Dead Zones
Most animals require oxygen to live—even animals that live under water. Water contains all sorts of dissolved substances, including oxygen gas. The most important source of dissolved oxygen in water is the atmosphere. Wind and wave action help oxygen mix into the water. Two other important sources of dissolved oxygen are phytoplankton and SAV. They photosynthesize and release oxygen into the water. Rivers flowing into the Bay carry a fresh supply of oxygen to the Bay's water.
This graph indicates the percentage of water in the Chesapeake Bay that has enough dissolved oxygen to support a healthy ecological community.
As water flows over the gills of an aquatic animal, dissolved oxygen is absorbed and enters its bloodstream. The blood carries the oxygen to every cell of its body. Those cells require oxygen for the process of cellular respiration. Dissolved oxygen is an important abiotic factor that helps maintain a diverse ecological community in the Chesapeake Bay.
Runoff from land carries sediment and nutrients into the Bay. However, sometimes the runoff contains high levels of fertilizers washed from farms, suburban lawns, golf courses, and sewage plants. Fertilizers contain large amounts of nitrogen and phosphorous, which provide nutrients for the algae in the Bay and can cause a sudden population explosion called an algal bloom. Any increase in these nutrients is called eutrophication.
Algal blooms increase turbidity in the Bay. The cloudy water often takes on a thick, pea-green appearance and blocks out the sunlight. That means the SAV cannot photosynthesize. Without the ability to make food, the SAV dies.
Aquatic ecosystems cannot sustain a runaway algal population for long because the algae use up all the resources. The algae begin to die and sink to the bottom. The dead algae and SAV become food for microbial decomposers. This causes another imbalance. With all the dead and decaying matter to consume, the microbes multiply, causing another population explosion. This time, the decomposers use up all the oxygen. Oxygen is an important limiting factor. Without it, the ecosystem collapses and the area becomes a dead zone where very few organisms can survive. Most areas of the Bay with low dissolved oxygen are the result of a complex interaction among natural and human-made factors.
Dead zones reduce usable habitat for many species in the Bay. This causes an increase in competition, predation, and transmission of diseases. Dead zones can also fragment populations, separating interdependent populations from each other. Winds can push dead zones from the middle of the Bay into shallower water, trapping and suffocating oxygen-dependent organisms.
Development Pressures
Before the Europeans arrived in the 17th century, Chesapeake Bay supported a population of Native Americans who depended on the Bay for modest amounts of seafood. The impact to the ecological balance was minimal and the Bay naturally replenished resources taken from the water. However, as the human population increased, critical habitats in the watershed were destroyed and the competition for the resources in the Bay became overwhelming. Natural buffers and balances failed. The ecosystem degraded.
Today, the population of the Chesapeake watershed has grown to over 16 million people! Many live in waterfront cities that are among the largest along the east coast. Scientists use the term population density to describe the concentration of a species. Cities, such as Baltimore, in which a large number of individuals live in a small area, have a high population density. Rural areas around the Bay have a low population density, but a very high influence on the health of the Bay. Improperly treated sewage, excessive runoff, increased animal manure, soil erosion, and heavy use of fertilizers add to the pollution and poor water quality of the Chesapeake Bay.
When pollution comes from a single known source, it is considered a point source. Pollution from a particular gas station or sewage plant are examples of point source pollution. When the pollution does not come from a single known source, it is a non-point source. Examples of non-point sources are farmlands and the system of storm drains in a city. Both types of pollution are responsible for degrading the quality of water in the tributaries and the estuary.
Eutrophication and increased instances of algal blooms are a global problem, as seen here in the waters surrounding Lake Starnberg in Upper Bavaria, Germany.