve to less than one percent of its historic numbers. The living oyster opens its shell, drawing water filled with plankton and detritus along its gills to the mouth. Scientists say that at their peak, the oyster population of the Chesapeake could filter the entire volume of water in the bay in 3 to 6 days. Whether or not that is true, it is certain that the bay lost not only its economic mainstay, but also a significant filter for its waters when the oyster reefs were wiped out.

Although the once-expansive reefs are gone, oyster communities still exist in the bay where the bottom is firm enough and where siltation is not a problem. The location of these communities fluctuates with freezing waters, heavy freshwater runoff, and sedimentation, which oysters cannot tolerate. In the Chesapeake Bay, most oyster bars and clusters are located in the transition zone between tidal wetlands and open waters. Researchers with the Virginia Marine Resources Commission are also involved in promising experimental projects to build artificial reefs of oyster shells, which will enable oysters to establish colonies that are above the siltation and low oxygen in the bottom substrate. The new reefs are turning out to be fish attractants as well, a benefit that is getting the attention of the bay’s anglers.
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Deep Water

Along most of the shoreline of the Chesapeake, water depths do not exceed 10 feet. Between the subtidal shallows and the channels of the tributary rivers, the water is about 30 feet deep. Tributary rivers have gouged out channels of 60 foot depths or more, while the ancestral riverbed carved by the Susquehanna down the belly of the Chesapeake Bay is more than 100 feet deep for much of its length.

As the bay waters drop below 10 feet, less sunlight can penetrate to the bottom, and grasses disappear. Detritus from tidal wetlands and underwater grass beds is carried into deeper waters by the currents. Phytoplankton, those microscopic plants at the mercy of the currents, is the only plant life that exists in these deeper realms. Animal life that swims or floats with the currents includes jellyfish and the larvae and eggs of various fishes and invertebrates.

Swimming in the water column at various depths are the bay anchovy and the Atlantic silverside—the two most abundant baitfish in the Chesapeake—along with many larger fish. Charter boat captains spend lifetimes learning the habits and patterns of both migratory and resident fish in the Chesapeake Bay. In addition to feisty bluefish and striped bass, anglers seasonally catch flounder, gray trout, croaker, spot, red and black drum, tautog, and cobia. Even the habits and seasons of baitfish are important, because where there are minnows, menhaden, mullet, anchovies, shad, and herring, there also will be gamefish.

Deeper waters are a collection point for sedimentation, which creates a hostile environment for bottom-dwelling (benthic) invertebrates that are able to live in shallower water. Lower amounts of oxygen, especially during summer months, add to the inhospitable environment. However, certain fish such as flounder and croaker are well adapted to life as bottom dwellers. Flounder, also called flatfish or doormats, undergo a metamorphosis after they are hatched, with one eye rotating around to the same side as the other eye, and the swimming orientation changing to a horizontal position instead of a vertical position. The fish can lie on the bottom and cover itself with silt to hide and wait to ambush its prey.


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