Science Briefs

Sediment Starvation Destroys New York City Marshes' Resistance to Sea Level Rise

In a particularly vulnerable geographic position, New York City's population, infrastructure, and economy are at risk due to sea level rise. The steady loss of marshes in NYC's Jamaica Bay is typical of many urban estuaries, where the threat of rising sea level is enhanced by urban encroachment, pollution, and upriver dams.

Landsat image of Jamaica Bay

Jamaica Bay as imaged by satellite in August 2016. (Credit: Landsat/OLI via NASA Earth Observatory)

Essential to the restoration and preservation of these key wetlands is an understanding of their sedimentation. Learning how they have kept pace with sea level over the past few centuries is essential to unravelling their present predicament and potential for survival in the future.

We present here a reconstruction of the history of mineral and organic sediment fluxes in Jamaica Bay marshes over three centuries using a combination of density measurements and a detailed accretion model from several cores extracted from the marshes with a Dachnowski side-opening corer in 2014.

We constructed a robust, detailed and complex record of marsh accretion rate through time. Organic and inorganic sediment fluxes were calculated using loss on ignition (LOI; a laboratory method that estimates the amount of organic material in a sample) and an age model based on nine age−depth tie points over the past 350 years. These tie points include markers of land use change (ragweed pollen, charcoal), pollution by heavy metals (lead, copper, and zinc) and nitrogen, the effects of federal and local environmental legislation, and marsh restoration.

Photo of four people in Jo Co Marsh examining a Dachnowski corer

Extracting a sediment core from Jo Co Marsh, Jamaica Bay, using a Dachnowski side-opening corer to eliminate compaction. (Credit: Dorothy Peteet)

Both ragweed (Ambrosia) pollen and charcoal are reliable indicators of landscape disturbance in the Hudson Valley — first by European settlers, around 1650 CE, and later by late 18th and early 19th centuries industrialization. Charcoal content rises with the industrialization of New York in the years following the Depression, and remains high until a core depth of about 24±2 cm, coincident with the passage of the Clean Air Act in 1963.

Our original hypothesis was that the accretion rate would decline concurrently with shoreline hardening, but we found instead, surprisingly, that marsh accretion continues to exceed sea level rise, even as mineral sediment flux declines toward the present (see graphs). USGS maps show that originally 18 tributaries contributed to the mineral sediment in Jamaica Bay, but only eight now remain.

We found that since 1800 CE, urban development dramatically reduced the input of marsh-stabilizing mineral sediment. Extensive dredging of channels and excavation of Grassy Bay borrow pit for JFK airport construction exacerbated the problem.

Graphs of a) sediment flux to Jo Co Marsh, and b) estimated accretion rate

Upper graph: The flux of organic (open circles) and inorganic (filled circles) sediment to Jo Co Marsh in grams per square meter per year (note that the y axis is logarithmic). Lower graph: The estimated accretion rate for Jo Co, and heavy line is the median while the gray polygon encloses the 95% confidence intervals surrounding the estimate. (Credit: Peteet et al. [2018])

However, as mineral flux decreased, surprisingly, organic matter flux increased. Population growth contributed to nitrogen increase in the 1800s. Our measurement of nitrogen content and nitrogen isotopes then indicated that in 1903 wastewater treatment plants dramatically increased the nitrogen loads to the bay. Zinc declines in 1972 are indicative of the Clean Water Act effects on cleaning up the Bay. Tertiary treatment of water in 1998 shows a decline in nitrogen content percentage while the heavy nitrogen isotopes remain.

Thus, examining the human history of pollution and our positive efforts at cleaning up the Bay, along with bulk density, LOI, and our age−depth model, we calculated the flux of organic and inorganic matter at Jo Co Marsh, in the eastern part of Jamaica Bay close to the airport. We found that the overall accretion rate has accelerated since industrialization. While the flux of mineral sediment decreased, organic matter flux increased to compensate.

Either marsh plants have increased their growth rate or decomposition has slowed, allowing the marsh surface to keep pace with rising sea level. However, the reduced mineral content causes structural weakness and edge failure of the marsh, leading to spatial loss of this important habitat and shoreline protection.

Marsh integrity now requires mineral sediment addition to both marshes and subsurface channels and borrow pits, a solution applicable to drowning estuaries worldwide. Further integration of this marsh mineral/organic accretion history with modeling provides parameters for Jamaica Bay marsh preservation, and a methodology for similar locales around the globe facing sea level rise.

Reference

Peteet, D.M., J. Nichols, T. Kenna, C. Chang, J. Browne, M. Reza, S. Kovari, L. Liberman, and S. Stern-Protz, 2018: Sediment starvation destroys New York City marshes' resistance to sea level rise. Proc. Natl. Acad. Sci., 115, no. 41, 10281-10286, doi:10.1073/pnas.1715392115.

Please address all inquiries about this research to Dr. Dorothy Peteet.

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