indicator

Embayment Water Clarity

Year	Alewife Cove (m)	Mystic Harbor (m)	Niantic River (m)	Stonington Harbor (m)	Wequetequock Cove (m)	Goal (m)
2018	1.03	0.67	1.24	2.03	0.61	0.7
Year 2018 Alewife Cove (m) 1.03 Mystic Harbor (m) 0.67 Niantic River (m) 1.24 Stonington Harbor (m) 2.03 Wequetequock Cove (m) 0.61 Goal (m) 0.7
2019	0.69	1.20	1.40	1.57	1.16	0.7
Year 2019 Alewife Cove (m) 0.69 Mystic Harbor (m) 1.20 Niantic River (m) 1.40 Stonington Harbor (m) 1.57 Wequetequock Cove (m) 1.16 Goal (m) 0.7
2020	1.25	1.36	1.59	1.46	1.37	0.7
Year 2020 Alewife Cove (m) 1.25 Mystic Harbor (m) 1.36 Niantic River (m) 1.59 Stonington Harbor (m) 1.46 Wequetequock Cove (m) 1.37 Goal (m) 0.7
2021	0.90	0.99	1.51	0.81	1.68	0.7
Year 2021 Alewife Cove (m) 0.90 Mystic Harbor (m) 0.99 Niantic River (m) 1.51 Stonington Harbor (m) 0.81 Wequetequock Cove (m) 1.68 Goal (m) 0.7

Year	Connecticut River (m)	Goldsmiths Inlet (m)	Housatonic River (m)	Mattituck Creek (m)	Mystic River (m)	Nissequogue River (m)	Northport Harbor (m)	Port Jefferson Harbor (m)	Goal (m)
2018	0.75	1.15	0.48	1.07	1.39	0.78	0.65	0.62	0.70
Year 2018 Connecticut River (m) 0.75 Goldsmiths Inlet (m) 1.15 Housatonic River (m) 0.48 Mattituck Creek (m) 1.07 Mystic River (m) 1.39 Nissequogue River (m) 0.78 Northport Harbor (m) 0.65 Port Jefferson Harbor (m) 0.62 Goal (m) 0.70
2019	0.84	1.13	0.59	0.98	0.87	1.51	0.82	1.22	0.70
Year 2019 Connecticut River (m) 0.84 Goldsmiths Inlet (m) 1.13 Housatonic River (m) 0.59 Mattituck Creek (m) 0.98 Mystic River (m) 0.87 Nissequogue River (m) 1.51 Northport Harbor (m) 0.82 Port Jefferson Harbor (m) 1.22 Goal (m) 0.70
2020	0.84	1.18	0.61	0.98	1.57	1.30	0.93	1.40	0.70
Year 2020 Connecticut River (m) 0.84 Goldsmiths Inlet (m) 1.18 Housatonic River (m) 0.61 Mattituck Creek (m) 0.98 Mystic River (m) 1.57 Nissequogue River (m) 1.30 Northport Harbor (m) 0.93 Port Jefferson Harbor (m) 1.40 Goal (m) 0.70
2021	0.41	0.98	0.68	0.89	1.39	0.97	0.64	1.15	0.70
Year 2021 Connecticut River (m) 0.41 Goldsmiths Inlet (m) 0.98 Housatonic River (m) 0.68 Mattituck Creek (m) 0.89 Mystic River (m) 1.39 Nissequogue River (m) 0.97 Northport Harbor (m) 0.64 Port Jefferson Harbor (m) 1.15 Goal (m) 0.70

Year	Connecticut River (1/m)	Goldsmiths Inlet (1/m)	Housatonic River (1/m)	Mattituck Creek (1/m)	Mystic River (1/m)	Nissequogue River (1/m)	Northport Harbor (1/m)	Port Jefferson Harbor (1/m)	Alewife Cove (1/m)	Mystic Harbor (1/m)	Niantic River (1/m)	Stonington Harbor (1/m)	Wequetequock Cove (1/m)	Goal (1/m)
2018	2.27	1.48	3.55	1.59	1.22	2.19	2.63	2.74	1.65	2.55	1.42	0.84	2.79	0.70
Year 2018 Connecticut River (1/m) 2.27 Goldsmiths Inlet (1/m) 1.48 Housatonic River (1/m) 3.55 Mattituck Creek (1/m) 1.59 Mystic River (1/m) 1.22 Nissequogue River (1/m) 2.19 Northport Harbor (1/m) 2.63 Port Jefferson Harbor (1/m) 2.74 Alewife Cove (1/m) 1.65 Mystic Harbor (1/m) 2.55 Niantic River (1/m) 1.42 Stonington Harbor (1/m) 0.84 Wequetequock Cove (1/m) 2.79 Goal (1/m) 0.70
2019	2.03	1.51	2.89	1.73	1.95	1.13	2.08	1.41	2.48	1.42	1.22	1.08	1.47	0.70
Year 2019 Connecticut River (1/m) 2.03 Goldsmiths Inlet (1/m) 1.51 Housatonic River (1/m) 2.89 Mattituck Creek (1/m) 1.73 Mystic River (1/m) 1.95 Nissequogue River (1/m) 1.13 Northport Harbor (1/m) 2.08 Port Jefferson Harbor (1/m) 1.41 Alewife Cove (1/m) 2.48 Mystic Harbor (1/m) 1.42 Niantic River (1/m) 1.22 Stonington Harbor (1/m) 1.08 Wequetequock Cove (1/m) 1.47 Goal (1/m) 0.70
2020	2.03	1.44	2.79	1.73	1.08	1.30	1.83	1.23	1.36	1.25	1.10	1.20	1.24	0.70
Year 2020 Connecticut River (1/m) 2.03 Goldsmiths Inlet (1/m) 1.44 Housatonic River (1/m) 2.79 Mattituck Creek (1/m) 1.73 Mystic River (1/m) 1.08 Nissequogue River (1/m) 1.30 Northport Harbor (1/m) 1.83 Port Jefferson Harbor (1/m) 1.23 Alewife Cove (1/m) 1.36 Mystic Harbor (1/m) 1.25 Niantic River (1/m) 1.10 Stonington Harbor (1/m) 1.20 Wequetequock Cove (1/m) 1.24 Goal (1/m) 0.70
2021	4.12	1.74	2.52	1.91	1.22	1.75	2.65	1.47	1.88	1.71	1.15	2.11	1.01	0.70
Year 2021 Connecticut River (1/m) 4.12 Goldsmiths Inlet (1/m) 1.74 Housatonic River (1/m) 2.52 Mattituck Creek (1/m) 1.91 Mystic River (1/m) 1.22 Nissequogue River (1/m) 1.75 Northport Harbor (1/m) 2.65 Port Jefferson Harbor (1/m) 1.47 Alewife Cove (1/m) 1.88 Mystic Harbor (1/m) 1.71 Niantic River (1/m) 1.15 Stonington Harbor (1/m) 2.11 Wequetequock Cove (1/m) 1.01 Goal (1/m) 0.70

Focus Question

Do Long Island Sound’s embayments support healthy eelgrass beds?

Answer: Yes, but only in some areas. Tracking water clarity in Long Island Sound’s embayments, or bays and harbors, indicates how healthy those areas are for eelgrass since eelgrass requires clear water to grow. Knowing the embayments’ water clarity will guide the Partnership’s efforts to restore and protect areas where eelgrass can grow.

What Was Measured

Secchi disk depth and light attenuation coefficient from Save the Sound’s Unified Waters Study.

A Secchi disk is a black and white disk which is lowered into the water until you can no longer differentiate the black and white quarters of the disk. This measures the Secchi depth and is an indicator of water clarity. The higher the Secchi disk depth value, the clearer the water.

Light attenuation coefficient is a way to measure water clarity specific to eelgrass growth. The lower the value of the light attenuation coefficient, the clearer the water. In Long Island Sound, the light attenuation coefficient necessary for eelgrass growth is less than 0.7. Because eelgrass requires a specific light attenuation coefficient value for growth, this measurement helps environmental scientists and managers better understand the water quality conditions so that they can protect and restore eelgrass.

Scientist measuring water clarity with a Secchi disk.

Status and Trends

Fair

Uncertain

Tracking Embayment Water Clarity helps provide an indication of whether the Partnership is making progress on the Coastal Habitat Objective to protect, enhance, and assess the extent and health of coastal habitats, specifically progress related to eelgrass meadow habitat since eelgrass requires clear waters to grow.

Eelgrass meadows provide important ecosystem services that benefit plants, animals, and humans around the Sound. Eelgrass meadow ecosystem services include:

Supporting biodiversity: Eelgrass meadows provide food and habitat for countless fish, birds, shellfish, and other creatures.
Water quality improvements: Eelgrass meadows add much-needed oxygen to the water and filter extra nutrients like nitrogen out of the water. Eelgrass roots clear up cloudy water by trapping sediments like sand and soil.
Reduces wave energy: Eelgrass meadows can make big waves less powerful, which reduces damage to shoreline habitats from storms and human development.

To keep the Sound’s eelgrass population healthy, the Partnership invests in protecting embayments where eelgrass already grows and restoring embayments where eelgrass has the potential to grow. Dr. Jamie Vaudrey and colleagues created the eelgrass habitat suitability index (see map below) to highlight embayments suitable for eelgrass based on the growing conditions eelgrass needs, like clear water. The Partnership tracks water clarity in embayments that are suitable for eelgrass using Secchi disk depth and light attenuation coefficient.

Map of eelgrass habitat suitability index in Long Island Sound.

The Partnership tracks water clarity in eelgrass-suitable embayments where eelgrass is currently present:

Alewife Cove (Connecticut)
Mystic Harbor (Connecticut)
Niantic River (Connecticut)
Stonington Harbor (Connecticut)
Wequetequock Cove (Connecticut)

And water clarity in eelgrass-suitable embayments where eelgrass is currently absent:

Connecticut River (Connecticut)
Housatonic River, (Connecticut)
Mystic River (Connecticut)
Goldsmith’s Inlet (New York)
Mattituck Creek (New York)
Nissequogue River (New York)
Northport Harbor (New York)
Port Jefferson Harbor (New York)

In Long Island Sound, the Secchi disk depth necessary for eelgrass growth is greater than 0.7 meters. The higher the Secchi disk depth value, the clearer the water. In 2021, eelgrass-suitable embayments with eelgrass present had Secchi disk depths between 0.8 – 1.7 meters. In 2021, eelgrass-suitable embayments with eelgrass absent had Secchi disk depths between 0.4 – 1.4 meters.

In addition to secchi disk depth, we can also calculate light attenuation coefficient which is specific to eelgrass growth. In Long Island Sound, the light attenuation coefficient necessary for growth is less than 0.7 (i.e., the lower the value, the clearer the water is). The light attenuation coefficient for eelgrass present and absent embayments were 1.01 – 2.11 and 1.22 – 4.12, respectively.

Eelgrass meadow in Fishers Island, New York (photo credit: Cayla Sullivan).

Some embayments show really clear waters, as represented by high Secchi disk depth values (i.e. greater than 0.7 meters), but eelgrass is still not present. This tells the Partnership that there may be other stressors in these embayment that make it hard for eelgrass to grow. Sometimes these embayments have high nutrient levels that cause other plants, like algae, to overgrow the eelgrass. Nutrients may be too high in these embayments due to nitrogen pollution. The Partnership is currently monitoring the amount of nitrogen in these locations to address these issues.

By knowing the water clarity in embayments where eelgrass grows or may grow, the Partnership can understand where and how to best restore and protect the eelgrass.