The short version
Long Island, like Cape Cod, has lost most of its eelgrass as nitrogen from septic systems and sewage has clouded its bays. In this report, Liana Simpson and Soren Dahl of the New York State Department of Environmental Conservation reviewed the science on what eelgrass needs to survive. They asked whether eelgrass could serve as a yardstick for Long Island’s nitrogen cleanup plans. Their answer: light reaching the bottom is the most reliable link between water quality and eelgrass health, and targets should be based on local data.
What the researchers did
This is a literature review prepared to support Long Island nitrogen planning. The authors summarized published research on how eelgrass responds to several conditions.
- They covered nitrogen, light and water clarity, algae, temperature and sediment, and compiled published threshold values in reference tables.
- They described the status of eelgrass in Long Island Sound, the Peconic Estuary and the South Shore bays.
- They included a case study of Tampa Bay, Florida, where seagrass recovered after nitrogen loads were cut.
What they found
- Steep losses in New York. Records suggest about 200,000 acres of seagrass grew in New York waters in the 1930s; only 21,803 acres remained as of 2009. In the Peconic Estuary, coverage fell from less than 1,600 acres in 2000 to under 1,000 acres by 2014.
- Light is the key. Eelgrass can need more than 20%, and up to 37%, of the sunlight at the surface to survive. Algae in the water, seaweed on the bottom and growths on the leaves all cut that light.
- Nitrogen thresholds from New England. A study of 62 New England estuaries found eelgrass at considerable risk when nitrogen loading exceeded 50 kg per hectare per year. A study of 19 Massachusetts estuaries found healthy eelgrass where tidally averaged total nitrogen was below 0.34 mg/L.
- Too much nitrogen can harm eelgrass directly. Beyond shading, high levels of nitrate and ammonium can weaken or kill eelgrass, especially when combined with warm water and low light.
- Recovery happens, but slowly. After treated wastewater was diverted from Mumford Cove in Connecticut, eelgrass returned across nearly half the cove, but it took nearly 15 years. Restoration by planting has succeeded less than 40% of the time worldwide.
What it means for Waquoit Bay today
Massachusetts already uses eelgrass as a benchmark: the state’s nitrogen threshold for Waquoit Bay’s main basin, 0.38 mg/L, was set to bring eelgrass back (see our explainer on the threshold report). That target sits close to the eelgrass thresholds this review cites from elsewhere, such as 0.34 mg/L from the Massachusetts estuary study and 0.4 mg/L for New York’s Peconic Estuary.
The report also offers two cautions for Waquoit Bay. Established eelgrass beds tolerate worse conditions than new plants trying to take root, so even better conditions are likely needed for recolonization
(Simpson & Dahl). And where no eelgrass remains nearby, as in most of Waquoit Bay, recovery may require replanting, using plants chosen for genetic diversity and tolerance of local conditions.
Read the report
Simpson, L., & Dahl, S. (2017). Eelgrass and Water Quality: A Prospective Indicator for Long Island Nitrogen Pollution Management Planning. New York State Department of Environmental Conservation, Division of Marine Resources. The report itself carries no date; the date comes from the official PDF’s metadata, which shows this revised version was created and posted by the agency on December 6, 2017. No DOI. Official copy: dec.ny.gov.
License: Published by the New York State Department of Environmental Conservation. State reports are not public domain, so we link to the official state copy rather than hosting it here.
Drafted with AI assistance from the linked source.
