Tidal Water and Habitat Quality Monitoring
The physical/chemical component of the Maryland Chesapeake Bay Water Quality Monitoring Program consists of data collected 14 times a year at 22 stations located in Marylands Chesapeake Bay mainstem and 12 to 20 times a year at 55 stations sampled in the tidal tributaries. This program assesses the water quality by evaluating the levels of nutrients and closely related habitat impacts such as dissolved oxygen and water clarity. One of the main goals of the Chesapeake Bay restoration is to reduce the impacts of excess nutrients on the Bay and these measures provide some of the most direct linkages to management programs that are achieving this goal. The Chesapeake Bay Program jurisdictions have agreed to reduce nitrogen, phosphorus and sediment pollution to the Bay. Reduction in nutrients and sediments will result in improvements in dissolved oxygen levels and in the habitat for submerged aquatic vegetation (SAV). This monitoring also determines attainment of non-attainment of water quality criteria.
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How Excess Nutrients Harm Freshwater and
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
How Excess Nutrients Harm the Bay
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.
Definition of nutrient loads versus
The Tributary Strategy goals refer to nitrogen, phosphorus and sediment loads, but concentration data are used in assessing status and trends. Results from laboratory analyses of water samples are reported in terms of concentration with a known level of precision and accuracy.
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 grasswild 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 levels are affected by a number of factors, including:
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 lay 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 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.
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
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