"Mitigate or migrate?": Marsh loss and sea level around Chesapeake Bay
Shortcut URL: https://serc.carleton.edu/69105
Location
Continent: North American
Country: USA
State/Province:Maryland
City/Town:
UTM coordinates and datum: none
Setting
Climate Setting: Humid
Tectonic setting: Passive Margin
Type: Process
Figure 1. Map showing the location of the Blackwater National Wildlife Refuge in Dorchester County, MD (orange) Details
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Figure 4. A USGS drill rig fords a flooded section of road. Flooded roads and yards are a common sight today. Details
Description
The Blackwater National Wildlife Refuge (BNWR) on the Eastern Shore of Maryland, USA is a major destination for bird-watchers worldwide. Set aside as a refuge in 1938, BNWR (Fig 1) is part of a one of the largest protected marsh ecosystems in the Northeastern U.S. and was established for of its importance to thousands of waterfowl who stop here during their biannual migration north and south along the Atlantic seaboard. But the marshland that historically characterized BNWR is rather short-lived over geomorphic timescales. This area was high and dry ~18,000 years ago when sea levels were hundreds of feet lower and an ice sheet extended as far south as Long Island. A cold, tundra-like environment prevailed, and the closest marsh would have been miles away on the present-day continental shelf. But as climates warmed into the Holocene and the Chesapeake Bay estuary filled with sediment and water, extensive marshes were established along the rim of the Bay as a transitional zone between open water and upland forests. A thriving fishery and lush forests attracted European settlers and supported many generations over a few centuries.
But over the past few decades, residents and refuge managers of BNWR have been reminded of the ephemeral nature of this unique marsh ecosystem. Despite considerable management efforts, repeat aerial imagery clearly shows that marshland in this low-relief landscape has been rapidly converting to an open-water morphology over human timescales (Fig 2-3). The loss of marsh is predominantly attributed to the locally accelerated rates of relative sea level (RSL) rise. Both natural and human-induced changes in climate are causing an average rise of global sea level on the order of ~1.8 mm/yr. In general, marshes are able to keep pace with this rise via biomass accumulation and sediment trapping; however, the rates of RSL rise around BNWR are so high that they are outpacing marsh accretion. Tide gauge data indicate ~30 cm of RSL rise over the past century, which translates to a rate of approximately 3.44 mm/yr—almost twice that of the global rate of sea level rise. This discrepancy is due in large part to the effect of a simultaneous lowering, or subsidence, of the land surface in this part of the Mid-Atlantic at a rate of approximately 1.7 mm/yr. To further exacerbate the issue, a non-native rodent called nutria (Myocastor coypus) has caused rapid loss of marsh infrastructure by eating marsh grasses, which trap and generate sediment, and destroying root mats. While the regional nutria population has been eradicated, their legacy can still be seen in a positive feedback mechanism of marsh loss in BNWR: With greater losses in marshland, the fetch over open water lengthens and amplifies shoreline erosion, particularly during storm surges.
Coast-proximal marsh loss is not nearly as dramatic in areas where subsidence is less pronounced, and this begs the question "why are subsidence rates so high here?" Several factors including deep-seated tectonics, eustatic sea level rise and fall from the growth and collapse of ice sheets on land and in water, compaction, and land use have been involved in the lowering of the land surface in focused areas around this region, and they are relevant at time scales varying from millions of years to hundreds of years:
- The Salisbury Embayment, a sediment sink occupying this region during most of the Tertiary, may have forced downwarping of the underlying stratigraphy over induced vertical displacements on the order of 103 meters / 300 million years.
- Peripheral adjustments along the margin of an impact crater that struck the mouth of the Chesapeake Bay in the early Miocene may have induced adjustment at a scale of 102 m / 30 million years.
- Eustatic adjustments from the rise and collapse of a glacial forebulge from the Laurentide Ice Sheet just ~165 mi north of BNWR may have caused 101 m of adjustment over 30 thousand year cycles.
- Rising sea levels increase the weight of water overlying Holocene bay-fill mud; this forces compaction on the order of ~1 meter/thousand years.
- Increased pumping of the local aquifer for agriculture may be drawing down the land surface at rates of ~5 meters/300 years.
Whereas each of these scenarios likely explains some portion of land subsidence, each requires its own strategy for evaluation, and the observed relative motion of the land surface is likely the cumulative effect of several of these factors. Even if geologists could tease out the individual effects from each of these factors, their effects are already set in motion and are by-and-large irreversible. As awareness and concern over marsh loss grows, the question posed by managers and society at large is "now what?"; Do we attempt to mitigate the problem or should we allow this landscape to adapt to a new normal via marsh migration to locations inland? A marsh mitigation strategy commonly considered involves artificially building marsh elevations using dredge spoil, a process that is expensive and lacks guaranteed success. This technique has been tested at several sites within BNWR, and though there were signs of improvement, they were quite local and vastly out of scale with the regional challenges. Meanwhile, marsh habitat is being lost and residents are forced off long-held family property with deep sentimental value.
The stratigraphy of the area indicates many periods throughout the Pleistocene in which the sea level rose substantially higher than the present. Extensive pollen and biostratigraphic records collected from sediment cores show that the flora and fauna have migrated successfully with each iteration. So the current rise of sea level, though accelerated by anthropogenic effects, is nothing new. But it is the first in which modern society is faced with the daunting task of adaptation (Fig 4), and unlike many species that came before us, we have to deal with private property boundaries and contrasting interests of stakeholders. As the seas continue to rise and geomorphologists work to inform society on the reasons and rates behind these phenomena, we will see just how effective we humans are as a species in adapting to a new normal.
But over the past few decades, residents and refuge managers of BNWR have been reminded of the ephemeral nature of this unique marsh ecosystem. Despite considerable management efforts, repeat aerial imagery clearly shows that marshland in this low-relief landscape has been rapidly converting to an open-water morphology over human timescales (Fig 2-3). The loss of marsh is predominantly attributed to the locally accelerated rates of relative sea level (RSL) rise. Both natural and human-induced changes in climate are causing an average rise of global sea level on the order of ~1.8 mm/yr. In general, marshes are able to keep pace with this rise via biomass accumulation and sediment trapping; however, the rates of RSL rise around BNWR are so high that they are outpacing marsh accretion. Tide gauge data indicate ~30 cm of RSL rise over the past century, which translates to a rate of approximately 3.44 mm/yr—almost twice that of the global rate of sea level rise. This discrepancy is due in large part to the effect of a simultaneous lowering, or subsidence, of the land surface in this part of the Mid-Atlantic at a rate of approximately 1.7 mm/yr. To further exacerbate the issue, a non-native rodent called nutria (Myocastor coypus) has caused rapid loss of marsh infrastructure by eating marsh grasses, which trap and generate sediment, and destroying root mats. While the regional nutria population has been eradicated, their legacy can still be seen in a positive feedback mechanism of marsh loss in BNWR: With greater losses in marshland, the fetch over open water lengthens and amplifies shoreline erosion, particularly during storm surges.
Coast-proximal marsh loss is not nearly as dramatic in areas where subsidence is less pronounced, and this begs the question "why are subsidence rates so high here?" Several factors including deep-seated tectonics, eustatic sea level rise and fall from the growth and collapse of ice sheets on land and in water, compaction, and land use have been involved in the lowering of the land surface in focused areas around this region, and they are relevant at time scales varying from millions of years to hundreds of years:
- The Salisbury Embayment, a sediment sink occupying this region during most of the Tertiary, may have forced downwarping of the underlying stratigraphy over induced vertical displacements on the order of 103 meters / 300 million years.
- Peripheral adjustments along the margin of an impact crater that struck the mouth of the Chesapeake Bay in the early Miocene may have induced adjustment at a scale of 102 m / 30 million years.
- Eustatic adjustments from the rise and collapse of a glacial forebulge from the Laurentide Ice Sheet just ~165 mi north of BNWR may have caused 101 m of adjustment over 30 thousand year cycles.
- Rising sea levels increase the weight of water overlying Holocene bay-fill mud; this forces compaction on the order of ~1 meter/thousand years.
- Increased pumping of the local aquifer for agriculture may be drawing down the land surface at rates of ~5 meters/300 years.
Whereas each of these scenarios likely explains some portion of land subsidence, each requires its own strategy for evaluation, and the observed relative motion of the land surface is likely the cumulative effect of several of these factors. Even if geologists could tease out the individual effects from each of these factors, their effects are already set in motion and are by-and-large irreversible. As awareness and concern over marsh loss grows, the question posed by managers and society at large is "now what?"; Do we attempt to mitigate the problem or should we allow this landscape to adapt to a new normal via marsh migration to locations inland? A marsh mitigation strategy commonly considered involves artificially building marsh elevations using dredge spoil, a process that is expensive and lacks guaranteed success. This technique has been tested at several sites within BNWR, and though there were signs of improvement, they were quite local and vastly out of scale with the regional challenges. Meanwhile, marsh habitat is being lost and residents are forced off long-held family property with deep sentimental value.
The stratigraphy of the area indicates many periods throughout the Pleistocene in which the sea level rose substantially higher than the present. Extensive pollen and biostratigraphic records collected from sediment cores show that the flora and fauna have migrated successfully with each iteration. So the current rise of sea level, though accelerated by anthropogenic effects, is nothing new. But it is the first in which modern society is faced with the daunting task of adaptation (Fig 4), and unlike many species that came before us, we have to deal with private property boundaries and contrasting interests of stakeholders. As the seas continue to rise and geomorphologists work to inform society on the reasons and rates behind these phenomena, we will see just how effective we humans are as a species in adapting to a new normal.
Associated References
- Boon J.D., J. M. Brubaker, D.R. Forrest. 2010. Chesapeake Bay Land Subsidence and Sea Level Change: An Evaluation of Past and Present Trends and Future Outlook. Special Report No. 425 in Applied Marine Science and Ocean Engineering to the U.S. Army Corps of Engineers Norfolk District. Virginia Institute of Marine Science. Gloucester Point, Virginia.
- Engelhart, S.E., B.P. Horton, B.C. Douglas, W.R. Peltier, and T.E. Törnqvist, 2009. Spatial variability of late Holocene and 20th century sea-level rise along the Atlantic coast of the United States. Geology, 37: 1115-1118; doi: 10.1130/G30360A.1.
- Miller, L., and B.C. Douglas. 2004. Mass and volume contributions to twentieth-century global sea level rise. Nature 428:406-409.
- Powars, D.S., 2000, The effects of the Chesapeake Bay impact crater on the geologic framework and the correlation of hydrogeologic units of southeastern Virginia, south of the James River: U.S. Geological Survey Professional Paper 1622, 53 p., 1 oversize pl.
- Stevenson, J. C., M. S. Kearney, and E. W. Koch. 2002. Impacts of sea level rise on tidal wetlands and shallow water habitats: a case study from Chesapeake Bay. American Fisheries Society Symposium 32: 23-36.
- Stevenson, J. C., M. S. Kearney, and E. C. Pendleton. 1985. Sedimentation and erosion in a Chesapeake Bay brackish marsh system. Marine Geology 67: 213-235.