Atlantic Sea Level Rise, Lagoonal Marsh Loss, and Wildlife Habitat Implications
R. Michael Erwin
WHAT ARE THE RESEARCH NEEDS?
Along the mid-Atlantic coast, relative sea level rise estimates of ca. 0.4 m (i.e, 4 mm/yr) by the year 2100 is coupled with estimates of only ca. 2.0 mm/yr marsh accretion rates, resulting in potential large-scale submergence especially of lagoonal marsh areas. This loss of wetlands has major implications to almost 100 species of migratory waterbirds. Marsh lagoonal islands are very important nesting habitat for American Black Ducks (a species of special concern to the USFWS), the largest concentrations of Laughing Gulls and Forsters Terns along the Atlantic coast, the declining Gull-billed Tern, Common Terns, Willets, Clapper and Black Rails. Moving to fringe marshes (which may actually increase in area in some regions) is often not an option for nesting birds because of human developments, disturbance, and introduced mammals, including domestic pets. On the positive side, feeding habitat for some species, especially shorebirds and wading birds, may increase as wetlands change to intertidal and mudflat areas. Additional empirical data are needed to determine local and regional variations in marsh elevational changes (not simply surface accretion), sea level changes, topographic profile changes on marsh islands, microhabitat alterations as water levels change, and species usages of different microhabitats such as mudflats, pannes, tidal creeks, and Spartina meadows among seasons. In this way, the relative risk to many species of waterbirds can be assessed over both short and long terms.
Recent projections about regional, national, or global changes in relative sea level rise have caused considerable concern not only among coastal scientists, but coastal zone planners and the multitudes of people living in the coastal zone; more than 50% of all U.S. citizens will live within 50 km of the coast by the year 2010. Although most of the media concern involves economic and social impacts of the potential shifts in the land-water interface, coastal ecologists in the mid-Atlantic region are gravely concerned about the implications of a 0.4 m (or 4 mm/yr) rise in relative sea level (estimate from Nicholls and Leatherman 1996, Coastal Manage. 24). It appears that the ability of coastal wetlands to keep up the vertical pace (through marsh accretion) with sea level rise is complicated by local and regional variations (due to numerous natural and anthropogenic causes) as well as the position of the wetland relative to the barrier island-mainland gradient. For example, marshes attached to mainland areas may in fact increase in area with gradual sea level rise, whereas lagoonal marshes may rapidly erode (see Fig. 1 for a Virginia example) and/or submerge. How widespread this lagoonal marsh submergence (Hayden et al. 1991, BioScience 41) is expected to be along the coast is subject to debate. In some areas, such as Louisiana and in the middle parts of the Chesapeake Bay in Maryland, little question remains that sea level rise and subsidence are causing rapid reversion of marsh to open water. Marsh accretion simply is insufficient to keep up. In other areas, there appear to be either few or no data (New Jersey and Virginia) or there is marked variability in measurements (Georgia) (see Fig. 2).
For the sake of this presentation, I assume that for most of the mid-Atlantic coast lagoonal marsh accretion rates are approximately 1.5 to 2.5 mm/yr, whereas RSLR is ca. 4 mm/yr. Thus, in 100 years, lagoonal marshes will experience a net elevational loss of about 20 cm. Therefore, at current tidal ranges in the mid-Atlantic, all lagoonal marsh areas, as well as much of the fringing marshes, will be submerged between mid- and high tides on each cycle. I also restrict my scenario to the endpoint of the 100 years, not the intervening periods with gradual changes in area and microhabitat shifts. This latter area is the subject of a current research proposal (see below under The Intermediate Scenarios).
|Figure 1. Example of net marsh loss at a lagoonal marsh island (Chimney Pole Marsh) in coastal Virginia, showing marsh loss (red) and marsh gain (black) from 1949-1990. White areas are regions of no net change. Marsh loss to open water exceeded 10% during the period (from Kastler and Wiberg 1996, Estuarine, Coastal and Shelf Science 42).|
The Long Term Scenario
This first scenario is based on a one-time incremental increase of the 100 year expected RSLR:
The coastal marshes of the mid-Atlantic are an extremely important habitat for many species of wildlife. Although this region is critical for many populations of marine and estuarine fishes, invertebrates (blue crab, oyster, fiddler crab), diamondback terrapins and many other species, I focus on migratory bird populations in this report. Nearly 100 species of migratory waterfowl, marsh and shorebirds, or colonial species spend a portion of their life cycles in these lagoonal marshes, either nesting, feeding, or resting (Table 1). Population sizes can often be immense - in winter, one species alone, the Greater Snow Goose may exceed 100,000 individuals among southern New Jersey, Delaware, and Maryland. Waterfowl totals in winter from New Jersey south to Virginia often exceed one-quarter million birds. In summer, nesting numbers may also be impressive (Table 2). Colonially nesting species such as Laughing Gulls often exceed 120,000 individuals, mostly in New Jersey and Virginia, where colonies sometimes reach 5000 individuals. Forsters Terns also nest in very large numbers; the mid-Atlantic populations of these two species represent the majority of the entire Atlantic coast population (Table 2). Although the data are not at all precise, estimates of nesting densities of the more cryptic, non-colonial species such as the American Black Duck, Willet, and Clapper Rail suggest that these species are also major components of the marsh trophic web (Table 3).
Lagoonal marshes have become increasingly important as nesting habitat for American Black Ducks (a species of management concern to the USFWS), gulls, terns, and Black Skimmers as barrier island habitats have either become developed (e.g. New Jersey and Maryland) or as human disturbance and/or mammals have increased in intensity and distribution (Erwin 1980, Biol. Conserv. 18). Most of the barrier islands from New Jersey south to Virginia now support populations of raccoons, red foxes, and/or Norway rats, all major predators on bird nests. In some areas, foxes and raccoons have expanded in the past decade. Some species, such as Herring Gulls and Common Terns, can easily shift habitats away from the disturbed barrier islands but other species are more specialized in their habitat requirements. As lagoonal marshes are reduced in area, marsh-specialist Clapper Rails, the rare Black Rail, Least Bitterns, Forsters Terns, Laughing Gulls and others will find generally lower quality habitats in the remaining fringe marshes; these fringe marshes are susceptible to invasion by the predators mentioned above in addition to dogs and cats associated with human development. Thus, some populations that are now productive (or sources of recruits) may become sinks if they are forced to nest in suboptimal habitats.
Not all of the species included in Table 1 will be adversely affected - in fact, many may be benefitted by the reversion of Spartina alterniflora/patens marsh area to unvegetated flats, then open water. These habitats are important feeding habitats for wading birds, about 20 species of migrating shorebirds, and dabbling ducks such as Mallards and Black Ducks. Therefore, losses of nesting habitat for the ducks and some colonial species may be partially compensated for in some species by increases in feeding habitats. For shorebirds that migrate through the mid-Atlantic during both north- and southbound migration periods (the vast majority only stopping for refueling), the increased amount of intertidal wetlands and open mudflats may increase their carrying capacity. Of course, calculating area change becomes complicated by hypsometry (or configuration of the tidal basin), i.e., currently used mudflats and intertidal areas will become more deeply inundated, rendering them unavailable to most short-legged shorebirds.
|Figure 2. Marsh accretion rates (mm/yr) and local relative sea level rise (RSLR) along the Atlantic and Gulf coasts (from National Academy of Sciences 1987, pp 67-68). Bars having two colors indicate the range if more than one study was reported.|
|Table 1. Number of avian species dependent upon emergent saltmarshes for nesting, feeding, and/or resting in the mid-Atlantic coastal region.|
|Marsh birds (rails, etc.)||3||8||1|
Table 2. Estimates of breeding populations of key marsh-nesting colonial waterbirds at risk from habitat loss due to coastal sea level rise, from New Jersey to Virginia.
|Species||No. Individualsa||% in marsh||% of Atlantic Coast
|a Data from Spendelow and Patton 1988, [USFWS report, BR 88(5)] for DE, MD, NJ, and from B.D. Watts 1994, (final report to Va. Dept. Game & Inland Fisheries) for VA.|
Table 3. Estimates of breeding densities of cryptic marsh birds in mid-Atlantic coastal marshes, New Jersey to Virginia.
|Species||Estimated densitiesa||Estimated populationb|
|Clapper Rail||0.67 nests per ha||142,000 birds|
|Willet||0.48 nests per ha||102,000 birds|
|American Black Duck||0.7 (VA) - 4.7 (NJ) prs/sq km||13 - 14000 birds|
|a Conservative estimates using averages of lowest densities
(if a range was given) for 4 studies in NJ and VA (rail), 2 studies in NJ and VA (Willet),
and two state surveys for Black Ducks.
b Conservative rail and willet estimates using total coastal wetland area, NJ to VA (Tiner 1987, Mid-Atlantic Wetlands, USFWS) of ca. 212,500 ha, assuming only about half is suitable as nesting habitat (106,000 ha), and extrapolating the estimated nesting densities from column 2. Black Duck densities are based on 1 sq.-km plot surveys where only the coastal marsh stratum data are presented (project of the Atlantic Coast Joint Venture, data from T. Nichols, NJ, and G. Costanzo, VA).
The Intermediate Scenarios:
Modeling Changes and Research Needs
Under this second scenario, the marsh changes are gradual through time, reflecting the ca. 2 mm per year differential between sea level rise and marsh accretion rates.
WHAT ARE THE RESEARCH NEEDS?
Knowing marsh surface accretion rates is not sufficient to capture the dynamics of marsh elevation changes because of the dynamics of below-surface processes (Cahoon and Lynch, in press, Mangroves and Salt Marshes). There appear to be substantial variations in both below and above-ground elevational changes (see Fig. 2). The degree to which these vary both within- and among-regions in the mid-Atlantic is crucial to our understanding the magnitude of the potential impact of RSLR. In similar fashion, water levels and tidal amplitudes are expected to vary both locally and regionally with increasing sea levels.
Translation is necessary to convert relative changes in sea level and marsh elevations into areal changes. In addition to tracking areal changes in marshes, however, the shifts in microhabitats from, for example, 100% Spartina cover to one with 20% tidal creeks, 30% pannes, 30% Spartina, and 20% open water, represents a significant change to migratory waterbirds. Whether these changes occur in some predictable manner among regions requires topographic analyses and modeling as well as comparisons of remote sensing imagery over a period of decades.
Finally, except for inventories of nesting colonial waterbirds with accurate locations of colony sites, few data exist for waterfowl, shorebirds, or rails that indicate which marsh complexes and which microhabitats are used the most during specific times of the year. In general, where survey data are available for these groups of birds, they are performed at scales that are too large to be useful in making habitat assessments.
The Biological Resources Division is proposing a 5 + year project that encompasses the following: selecting estuaries that support significant wildlife resources in the mid-Atlantic region that contain extensive marsh lagoonal islands, establishing permanent Sedimentation-Erosion Tables (SETs) and tide gauges that will monitor water and marsh elevational changes at a network of locations, and monitoring habitat usage by migratory waterbirds at selected marsh sampling sites during breeding, migration, and wintering periods. Application of GIS technology to marsh surfaces will allow topographic modeling. In this manner, we will be able to project physical changes in these estuaries to ecological changes in marshes that can be evaluated from the perspective of wildlife value.