What Do They Measure in the Chesapeake Bay?

We hear a lot about the “health of the Chesapeake Bay”, but what does that really mean?  The health of the bay encompasses three categories; water quality, habitats and lower food web, and fish and shellfish.  These three categories are combined to give you the overall “Bay Health” score, which, as of 2008 is 38%, with 100% being a fully restored ecosystem.  Since the dairy industry is continually asked to do more to improve the water in the Chesapeake Bay it is important that we understand how we are being evaluated.  

“Water quality” gains the most attention in many bay health discussions and is directly impacted by agricultural practices.  With respect to the Chesapeake Bay, water quality encompasses four specific parameters:  dissolved oxygen levels, water clarity, chlorophyll a levels, and chemical contaminants.  Unfortunately, the long- and short-term trends show that only 21% of the water quality goals have been reached and they are not moving in the right direction. 

Dissolved oxygen is the amount of oxygen in water.  Just like animals on land, fish and other aquatic organisms need oxygen to survive, especially during spring spawning in the shallow waters.  Oxygen in the bay water comes from infiltration from the air, underwater grasses (which work much like plants on land), and river and ocean water.  Settling and decomposition of algae is the main cause for low dissolved oxygen levels in the Chesapeake Bay.  Algae are a food source for many organisms in the Chesapeake Bay, but if there is more algae than the fish and aquatic organisms can eat it will grow out of control and eventually die.  When it dies it decomposes.  The decomposition consumes much of the oxygen in the water creating an environment where fish and other aquatic organisms cannot survive. These algae blooms are caused by excessive nutrients (nitrogen and phosphorus) that enter the bay from rivers and streams.  Therefore, dissolved oxygen levels are a very good indicator of nutrient loading to the Chesapeake Bay.

Water clarity is a measure of the amount of sunlight that can penetrate the water.  Just like land plants, aquatic grasses need light to grow, and light is also important to aquatic organisms so they can see prey and avoid predators.  Sediment and plankton are two of the main causes of decreased clarity, but algae blooms can also decrease water clarity.  Therefore, water clarity is influenced by the amount of nutrients reaching the bay as well as the sediment that leaves the land.

Chlorophyll a is a green pigment that allows plants to grow, much like plants on land, but a is predominant in algae.  Therefore, chlorophyll a is a direct measurement of algae in the bay.  As mentioned previously, algae is the base of the food chain in the bay so it generally a beneficial organism, but if there is more than can be utilized it can decrease clarity and reduce oxygen in the water.  It is also a very good indicator of nutrient loading.

Chemical contaminants are the last component included in the overall water quality score.  Chemicals are monitored on 89 tidal water segments.  Metals, like Mercury, polychlorinated biphenyls (PCBs), hydrocarbons from gas, oil, and coal burning, and herbicide and pesticide residues are some of the major chemicals that are being monitored.  Unlike the other water quality contaminants this one can have a direct impact on human health because fish and aquatic organisms tend to concentrate chemicals.  Therefore, warnings are in place on the amount of fish people should eat from the bay. 

Obviously not all of these water quality conditions are caused by agriculture, but agriculture does play a role and understanding the score card will make it easier to play by the rules.