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Eelgrass on decline in RI waters, report says
July 6, 2017, WAKEFIELD – A recently-completed report from the URI Environmental Data Center (EDC) shows eelgrass and other submerged aquatic vegetation (SAV), protected by the R.I. Coastal Resources Management Council (CRMC), is on the decline in most of the salt ponds and in Narragansett Bay.
Eelgrass (Zostera marina) – a rooted, flowering plant that grows beneath the water’s surface in small beds or larger meadows – and other species of submerged aquatic vegetation play a vital role in the function of coastal ecosystems. Eelgrass, the most common type of SAV in Rhode Island, serves many ecological functions, including supporting the marine food web and providing important habitat for many species of fish and invertebrates. Fish such as tautog lay their eggs on the surface of eelgrass leaves. Shellfish such as bay scallops, quahogs, blue crabs and lobsters can be found in eelgrass beds and some species of waterfowl feed on eelgrass. Eelgrass can also provide protection of the shoreline by dampening wave energy and holding marine sediments in place.
In 1997, the Atlantic States Marine Fisheries Commission established a policy on the assessment, protection and study of SAV as a recommendation for all member states. In Rhode Island, submerged aquatic vegetation has been deemed a critical marine resource and it is protected by both federal and state legislation – the Clean Water Act and RI Coastal Resources Management Plan, Section 300.18, respectively.
In 2016, the CRMC provided funding to the URI EDC and the United States Geological Survey (USGS) to acquire aerial photographs of the southern coastal ponds, Narragansett Bay and Block Island. The EDC used the resulting images to create SAV maps for the entire Rhode Island coast, as well as a written report that summarizes trends in SAV growth.
“Mapping the distribution and extent of eelgrass is a critical first step in understanding, managing, and protecting shallow, subtidal estuarine habitats,” the report states.
SAV beds can be degraded by poor water quality, disease, algal blooms, or physically damaged by human activity like shallow-water boating, dredging and filling associated with maintaining navigation channels and basins, and the construction of in-water structures such as docks, which can shade the beds. For these reasons the CRMC has created policies and regulations within its regulatory program to protect SAV by limiting or prohibiting certain activities in areas where SAV is present.
Having accurate maps of where SAV is growing in Rhode Island’s coastal waters is crucial to its protection. The CRMC is part of the RI Eelgrass Mapping Task Force, which is led by the URI EDC, the Narragansett Bay National Estuarine Research Reserve (NBNERR) and Save The Bay. This group has coordinated previous eelgrass mapping efforts and developed a long-term plan for mapping and monitoring SAV throughout the state’s coastal waters. With previous mapping done in 2007, 2012 and now 2016, the EDC was able to examine trends to determine where and to what degree SAV beds are expanding or shrinking in size throughout Narragansett Bay and the coastal ponds.
The CRMC aided the EDC in its efforts to “ground-truth” the eelgrass maps, which involved taking boats and kayaks out in the summer and early fall of 2016 to confirm the presence or absence of eelgrass with underwater cameras and GPS equipment. The resulting maps and report will greatly aid in coastal decision-making.
A total of 1,144 acres of SAV were documented and ground-truthed as a result of the 2016 study, representing an 18 percent decrease of SAV acreage in Rhode Island coastal waters from 2012. A map of the 2012 and 2016 delineations is available at https://tinyurl.com/lqjam4p. According to the report, most of the SAV in the study area – 91 percent – was eelgrass and eelgrass surveying was the focus of the ground-truthing field work. Between 2012 and 2016, SAV acreage declined at most sites that were analyzed. The sites with the largest decline in acreage between the two years are Quonochontaug (52 percent decline), and Point Judith Ponds (48 percent), and Little Narragansett Bay (25 percent).
Jamestown also had a decrease of eelgrass acreage (19 percent) but the site and Ninigret Pond continue to have the most eelgrass of any sites statewide, the report said. Ninigret was also the only coastal pond to not have a decrease in acreage. The Narrow River was the only site in the study area that experienced a large increase in eelgrass acreage from 2012 to 2016 (a 45 percent increase).
With the completion of the 2016 mapping effort, the CRMC and its partners now have consistent and comparable data for three years over a 10-year period for Narragansett Bay. There are now four sets of data for the coastal ponds, dating back to 1999. According to the report, SAV in the coastal ponds has been decreasing at a rate of almost 23 acres a year. Quonochontaug Pond has seen a dramatic decrease of 61 percent of its eelgrass beds since 2009, at a rate of eight acres per year. Potter Pond had a decrease of 39 percent from 1999 to 2009, but it has maintained approximately 70 acres of SAV since that time. An exception worth noting is Ninigret, which has had approximately 200 acres of SAV every year since 2009, an increase of 42-acres since 1999. And from 2012 to 2016, the eelgrass acreage has increased 48 percent in the Narrow River.
“We are fortunate to work with partners like the URI EDC who produce such high-quality data products and analysis,” said Caitlin Chaffee, policy analyst with the CRMC. “At this point it’s still a bit early to pinpoint a single cause, but having long-term datasets is key to determining what might be causing the trends we are seeing in SAV growth, so that we can adjust our policies and management actions accordingly.”