How are the scores calculated?

Overall Health Index

The Long Island Sound report card compares 4 indicators (dissolved oxygen, nutrients, chlorophyll a, water clarity) to scientifically derived thresholds or goals. These indicators are combined into an Overall Health Index, which is presented as a subregion percent 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.


Nutrients (nitrogen and phosphorus) are the building blocks for life, but too much in a natural system can lead to problems. Our communities contribute excess nutrients to the Sound from wastewater, septic systems, fertilizer, and fossil fuel burning. Nitrogen fertilizes excessive growth of plant-like organisms (phytoplankton and seaweed), leading to algae blooms. As these organisms and the animals that feed on them respire, die, and decompose, oxygen in the water is depleted. Our other indicators capture the impact of high nutrient inputs (low DO, high chlorophyll a, low water clarity). When conditions are very bad, we sometimes see fish kills and harmful algal blooms.

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.