Excess amounts of nutrients are the most extensive pollution problem affecting the Chesapeake Bay. Just as nutrient fertilizers are used to promote plant growth on our lawns and farm fields, nutrients in streams and the Bay encourage the growth of aquatic plants. Aquatic plants can be one of two major forms:
- plants with stems and leaves or
- simpler, microscopic plants called algae.
Although some aquatic plants are beneficial and provide food, oxygen, and habitat, excessive nutrients may result in thick growths of aquatic plants (especially algae) that contribute to an unhealthy environment.
The most important nutrients affecting aquatic plant growth are nitrogen and phosphorus. Plant and animal matter (including animal and human waste), fertilizer, and even car and power plant exhaust, all contain nutrients. When these nutrient sources are not controlled, excess nutrients find their way into the groundwater, creeks, rivers and, eventually, the Bay.
How Excess Nutrients Harm Streams and Rivers
In freshwater ponds, lakes, reservoirs, streams and rivers, too much algal growth resulting from high levels of nutrients may degrade the physical habitat of the waterway and may directly affect the health of fish and other aquatic life. For example, heavy growths of algae in streams may form dense mats on the water surface or on the stream bottom. These algal mats reduce the amount and diversity of shelter and nursery areas for small fish and aquatic invertebrates, such as mayflies and crayfish.
Algae floating in the water may shade the bottom, thus reducing the growth of important submerged plants rooted on the bottom. In addition, nutrients from nonpoint source runoff are often associated with high levels of suspended sediments eroded from the land surface or streambanks. These suspended sediments degrade water quality by clouding the water, making it difficult for aquatic organisms to breathe and find food. These suspended sediments settle and smother bottom habitats where aquatic invertebrates and fish find shelter.
How Excess Nutrients Harm the Bay
As in the freshwater environment, excess nutrients can cause too much algal growth in the Bay. Too much algae clouds the water and blocks the light needed by underwater plants called bay grasses or "submerged aquatic vegetation" (SAV). Certain algae also can coat the leaves of the bay grasses, further reducing the amount of light that reaches these plants. Bay grass beds are essential because they provide food, shelter, and nursery areas for fish, blue crabs, and other aquatic animals.
Too much algae can cause other problems in the Bay and its tidal tributaries. When algae die, they settle to the bottom and are consumed by bacteria during the decomposition process. This process consumes oxygen, depleting it from bottom waters. The resulting low dissolved oxygen concentrations drive fish and blue crabs from their preferred habitat and can kill clams, worms, and other small bottom organisms on which crabs feed.
Salinity, the concentration of salts in the water, is extremely important to aquatic life. In the Chesapeake Bay and its tributaries, salinities range from less than 0.5 parts per thousand (ppt) in non-tidal and tidal fresh areas to about 30 ppt at the mouth of the bay. Ocean water is generally around 36 ppt. Salinities are higher on the eastern side of the bay than on the western side, due in part to the Coriolis effect of the earth’s rotation and in part because more freshwater is discharged from rivers along the western shore of the Bay.
Salinities are highest after dry weather periods and lowest after wet weather or snowmelt periods. This is why salinities are usually lowest in April and May, after the spring rains, and increase in August and September after the summer, which is usually a drier period. In rainy years salinities are lower than usual, and in drought years, they are higher.
Many plant and animal species can live only in a restricted salinity range. For example, the bay grass wild celery lives in fresh to brackish reaches of the bay and its tributaries, while eelgrass lives only in higher salinity areas of the bay. Thus changes in salinity determine what plant and animal species live in a given area. Abrupt changes in salinity can be particularly harmful to many species.
Dissolved oxygen is required by all aquatic animals. Low dissolved oxygen levels (hypoxia) can impair animal growth or reproduction, and the complete lack of oxygen (anoxia) will kill animals. Animals with limited mobility such as oysters and clams are particularly vulnerable to hypoxic or anoxic conditions. Even fast swimmers such as fish can become trapped in areas with low oxygen or no oxygen.
Dissolved oxygen levels are affected by a number of factors, including:
- water temperature
- how well mixed the water is (for example, the presence or absence of a pycnocline)
- how much oxygen is being produced by biological processes (such as photosynthesis by plants or protists), and
- how much oxygen is being used up by abiotic and biological processes (e.g., respiration or the decomposition of organic matter such as dead phytoplankton) in the water column or in the sediments and, at the sediment-water interface.
The pycnocline is the place in the water column where there is an abrupt change in density due to a change in salinity and/or temperature. The lower layer is saltier, colder, and denser than the surface layer and forms a "salt wedge," bringing salty water up the bay and its tributaries from the ocean. If the pycnocline is very pronounced, it serves as a partial barrier between the upper layer and the lower layer, and little mixing occurs. This means that oxygen from the surface does not get down to the bottom layer, allowing the bottom layer to become depleted of oxygen.
High levels of algae produce oxygen during photosynthesis while the cells are alive and in the presence of sunlight. However, during the night photosynthesis shuts down, and these same cells use up the available oxygen in the process of respiration. After the algal cells die, they sink and bacteria decompose them; the decomposition process also uses up oxygen from the water column or sediment layer. If a pycnocline is present, dead algal cells fall to the lower layer (below the pycnocline) where they are decomposed. Thus, the bottom waters can become rapidly depleted of oxygen, and oxygen from the upper layer is not well mixed into the lower layer because the pycnocline serves as a partial barrier.
In the Chesapeake Bay, dissolved oxygen levels are usually lowest in bottom waters during the summer months. Oxygen availability can drop to low levels that are likely to harm aquatic animals. Dissolved oxygen levels in the summer bottom waters below 2 mg/L are considered "POOR," and oxygen levels above 5 mg/L are considered "GOOD."
Water temperature affects the rate of growth and reproduction of aquatic animals. Extremes in temperature can kill animals. Water temperatures are affected by air temperatures, rainfall, flow, physiography of the waterbody, and the amount of development in the watershed.
Water temperatures are warmest during the late summer months and coolest during the late winter months. Deeper waters warm up and and cool down slowly. In contrast, small non-tidal tributary reaches and shallow areas can change temperature relatively rapidly.
pH is the measure of acidity (low pH) or alkalinity (high pH) of the water. The average pH in the Chesapeake Bay tidal areas is generally between 7 and 9. During an algae bloom, pH is higher (more alkaline).
The pH scale is the negative logarithmic scale of the activity of hydrogen ions in water. A pH of 7 is neutral, and means there is a concentration of hydrogen atoms of 10-7. A pH of 9 is somewhat alkaline, and has a concentration of 10-9. A pH of 6 is somewhat acidic, and indicates a concentration of 10-6. For example, a pH of 6 has 10 times as many hydrogen atoms as a pH of 7, and thus can be considered 10 times more acidic. A pH of 7 has 10 times as many hydrogen atoms as a pH of 8, and 100 times as many hydrogen atoms as a pH of 9, and so forth.
Abundance of Algae
Algal abundance is estimated based on chlorophyll a measurements. High algal abundance can harm living resources such as bay grasses (SAV) and aquatic animals. Excess algae in the water column or growing on bay grasses can shade out the grasses. In addition, excess algae can cause reduced dissolved oxygen levels during the night (when they respire) and after they die (as they sink and are decomposed). Resulting low or no oxygen conditions can harm or kill aquatic animals, such clams and fish.