Saving Seagrass: Using Science to Find the Answers

New research points to nitrogen pollution as a threat to seagrass in southern New England and Long Island Sound.

(Click to enlarge) Fifty-five percent of all nitrogen pollution from eastern Connecticut’s Mystic River watershed is from residential cesspools and septic systems or agricultural fertilizer—36 and 19 percent, respectively. A 43 to 66 percent nitrogen reduction would be needed to support eelgrass growth in Mystic Harbor and Beebe Cove, where it historically occurred, and reduce the risk of losing the eelgrass meadows at Ram Island, just offshore.

Seagrasses provide shelter, feeding grounds and nursery habitat for thousands of ocean animals, including commercially and recreationally important fish and invertebrates. They stabilize coastal sediments, generate oxygen, improve water quality by absorbing nutrients and can store more carbon than forests. But around the world, seagrasses are dying and disappearing because of degraded water quality and development pressures.

In 2009, The Nature Conservancy received funding from the National Oceanic and Atmospheric Administration’s Community-based Restoration Program to research the causes of seagrass decline in the Southern New England and Long Island region.

The research was divided into two phases. The first phase, completed in 2012, focused on understanding how one species of seagrass—eelgrass (Zostera marina L.)—was responding to environmental conditions and then testing plants to determine eelgrass’ response to variations in water quality and temperature. Eelgrass samples were collected from 36 sites, spanning the region from Cape Cod to Long Island’s south shore bays, and tested for genetic diversity and tolerance to different stressors, such as nutrient and light levels. The best conditions for eelgrass growth included full sunlight; low organic soil matter; and normal, seasonal, ambient temperatures.

Nearly all of the seagrass sites sampled showed that nitrogen pollution was the major stressor; however, the sources of nitrogen differed among the sampling locations. At study sites in the coastal bays of Rhode Island, Connecticut and Long Island, seagrasses were primarily stressed by nitrogen pollution from sewage; while seagrasses sampled from Cape Cod and the eastern end of Long Island Sound were mainly affected by excess nitrogen from the atmosphere and fertilizers, in addition to sewage.

Picking up where the first phase left off, the research’s second phase focused on further evaluating human-made sources of stress, including the combined effects of nitrogen pollution and temperature. It then examined a series of targeted embayments throughout the Southern New England and New York study area where restoring enabling conditions would most likely preserve existing seagrass habitat or support recovery of lost habitat. (See graphic above for an example.)


Completed in spring 2014, the research’s second phase adds important information. The new findings indicate that:

  • Septic systems and cesspools are the predominant source of nitrogen pollution from non-point sources at most of the sites sampled throughout the Southern New England region and Long Island.
  • Understanding the sources of nitrogen, and the relative contributions from each source, is necessary to ensure resources and management practices are targeted correctly at the sources of the problem, which can vary from one bay to the next.
  • Additional research is needed to complete the picture of nitrogen loading sources entering Long Island Sound.
  • Even small increases in temperature can impact the long-term survival of the remaining eelgrass beds in the region and may explain why restoration attempts have been hampered in many estuaries.
  • Many of the bays studied are at high risk if nothing is done. However, if nitrogen pollution is reduced, the environmental conditions seagrass meadows and other coastal habitats need to thrive can be restored. By reducing the stresses on the populations of seagrasses we still have we increase the probability that they may be able to adapt to warmer temperatures.
  • We can reduce nitrogen pollution entering our waters by using better technologies that are available to treat our wastewater and by working with the agriculture industry, landscapers and homeowners to put best management practices that reduce the impacts of fertilizers into use.
  • Additional research is needed to identify and quantify sources of nitrogen at embayments that were not included in this study. Subsequently, methods should be developed that reduce nitrogen at its source.

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