The Coastal Bays Health Index, for each region, averages (i.e., equally weights) the six indicators used into a single overarching index score. This score evenly combines all six indicators (chlorophyll a, dissolved oxygen, total nitrogen, total phosphorus, seagrasses, and hard clam) into an overall assessment of health for each region. Each region's score is then area-weighted and averaged to produce the overall Coastal Bays grade.
Chlorophyll a is the green pigment in tiny marine algae (phytoplankton) that produces food. Measuring chlorophyll is based on the amount of phytoplankton (microalgae), which uses both nitrogen and phosphorus to grow. Too much algae in the water reduces water clarity, and decomposing algae leads to reduced dissolved oxygen. In a balanced ecosystem, phytoplankton provide food for fish, crabs, oysters, and worms. When too many nutrients are available, phytoplankton may grow out of control and form algal blooms that can harm fish, shellfish, mammals, birds, and even people.
The oxygen dissolved in water is critical to the survival of fish and shellfish living in it. All of the living creatures in the water need oxygen to survive but as dissolved oxygen levels decrease, it becomes harder for animals to get the oxygen they need to survive. Low dissolved oxygen is often the result of eutrophication, which occurs when there are too many nutrients (such as nitrogen and phosphorus) in the water which cause dense algal blooms to grow. When the algae die and decompose, the decomposition process uses up dissolved oxygen in the water, reducing the oxygen available for fish, and other organisms, which may become stressed or even die.
Nitrogen is important to all living things. Nutrients such as nitrogen occur naturally in both freshwater and saltwater. Plants and animals need nutrients to grow and survive. But when too much nitrogen enters the water it can fuel the growth of algae, creating dense blooms that block sunlight and reduce oxygen for fish and other organisms. Nitrogen runs off the land during rain events. Atmospheric nitrogen from industry settles on the water.
Total phosphorus is an indicator of too much phosphorus in the water. Phosphorus attaches to sediment particles, so phosphorus and sediment pollution are linked. Phosphorus is an essential nutrient for all plants and animals. But too much phosphorus in the water causes algae to grow in large, dense algal blooms, which depletes oxygen for fish and other marine organisms.
Seagrasses provide critical habitat to key species and can improve water clarity. Seagrasses are submerged plants found in shallow waters and are a critical part of the water that provide a number of benefits like buffering coastal communities from storms, removing pollution from the water, and providing shelter for animals.
Hard clams are a benthic organism that requires good water quality conditions to survive. Clam densities relatively close to or above the threshold indicate that the water quality and physical conditions are suitable for clam growth and reproduction.