The water appears fine on a late July morning at Norwalk Harbor. It’s the kind of morning that makes the Connecticut coastline seem precisely what it is: a region where the Sound runs south into Long Island, appearing serene and deep blue in the early light, where yachts are moored close to train terminals, and where commuter towns meet the coast. Nothing is revealed by the surface. However, water temperatures rising into ranges that the ecosystem under that surface was not designed for are being recorded by the monitoring buoys moored in the channels, something that the image cannot convey.
The rate of warming in Long Island Sound is almost four times that of the worldwide ocean. The Long Island Sound Study, an EPA-funded research and management collaboration that has been monitoring the estuary’s health since the late 1980s, is the source of this number, which is derived from decades of water temperature data gathered from monitoring stations run by academic institutions, environmental organizations, and other organizations.
Both the size of the number and the region it depicts make it remarkable. This is neither an isolated tropical reef system nor a remote body of polar water. Squeezed between Connecticut to the north and Long Island to the south, this semi-enclosed estuary is encircled by some of the densest suburban development in the country. The combined stresses of eight million people’s daily land use are directed into 1,300 square miles of coastal water.
One aspect of the issue is the enclosure. Long Island Sound is located in a geographic region that retains heat, in contrast to open ocean habitats that can release heat thru mixing with nearby ocean masses and colder deep water. In the shallower western regions close to New York City, summer heat builds up more quickly than the water can reach equilibrium. As the temperature rises, dissolved oxygen levels decrease because warmer water holds less oxygen than cooler water. The western Sound’s bottom waters have historically developed hypoxic zones during the hottest weeks of late summer, which are dead zones with insufficient oxygen to support fish, lobster, and the benthic invertebrates that make up the base of the food web. These incidents have been chronicled for many years. They are becoming increasingly widespread and common.
The suburban watershed’s nitrogen runoff further exacerbates the heat issue. Algal blooms are fueled by a chronic nitrogen load that is delivered by fertilizers from lawns, golf courses, and agricultural land, nutrient-rich discharge from outdated septic systems, and rainfall that washes across impermeable surfaces before draining into the Sound. As the blooms break down, they absorb oxygen, intensifying the hypoxia that warmed water is already producing. Another layer is added by the acidification dimension: local nitrogen chemistry speeds up the pH fall that results from the Sound’s surface absorption of atmospheric carbon dioxide.
The species composition changing throughout the estuary is indicative of the ecological response. American lobster has been migrating northward and into deeper, colder waters, which has historically sustained one of Connecticut’s most economically significant fisheries. The population that formerly made the western Sound a dependable location for lobsters has mostly disappeared from those regions. Another cold-water species with long-standing cultural and commercial significance in the area, the winter flounder, has seen a comparable loss. Warmer-water species, such as invading green crabs, which outcompete native shellfish and disturb the substrate ecology that young fish rely on, are moving in the opposite direction—northward, made possible by water that now fits them.
The length and density of Long Island Sound’s monitoring record make it especially valuable as a case study. In contrast to most ocean habitats, the Long Island Sound Study has been conducting standardized water quality measurements long enough to offer before-and-after comparisons. The trajectory of a coastal environment under compounding urban, agricultural, and climate pressure is depicted in that record in granular detail, and it does so in an area where enough people reside to make the data not abstract.
