Overall Health Index

The Chesapeake Bay report card compares 7 indicators (dissolved oxygen, nitrogen, phosphorus, chlorophyll a, water clarity, aquatic grasses, benthic community) to scientifically derived thresholds or goals. These indicators are combined into an Overall Health Index, which is presented as a subregion percent score. Other indicators (blue crab, bay anchovy, striped bass) presented on these pages are not included in the score.

Dissolved Oxygen

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.

Chlorophyll a

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.

Water Clarity

Water clarity is a measure of how much light penetrates though the water column. Water clarity is dependent upon the amount of particles (e.g. suspended sediment and plankton) and colored organic matter present. Clear water is especially critical for seagrasses since, like all plants, they need to be able to absorb the sun’s rays to grow. Excess sediment in the water reduces water clarity by blocking sunlight to seagrasses. Fish and other organisms in the water need aquatic grass habitat to thrive.

Aquatic Grasses

Aquatic grasses 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.

Benthic Community

The Benthic Index of Biotic Integrity (BIBI) measures the condition of the benthic community (e.g. clams and others) living in or on the soft bottom areas of the Bay. These organisms are a key food source for many species including perch, spot, and croaker.

Blue Crab

Blue crabs are an important living resource in the Bay. They are both predator and prey in the Bay's food web. They use aquatic grasses habitat to hide from predators and to mate and molt. Fishing for blue crabs in the Bay is a recreational and commercial past time.

Bay Anchovy

Bay anchovy is an important living resource in the Bay because it is one of the most abundant schooling fishes in the Bay. It is an important food source for top predators and eats zooplankton (tiny animals floating in the water column). Most bay anchovy do not migrate out of the Bay, but instead spend their whole life here.

Striped Bass

Striped bass, or rockfish, is a key top predator, and uses the Bay as an important spawning and nursery area. Striped bass is Maryland’s state fish and a popular commercial and recreational fishery.