Great South Bay After Sandy: A Storm Breach Brings Eelgrass Back, but Recovery Is Unsure

Great South Bay, off the south shore of Long Island, New York, was once clear enough to stand in chest-deep water and see seagrass growing around your feet. A National Park Service (NPS) feature, written by Jamie Remillard and Caroline Gottschalk Druschke of SEAcomm URI, tells how Hurricane Sandy changed it, drawing on the research of Bradley Peterson, an associate professor at Stony Brook University's School of Marine and Atmospheric Sciences.

A bay that lost its clams

In the early 1970s, the bay was known for abundant seagrass and a major shellfishery. Estimates say that in the mid-1970s about half the hard clams eaten in the United States came from this 151-square-mile bay. By the mid-1980s overharvesting and brown tides, frequent phytoplankton blooms that hurt water quality, had crashed the clam population. Brown tides, fed by septic systems and fertilizer runoff from the developed south shore, also stripped the bay of the seagrass that sheltered finfish and shellfish.

Peterson says the bay's clams had once been so numerous that together they could filter its entire volume in about four days. Without them, the water has been dense with phytoplankton for close to thirty years, the seafloor barely reached by sunlight and the water the color of chocolate milk.

What the monitoring shows

Peterson has studied the bay since 2007 as part of NPS's Inventory and Monitoring Program, which tracks how ecosystems change over time and respond to stresses such as pollution, storms, sea level rise or climate change. He works through the Northeast Coastal and Barrier Network, which tracks long-term "vital signs" at eight park sites. His team measures water temperature, salinity, light reaching the bottom, turbidity, dissolved oxygen and chlorophyll, and tracks seagrass size, area covered and density. When Sandy struck, he had six years of data.

Seagrasses and water quality are linked: grasses trap sediment and reduce turbidity, letting in more light, and they shelter juvenile fish and the shrimp and crustaceans that live on the seafloor, so grass and fish are directly tied. Eelgrass, which he likens to a forest under the water, grows five to ten times taller than widgeongrass and supports more life because it is structurally more complex.

Seagrass under optimal conditions (above) and shaded by large mats of macroalgae (below). NPS.

The breach

Sandy cut a breach in the Otis Pike Fire Island High Dune Wilderness, letting water pass freely between the bay and the Atlantic. Peterson saw at once that it could be a game changer: clear ocean water would flush polluted water and cool the bay near the breach, conditions eelgrass likes. With funding from the Department of the Interior's post-Sandy resilience program, he led student researchers in two studies from 2013 to 2016. By August 2016 he reported that eelgrass had responded rapidly and dramatically, with an explosion of growth west of the breach in areas where it had been absent for over a decade. Bay anchovy, silverside, three-spine stickleback, killifish, pipefish, juvenile summer flounder and tropical fish were sheltering in the new beds.

Many crustaceans take refuge in seagrass beds, like this well-camouflaged shrimp. NPS.

Not yet a recovery

Peterson cautions that eelgrass is on a tightrope. Pollution and brown tides persist in the central bay, so eelgrass is confined to shallow water near the breach, which can warm quickly in summer. Hard clams are growing faster on better food that the breach introduced, but it also brought lady crabs, potential clam predators, along with colder water and higher salinity that could encourage a clam parasite called QPX. He describes the bay as a changed system, and the authors say resilience can be judged only with time and careful monitoring.

Sources

• Jamie Remillard and Caroline Gottschalk Druschke (SEAcomm URI), for the National Park Service Northeast Coastal and Barrier Network, Exploring the Woods under the Water; the quoted researcher's words are paraphrased and the researchers' photographs are left out; rewritten in hubnx's own words.