Ecosystem Targets and Supporting Indicators
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The Embayment Water Clarity Indicator is a supporting environmental indicator of the Water Clarity Ecosystem Target which is to improve water clarity by 2035 to support healthy eelgrass communities and attainment of the eelgrass extent target. The Eelgrass Extent Ecosystem Target is to restore and maintain an additional 2,000 acres of eelgrass by 2035 from a 2012 baseline of 1,893 acres. Currently, the water clarity target reports on the average secchi depth in the basins of the open waters of the Long Island Sound; however, eelgrass is only found in the embayments (or bays and harbors) of the Sound. This growth is limited to shallow waters as eelgrass is highly dependent on light availability reaching the sediment bottom. This indicator reports on the secchi disk depth, an indicator of water clarity, in embayments where eelgrass is present and where eelgrass is absent but habitat suitability for growth is high. The habitat suitability is defined by the eelgrass habitat suitability index developed by Dr. Jamie Vaudrey and colleagues in 2013 (see map below). This index uses water quality conditions, including water clarity, to define areas ideal for eelgrass growth and thereby assist in the evaluation of potential restoration sites.
This indicator reports on the light availability in the embayments where eelgrass is present or where eelgrass is absent but habitat suitability is high. The secchi disk depth is a measurement of light availability. Typically, secchi disk depth is measured using a black and white disk which is lowered into the water until you can’t differentiate the black and white quarters of the disk anymore indicating how clear the water is. However, for this indicator, the secchi disk depth was converted from turbidity data collected by Save the Sound’s Unified Waters Study. Turbidity is a measurement of the total suspended solids in the water column which is measured by lowering instruments with multiple sensors through the water column from a boat. In Long Island Sound, the required secchi disk depth necessary for eelgrass growth in Long Island Sound is greater than 0.7 meters.
As mentioned before, this indicator reports on the secchi disk depth in embayments where eelgrass is present including Alewife Cove, Mystic Harbor, Niantic River, Stonington Harbor, and Wequetequock Cove in Connecticut, and where eelgrass is absent but has high habitat suitability including Connecticut River, Housatonic River, and Mystic River in Connecticut and Goldsmith’s Inlet, Mattituck Creek, Nissequogue River, Northport Harbor, and Port Jefferson Harbor in New York. In 2021, the range of secchi disk depth for the eelgrass present embayments was 0.8 – 1.7 meters whereas eelgrass absent embayments with high habitat suitability was 0.4 – 1.4 meters. In 2021, the embayment with the highest water clarity was Wequetequock Cove in Connecticut; where the lowest water clarity was Connecticut River in Connecticut. Currently we do not report on embayments where eelgrass is present in New York; however, this does not mean that eelgrass does not exist. In fact, the most pristine meadow in Long Island Sound is Fishers Island in New York (see image below). In 2024, Save the Sound will add this site to the Unified Waters Study so we can start to monitor the water quality conditions at this site.
It is important to note that some of embayments where the secchi disk depth are meeting the required threshold, but eelgrass is not present is an indication of another limiting factor to eelgrass growth. In addition to light availability, eelgrass is also dependent on the nutrients available in the embayment. In most cases, the nutrients in these embayments are too high and therefore promoting the growth of other species like algae. Nutrients may be too high in these embayments due to high nitrogen loading from both point and non-point sources. Long Island Sound is currently monitoring the nitrogen concentration in these locations to address these issues.
In addition to secchi disk depth, we can also calculate another water clarity measurement specific to eelgrass growth – light attenuation coefficient. 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). Because this measurement is specific to eelgrass, it helps environmental scientists and managers better understand the water quality conditions so that they can protect and restore eelgrass. The light attenuation coefficient for eelgrass present and absent embayments were 1.01 – 2.11 and 1.22 – 4.12, respectively.