Understanding the Bay Health Index
What is the Bay Health Index?
The Bay Health Index evaluates the nutrient-related health of more than 150 harbors, coves, salt ponds and rivers throughout Buzzards Bay and Vineyard Sound. It assigns waterways scores between 0-100, where 100 represents pristine conditions and 0 indicates severe nitrogen pollution.
The Coalition's Baywatchers program collects summer monitoring data, which the Marine Biological Laboratory analyzes. This assessment has guided regional water protection efforts for nearly three decades.
What does my Bay Health Score mean?
Poor (0-35)
Excessive nitrogen pollution creates unhealthy habitats. Waters show murky visibility under 2 feet, lack eelgrass beds replaced by algae mats, support few fish/shellfish species, and experience low dissolved oxygen. Severely compromised waterways may smell like rotten eggs with mucky sediments.
Fair (35-65)
These transitional areas face improving or worsening conditions. Cloudy water has reduced eelgrass, diminished aquatic diversity, and supports mats of algae. Periodic oxygen depletion and algal blooms stress marine life.
Good (65-100)
Minimal nitrogen pollution supports healthy ecosystems. Features include clear blue water with visibility exceeding 6 feet, lush eelgrass beds, high dissolved oxygen supporting sensitive species, and infrequent algal blooms.
What does the Bay Health Index measure?
The index combines five indicators into a single score that provides a snapshot of a waterway's health:
- Nitrogen (organic) – Organic forms of nitrogen from natural and human sources
- Nitrogen (inorganic) – Dissolved inorganic nitrogen directly usable by algae
- Dissolved oxygen – Oxygen levels critical for fish and shellfish survival
- Algal pigments – Chlorophyll concentration indicating algae abundance
- Water clarity – Secchi depth measuring how deep light penetrates
Note: The index does not include bacteria assessment and does not indicate whether water is safe for swimming or shellfish harvesting.
The Bay Health Index evaluates the nutrient-related health of more than 100 harbors, coves, salt ponds and rivers throughout Buzzards Bay and Vineyard Sound. It assigns waterways scores between 0-100, where 100 represents pristine conditions and 0 indicates severe nitrogen pollution.
The Coalition's Baywatchers program collects summer monitoring data, which the Marine Biological Laboratory analyzes. This assessment has guided regional water protection efforts for nearly three decades.
Poor (0-35)
Excessive nitrogen pollution creates unhealthy habitats. Waters show murky visibility under 2 feet, lack eelgrass beds replaced by algae mats, support few fish/shellfish species, and experience low dissolved oxygen.
Fair (35-65)
These transitional areas face improving or worsening conditions. Cloudy water has reduced eelgrass and supports mats of algae. Periodic oxygen depletion stresses marine life.
Good (65-100)
Minimal nitrogen pollution supports healthy ecosystems with clear blue water, lush eelgrass beds, and high dissolved oxygen.
The index combines five indicators into a single score:
- Nitrogen (organic) – Organic forms of nitrogen
- Nitrogen (inorganic) – Dissolved inorganic nitrogen
- Dissolved oxygen – Oxygen levels for fish survival
- Algal pigments – Chlorophyll concentration
- Water clarity – Light penetration depth
Note: The index does not indicate whether water is safe for swimming or shellfish harvesting.
Become a Baywatcher
Help the Buzzards Bay Coalition keep an eye on the health of the water in your community. Become a Baywatcher, and you can become a "citizen scientist" on the water this summer.
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