Geoscience Reference
In-Depth Information
These remarkable living structures have earned the eastern oyster praise as an ecosystem engineer. Reefs
perform a number of important ecological functions, serving as predator refuges as well as feeding and nesting
sites for a wide variety of species. With their rugged and ragged architecture, reefs contribute to habitat com-
plexity and thus enhance productivity and biodiversity. More than 300 species have been found living and
growing in the reefs.
Chesapeake Bay oyster reefs have among the highest densities of fish ever recorded outside of tropical coral
reefs and are home to a greater diversity of species than the unstructured soft-bottom habitat nearby. Some
fifty-seven species of fin fish have been spotted on oyster reefs, including white perch, black sea bass, striped
bass, and Atlantic silverside. The species that frequent oyster reefs fall into three groups: reef residents, such
as the blennies and gobies, which use it as a primary habitat; so-called facultative residents, which are often
found in and around reefs; and transient species, which are generally more far ranging but tend to forage on or
near reefs. Some residents, like the naked goby and striped blenny, become prey for transient species such as
striped bass. Generally, however, reefs serve as nursery habitats, necessary for the breeding, feeding, and
growth of gobies, blennies, skilletfish, and oyster toad fish. In winter, diving ducks and loons feed on and over
the reefs.
Large oyster reefs perform other ecological services, such as preventing shoreline erosion. But perhaps their
most important role is as a filtration system. As filter feeders, oysters pump 80 to 100 liters (21 to 26 gallons)
of water through their bodies daily. In the process, they glean phytoplankton as food and remove particulate
matter from the water column, which they transport to the bottom. The result is clearer water, which allows
more light to penetrate, enhancing the growth of sea grasses and bottom-dwelling algae. As we have already
seen, sea grass beds serve as important nursery habitats for blue crabs and striped bass. It is difficult to estim-
ate the overall positive effect of oyster reefs on the ecosystem, but some idea of the scale of the impact can be
deduced from this fact: at one time, oysters could recycle the entire water volume of the bay in 3.3 days, a pro-
cess that now takes 325 days.
Clearer water means more growth of submerged underwater vegetation, which depends upon water clarity
for photosynthesis. Some sixteen such sea grasses grow in the Chesapeake, each occupying a different part of
the bay, from the mouth to the head, depending upon its tolerance to varying salinities. Wild celery grows in
freshwater; sago, pondweed, redhead grass, and widgeon grass in the brackish waters of the mid-bay; and eel-
grass at the mouth of the bay, where waters are essentially salt. Declining water quality due to sedimentation
and algal growth has reduced visibility to 1 meter in many areas of the bay, with the consequence that sea
grass beds have declined in extent by as much as 90 percent since the 1970s.
Like oyster beds, sea grass beds act like bio or ecological engineers, providing multiple environmental ser-
vices. The grasses act as shock absorbers for wave energies, thus reducing shoreline erosion. Their undulating
blades also trap sediments and consolidate them around their roots. In addition, the grasses sop up nutrients
such as phosphorous and nitrogen, which pour into the bay from a host of sources, ranging from agricultural
runoff to urban sewage. Their ability to absorb these nutrients (which in excess act as pollutants) prevents algal
bloom in the spring. In the fall, when the grasses die back, they gradually release the nutrients into the bay,
again mitigating their potentially negative effects on water quality.
The sea grass beds also greatly enhance the physical complexity of the shoreline environment, and as a res-
ult become a hiding, feeding, and breeding place for a diversity of organisms. A handful of healthy sea grass is
alive with shrimp, little fishes and crabs, sea horses, and diamond back terrapins. Eelgrass blades also harbor a
rich epiphytic community—microalgae, hydroids, bryozoans, sponges, and barnacles—which use the blades as
support. Like oyster beds, sea grass beds serve as important nursery habitats for shellfish and finfish, and eel-
grass beds in particular are especially important to juvenile blue crabs, which use them to avoid predators. Sea
grasses are also an important source of food for resident and migratory species. Bay scallops depend upon eel-
grass beds for their detritus-based diet and also use it as a nursery area, attaching themselves to the blades until
