Photos of the Long Island Sound

Research & Monitoring

Status & Trends

LISS Ecosystem Targets and Supporting Indicators

Water Clarity

Improve water clarity by 2035 to support healthy eelgrass communities and attainment of the eelgrass extent target.

* Click labels in legend to hide data and adjust scale

Water Clarity in the open waters of the Sound is used for this ecoystem target because there is currently no systematic tracking of water clarity in embayments and near-shore waters where eelgrass can grow (see Challenges).

Open Waters
Western NarrowsEastern NarrowsWestern LISoundCentral LISoundEastern LISoundOverall
20131 (F)3 (C)4 (B)4 (B)5 (A)4 (B)
20151 (F)3 (C)5 (A)5 (A)5 (A)5 (A)

Status and Trends

In the open waters of Long Island Sound (where water quality is systematically tracked) there is a general trend of increasing water clarity from west to east in the main stem of Long Island Sound as shown in the water clarity chart for 2016 — from 1.35  meters (∼4.5 feet) in the station farthest west to 3.2  meters (∼10.5 feet) in the station farthest east.


There is currently no systematic tracking of water clarity where eelgrass can grow in Long Island Sound’s embayments and near-shore waters. The water clarity trends in the main stem of Long Island Sound (see chart above) may not reflect the water clarity of individual embayments where eelgrass is most likely to occur.

Embayment water clarity is measured in some Long Island Sound embayments by the Unified Water Study of Save the Sound but there is currently no systematic reporting of the information.

Not all the eastern Long Island Sound embayments, which are the most hospitable to eelgrass, are monitored for water clarity.

How is This Target Measured?        

Routine monitoring of water clarity is done monthly throughout the year and biweekly in the summer by the Connecticut Department of Energy and Environmental Protection (CTDEEP). Water clarity is measured by optical techniques including Secchi disks or photosynthetically active radiation (PAR) sensors.   

The chart at the top of the page shows the Secchi Disk Depth in meters at stations going from west to east in the main stem of Long Island Sound. Secchi Disk Depth is a measure of water clarity and generally increases from west to east in Long Island Sound, indicating clearer water in the east.  Water clarity in individual nearshore embayments which may be suitable for eelgrass may not reflect this pattern.


Water clarity is a measure of light penetration into the water of Long Island Sound and is important in nearshore water to allow the growth of eelgrass, which is an important habitat for numerous marine animals.

Eelgrass in Long Island Sound is currently limited to embayments in the extreme eastern Sound, having disappeared from most of its historic range.

Water clarity can be decreased by high phytoplankton abundance, sediment, or any other types of suspended particles in the water.

Additional Information

  1. 2012 Distribution of Eelgrass

This figure shows the 2012 distribution of eelgrass in eastern Long Island Sound in 2012, as determined by aerial surveys conducted by the US Fish and Wildlife Service (in red). Additional undetermined submerged aquatic vegetation (SAV) is shown in green.  In this area of eastern Long Island Sound eelgrass covers about 2,000 acres of the seabed along the Connecticut coast; Fishers Island, New York; and very limited parts of the shoreline of the North Fork of Long Island.  These 2,000 acres represents almost all the known eelgrass in Long Island Sound. Maximum Eelgrass.

2. Potential with Adequate Water Clarity

Dr. Jamie Vaudrey and colleagues* developed a model showing the maximum potential area of the bottom of Long Island Sound which could be colonized by eelgrass based on water depth, mean tidal amplitude, and percent of surface light reaching the bottom (the Exclusive Band). This assumes all other characteristics of the environment, such as water quality, are optimal, and the area would be reduced by sea level rise.  Currently only 1.28% (8.34 km2 of 651.8 km2) of this potential area is colonized by eelgrass.

*Vaudrey, J.M.P., J. Eddings, C. Pickerell, L. Brousseau., and C. Yarish. 2013. Development and application of a GIS-based Long Island Sound Eelgrass Habitat Suitability Index Model: Final report submitted to the New England Interstate Water Pollution Control Commission and the Long Island Sound Study.


Katie O‘Brien-Clayton, CTDEEP

Evelyn Powers, IEC

Peter Linderoth, Save the Sound

Dr. Jamie Vaudrey, UConn

Source of Data

Water clarity data for the main stem of Long Island Sound from the Connecticut Department of Energy and Environmental Protection (primary data source), and the Interstate Environmental Commission (IEC) for Western Long Island Sound. Additional data for embayments will be sourced in the future through Save the Sound and Dr. Vaudrey.

Data Notes

  • The technical explanation on how the target was selected is found in Appendix B of the Comprehensive Conservation and Management Plan.




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