Wednesday, September 1, 2004
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The Bay
Source: Microsoft Encarta 2004 |
The Chesapeake Bay Watershed spans over 64,000 square miles and includes Maryland, Virginia, the District of Columbia, Delaware, Pennsylvania, West Virginia, and New York. Many rivers flow into it, most notably the Potomac and the Susquehanna. More than 15 million people live in the Watershed. Therefore, there is little wonder as to why the Bay is in such trouble!
The EPA has placed the Chesapeake Bay on its "impaired waters" list. That is because the Bay is a mess. Sediments flowing into its tributaries block sunlight for submerged aquatic vegetation (SAV) and carry phosphorus. Organic waste brings in lots of nitrogen. Excess nitrogen and phosphorus cause excess algae growth that further blocks sunlight. When SAV beds don't get sunlight, they die, like any vascular plant. When SAV beds die, oysters, blue crabs, and other Bay dwellers suffer. The algae will deplete the oxygen in the bottom waters, which is also a disaster for fish and bottom dwellers. They must find somewhere else to live, or they die. |
Over the 400 years or so that the Watershed has been developed, some natural defenses fail. Forests usually block nitrogen and phosphorus from flowing into the Bay. Now that many of them are gone and replaced with much less absorbent urban sprawl and farms, the nutrients are free to run right into the Bay and become too many for the ecosystem to handle. Organic wastes, including human waste, spill into the Bay and add to the already above-the-healthy-limit nitrogen content. Things are looking pretty bad for the Chesapeake.
What lives in the Bay? How do these nutrients work? How do geological and weather patterns affect the Bay? With the combination of endangered plants and animals, nitrogen cycles, water quality, light dependence, and soil management, what we have here is a hot subject for estuarine scientists!
How's Life?
Let's begin with the Bay Grasses, a.k.a Submerged Aquatic Vegetation (SAV). Lots of SAV live in the Chesapeake Bay. The Common Waterweed, Elodea canadensis, is found throughout North America, likes high concentrations of nitrogen and phosphorus, and while it prefers freshwater, it can stand brackish, slow-moving tributaries. Eelgrass, Zostera marina, on the other hand, prefers the high salinity in lower tributaries. It is considered the only true seagrass in the Chesapeake Bay, and blue crabs, Callinectes sapidus, rely on it for mating and hiding juveniles. Wild celery, Vallisnera americana, resembles eelgrass, although it prefers lower salinities, slow-moving water, and murky areas, and it can hold out well against strong currents.
SAV beds play very many roles in the lives of Bay animals and protists. They serve as a nice hiding place from predators. They photosynthesize and therefore add to the oxygen in the bottom waters so fish and other animals can breathe. They hold sediment in place, keeping turbidity down. Also, they thrive on the many nutrients in the Bay. It is sad that so many of these SAV beds are gone or severely destroyed due to pollution and other factors, since this is so detrimental to the other Bay residents. As already mentioned, the main problem is a surplus of nitrogen and phosphorus in the water that stimulates algae growth to above-normal levels.
Algae are very simple organisms, but there sure are a lot of them! There's also blue-green algae, but they are technically bacteria (cyanobacteria, actually) and play a minimal role in Bay processes. The focus here is on the phytoplankton, the plantlike protists floating around in the water including the very many taxa that make up true algae.
For the most part, all the algae does is block sunlight and deplete oxygen when it dies and decomposes. Over the past few years, however, the dinoflagellate alga Pfiesteria piscicida has terrorized the Bay. It secretes a deadly toxin and attaches to fish and kills them. Humans who swim in water with this vicious alga get very sick, but they have all recovered. There are also concerns about eating fish from water infested with P. piscicida.
Needless to say, there are plenty of fish varieties in the Chesapeake. Bluegills, Lepomis macrochirus in family Centrarchidae, prefer salinity below 18 ppt (parts per thousand) and spawn in freshwater. They feed on small arthropods, plants, and smaller fish, while water snakes, birds, and bigger fish feed on them. Atlantic menhaden, Brevoortia tyrannus in family Clupeidae, is an abundant estuarine fish, spawns in the ocean, and then moves into less saline waters for growth. They eat zooplankton, phytoplankton, and organic plant detritus. The very popular striped bass, Morone saxatilis, is anadromous, meaning it lives in the ocean but goes to fresher water to spawn. Also called rockfish, they eat crustaceans, fish, mussels, squids, and worms. And these are just to name a few!
Newly Caught Blue Crabs |
Of course, what is the Chesapeake Bay without the crustaceans? Blue crabs, Callinectes sapidus, are synonymous with the Bay and Maryland in general. As juveniles, they live in SAV beds to hide from predators, such as eel, rockfish, croakers, and many others. These crabs have a wide variety of food, from plants to worms, other crustaceans to detritus (organic or geological debris), dead fish to their own young. They have ten legs (hence the order Decapoda) and are, appropriately enough, blue and sometimes greenish. It is slowly becoming common knowledge in the Chesapeake area that the males' ventral side (apron) looks like the Washington Monument, and the females' apron looks like the Capitol dome. It is important to tell them apart to be sure to catch and eat only males. They serve as food for us as well as sharks, sea turtles, and other sea and bay dwellers. |
Blue crabs have very funny looking fellow-arthropods called horseshoe crabs, Limulus polyphemus. Actually, horseshoe crabs aren't crabs at all. Just look at them. Taxonomically, they're more like big, shelled spiders. They've looked about the same for a few hundred million years now and have book gills, leaving them to bear some paleontological significance. They're basically extant trilobites. They eat clams and worms. Horseshoe crabs can survive for a while outside of water. And they're quite fun to catch!
I can describe an oyster in one word: Delicious! That's the American oyster, Crassostrea virginica, and they used to fill up the Bay so much they were a hazard for the ships passing through in colonial times! Oysters are filter-feeders. They keep the Bay water clean by eating algae and nutrients. It does not help the current state of the Bay that there are so few oysters left now. Loss of habitat due to algal blooms has caused this, but not by itself. Pollutants kill young oysters. Also, two parasites, MSX (Haplosporidium nelsoni) and Dermo (Perkinsus marinus), are very lethal to oysters and have made their own significant contribution to the decline of these bivalves. Alongside the efforts of Maryland and Virginia to restore oyster reefs, there is also plenty of research on these two parasites, for which there are still no known remedies.
This by no means sums up all known life in the Bay. It is also an important habitat for birds, insects, zooplankton, marine mammals, sharks, and many more. There are just very many individuals of a wide variety of species whose lives depend on the health of the Chesapeake Bay.
Watch Your P's and N's
Now don't misunderstand. Nutrients themselves are good for the Bay and any ecosystem. The problem is having way too much of them. Nitrogen and phosphorus are constantly cycling through land, air, water, and organisms and are necessary for the growth of plants and algae. When there is too much of these nutrients, the algae grows and reproduces like crazy, forms algae blooms, and blocks the sunlight from reaching underwater plants. Sometimes it grows right on Bay grasses and other underwater plants. Dead algae invites bacterial degradation that uses up the deep water dissolved oxygen, causing hypoxia (low oxygen) and even anoxia (no oxygen!), and suffocates fish. This is happening across the Bay, so the fish and other fleeing organisms run out of places to go to be able to breathe. This over-enrichment of a body of water is called eutrophication. |
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Contaminants are the other main problem in the Bay. Chlordane, polychlorinated biphenyls, mercury, and other toxic chemicals bioaccumulate in many fish, making them unsafe to eat. These toxins enter the Bay via point sources (specific sewage and industrial waste flows, for example) or non-point sources, such as land run-off or atmospheric input.
Nutrients and their consequent algal blooms result in depletion of vegetation and dissolved oxygen. Contaminants, however, poison the environment directly. Chlordane, DDT, and many others have been banned because of their abilities to bioaccumulate, and they are carcinogenic. They kill animals and/or plants, or at least make them unsafe to eat. Often the water can become unsafe for swimming if the contamination is bad enough! Then again, the aforementioned Pfiesteria piscicida blooms have the same effect.
What is being done about reducing nutrient content? Wastewater management stations have performed Biological Nutrient Removal techniques in their secondary treatment practices, after they've taken care of solid materials. Phosphate detergents have been banned, also reducing the phosphorus that reaches the Bay. New farming techniques help reduce runoff, such as planting different crops every other row, which is called strip cropping. This way, the vegetation catches the nutrients before they get to the water.
Sunshine over the Chesapeake Bay |
How Deep, How Dark
As it is just about anywhere, light is a very important factor in the Chesapeake. The crisis with Submerged Aquatic Vegetation is their lack of sunlight due to algal blooms, algae sticking to them, suspended sediment, and other threats. However, the exact threat can be assessed. This is why Bay physicists calculate the Secchi depth, which is essentially the way to figure water clarity. The formula factors in suspended matter, such as plankton, sediment, and detritus. It involves a Secchi disk, which is a plate with black and white quadrants and a weight on the bottom. This plate is lowered into the water until it is no longer visible, and this is the Secchi depth. Shallower the depth, there is more turbidity and therefore more of a threat to Bay organisms. Deeper the Secchi depth, less turbidity and less threat. Water clarity may also be assessed using a device called a transmissometer.
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Salinity is also a very important physical factor. An estuary such as this usually has fluctuations in salt concentration, so its organisms are quite used to it. It is the extended periods of too low or too high salinities for a given area that causes certain life forms to suffer. These long changes can also draw some fishes and other animals into different habitats entirely.
As already mentioned above, dissolved oxygen content is critical. It is required above a certain concentration to keep organisms from being stressed or killed. Warmer water cannot dissolve oxygen as well as colder water. Dissolved oxygen content is higher during the afternoon, while the sun is out and all the SAV and algae are photosynthesizing. In areas with high enrichment (nutrient pollution), however, these fluctuations are greater and can be lethal to fish. Appropriately enough, deeper water experiences lower dissolved oxygen concentrations.
How can I forget pH? In the Bay and tributaries, algal blooms and photosynthesis raise the pH (making the water more basic or alkaline) and acid precipitation lowers the pH (making the water more acidic, obviously). Aquatic organisms like the water pH to be between 5.5 and 8.5, with 7 being neutral of course.
About The Proverbial Tray
The Chesapeake Bay is the largest estuary in the United States. What is an estuary? It's not freshwater. It's not saltwater. An estuary is somewhere in between, containing more saline (closer to ocean) and less saline (closer to tributary mouths) regions, which works quite well with anadromous species. In this case, the Chesapeake serves as the gathering place for millions of gallons of fresh water from rivers Susquehanna, Potomac, Choptank, Severn, James, Rappahannock, Occoquan, Shenandoah, Monocacy, Wicomico, Nanticoke, Patuxent, and numerous others.
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The Bay is only 10,000 years old. That's young for a geological structure! Originally, it was just the lower portion of the Susquehanna River, and over time the seawater reached up the river valley, submerging the shoreline until it formed the brackish Bay we know and love today. Since it is so shallow compared to its area, it is sometimes referred to as a "tray", where all of its tributaries pour in their waters and any other materials they carry. There are several islands in the Bay, including Smith Island, Tangier Island, Kent Island, and Tilghman Island, and they are filled with muddy wetlands and tall marsh grasses. Beautiful! |
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Summers are often very hot and humid. Some years there are thunderstorms every day. Others bring several weeks at a time of no rain at all. While the storms do allow for more mixing within the Bay, which is helpful against hypoxia in and of itself, they do wash more sediment and nutrients into the already enriched ecosystem. The dry summers don't wash so many particles into the Bay, but there is also little mixing. Every once in a while, we have a hurricane. Last year, we had Isabel, and back in 1999 we had Floyd. The Chesapeake region usually receives the weaker remains of hurricanes that reach North Carolina or even Florida, or those that did not keep their hurricane status by the time they hit the shore. Either way, it means torrential rainfall and lots of nutrient flow and mixing. The Maryland Department of Natural Resources keeps water quality stations along the Bay shore and along the major tributaries, where they regularly monitor water clarity, nutrient content, salinity, dissolved oxygen, and other factors. This way, they can more accurately assess the effects hurricanes and other extreme weather patterns have on the various facets of the Chesapeake Bay.
That's All For Now!
There is lot to the Chesapeake Bay, way more than even touched on here. Nor do the characteristics and ecological crises mentioned above occur only in the Chesapeake. See my sources and links below if you're interested in learning more.
Sources
Acknowledgements
- Russell Mader - Chesapeake Bay Program
- Deborah J. Rudy - Alliance for the Chesapeake Bay
- William Romano - Maryland Department of Natural Resources
Where to Find Related Research
- Chesapeake Bay Program (link above)
- Maryland Department of Natural Resources (link above)
- University of Maryland Center for Environmental Science, Chesapeake Biological Laboratory
- Virginia Institute of Marine Science (link above)
- Old Dominion University, Center for Coastal Physical Oceanography
- United States Geological Survey, Chesapeake Bay
- Clemson University, Department of Biological Science (striped bass)
- National Oceanic and Atmospheric Administration (NOAA)
- University of California at Santa Cruz, Department of Ocean Sciences
- University of Maryland, Wye Research and Education Center
- United States Department of Agriculture (USDA), Agricultural Research Service
- Louisiana State University, Wetland Biogeochemistry Institute
- North Carolina State University, Department of Marine, Earth, and Atmospheric Sciences
- University of Maryland, Center for Environmental Science, Horn Point Laboratory
- Salisbury University, Henson School of Science and Technology
- University of Delaware, College of Marine Studies
- And many more!
For More Information
If there is any information in this Article you know to be false or falsely implied, let me know.
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© 2004-2007 Katrina G. Moncure unless otherwise stated. Do not copy anything on this site without my permission. All rights reserved.
