The University of Rhode Island’s Environmental Data Center recently completed a major update of submerged aquatic vegetation (SAV) mapping for the Long Island Sound (LIS) and the Peconic Estuary (PEC), marking the first region-wide survey of this kind since 2017 for LIS and 2014 for the Peconic. The project, funded by the U.S. Environmental Protection Agency through NEIWPCC in collaboration with the Long Island Sound Partnership (LIS Partnership) and the Peconic Estuary Partnership (PEP), collected new high-resolution aerial imagery and underwater video surveys during the summer and fall of 2024. Since 2002, eelgrass meadows in the Long Island Sound have been intermittently monitored through aerial surveys and field observations. These surveys provide valuable insight for resource managers into the distribution of eelgrass habitat and how it changes over time.
Eelgrass (Zostera marina Linnaeus) is a common SAV species found in shallow subtidal environments of LIS and PEC. Eelgrass, and other underwater grasses such as widgeon grass (Ruppia maritima), form underwater meadows or beds that provide essential nursery habitat for fish and shellfish, stabilize sediments, improve water quality, and sequester “blue carbon.” Blue carbon refers to carbon dioxide that is captured and stored in coastal and marine ecosystems. Both the LIS Partnership and PEP Comprehensive Conservation and Management Plans highlight eelgrass as a species of critical concern. Because eelgrass is highly sensitive to water clarity and nutrient conditions, its distribution serves as a vital indicator of estuarine health.
Following National Oceanic Atmospheric Administration (NOAA) Coastal Management protocols and an approved EPA Quality Assurance Project Plan, both true color and infrared aerial photography were captured over the two estuaries in June and July. Field crews then collected underwater video surveys in September and October using GPS-linked cameras to validate eelgrass locations.
Analysts used Esri ArcGIS Pro software to interpret eelgrass beds from ortho-photography (aerial photographs that have had the distortion removed), incorporating field data and historical maps. More than 8,000 underwater video points informed the delineation of hundreds of SAV polygons – shapes drawn on digital maps that outline where SAV was observed – with an overall mapping accuracy of 84%. This process of capturing true-color aerial imagery over a large region and then interpreting images by eye with the aid of mapping software is referred to as “Tier 1” mapping. This mapping technique is part of a widely accepted seagrass monitoring framework that is used for quantifying the area and distribution over large regions.
In the Sound, 2,041 acres of eelgrass were mapped across 261 individual polygons, with the largest eelgrass meadow spanning 179 acres in Groton-Long Point, Connecticut. A comparison of the 2017 and 2024 LIS mapping found a net gain in eelgrass presence of 473 acres. Meanwhile, 800.6 acres of eelgrass were mapped across 112 individual polygons in the PEC. Here, the largest eelgrass meadow covers 132.8 acres and is located off Shelter Island in Coecles Harbor, New York.
Publicly accessible interactive maps, aerial imagery, and data are now available through Esri’s ArcGIS Online platform using the following links:
Regular Tier 1 assessments, ideally every one to three years, help track the recovery and resilience of these valuable underwater habitats, ensuring that managers and restoration partners can make informed decisions to protect and restore eelgrass meadows. The full report is available here and on the NEIWPCC Resource Library.