Rising seas could expand Hudson River estuaries

Fourteen percent of the freshwater tidelands along the lower reaches of the Hudson River in New York will be lost to sea level rise by the end of the century if current trends continue, according to a new study. But the study also finds that new wetlands could form in upland areas to result in a net increase in total wetland area in 2100—an illustration of the paradoxical environmental gains that might occur amid the disruption of climate change, if we humans plan carefully.

Researchers from Cornell University and the nonprofit Scenic Hudson used a computer model to simulate how rising seas are likely to alter the type and extent of wetlands in the Hudson River estuary, which stretches from Manhattan at the mouth of the river about 150 miles northward to Troy, just upriver from the state capital of Albany.

Currently about 7,000 acres of tidal wetlands are found along this stretch of the Hudson, about 80% of which are considered freshwater estuary. This ecosystem has been extensively altered by human activities, but it still supports a variety of rare plant communities and wildlife, including two endangered species of fish, the Atlantic sturgeon (Acipenser oxyrhinchus) and shortnose sturgeon (A. brevirostrum). The estuary also plays a role in water filtration, nutrient cycling, and buffering riverside communities from storms.

The researchers considered nine scenarios for the future of the estuary, based on the rate of sea level rise (high, medium, or low) and the rate of accretion (also high, medium, or low). Accretion refers to the rate at which deposited sediment and organic matter add new land area to a marsh.

Sea level around New York City is already rising faster than the “low sea-level rise” value in the model, so the researchers concentrated on the medium and high sea-level rise projections. The current trends in the Hudson River estuary most closely resemble the medium sea-level rise/low accretion scenario.

In this and all other medium and high sea-level rise scenarios, some current wetlands would be lost to permanent inundation by the end of the century, the researchers reported recently in PLoS ONE. But if marshes are allowed to migrate into upland areas, these losses could be offset and the extent of tidal wetland in 2100 could be even greater than it is today, totaling between 8,100 and 10,900 acres.

“That’s fantastic,” says study team member Magdeline Laba, a scientist at Cornell University. “I never thought that would be a result. These marshes are amazingly resilient.”

The results suggest that conservation planners need to secure protection not just for current marshes but also for areas where future wetlands are likely to be. In the current trend scenario, just under half of new wetland would form in currently protected areas.

But if sea level rises faster than its current rate, new wetland will represent two-thirds of the estuary’s total area in 2100. And there will be more conflict with developed areas, with buildings or roads blocking the development of new marshes.

The researchers also assessed where wetland areas would persist mostly in their current state and where they would be likely to change class—high marsh becoming low marsh, or low marsh converting to tidal flat. Under the current trend scenario, for example, 37% of wetlands would remain in their current class. It’s especially important to conserve these fully resilient wetlands, the researchers say, because they will provide a bridge between the current and future estuary.

Scenic Hudson has used these findings to create an action plan to protect current and future wetlands along the river. – Sarah DeWeerdt | 5 July 2016

Source: Tabak N.M. et al. “Simulating the effects of sea level rise on the resilience and migration of tidal wetlands along the Hudson River.” PLoS ONE DOI: 10.1371/journal.pone.0152437

Header image: The Esopus Estuary near Saugerties, New York. Credit: Matthew and Heather via Flickr.