Gulf of Maine Coastal Habitats and Climate Change:

what does the future hold?

Michele Dionne

Wells National Estuarine Research Reserve

Wells, Maine

Summary

Gulf of Maine Coastal Habitats

The Gulf of Maine coastline is formed of an impressive array of intertidal and subtidal habitat types, configured within a highly varied coastal geomorphology. It could be argued that the Maine coastline exhibits the most complex array of habitats within the Gulf region, if not the entire eastern seaboard. For the purposes of discussing fish and shellfish coastal inhabitants, these habitats can be grouped according to their most obvious structural features: tidal marsh, marsh channel/creek, open estuarine channel, eelgrass bed, macroalgal bed, mudflat, sandflat, shellfish reef, ledge, cobble, gravel. Collectively, these habitat types are referred to as wetlands when they occur within the intertidal zone (Cowardin et al. 1979). In this discussion I will place special emphasis on tidal marsh estuaries and eelgrass systems, as they are likely to be the most vulnerable to the effects of climate change. The scenarios described below for these two habitats are generally applicable to other coastal habitat types.

Coastal Habitat Fish Associations

Of the many wetland habitat types that occur in the Gulf of Maine, few have been well studied with respect to their fish fauna. Studies of tidal marsh systems have documented 50 fish species occurring in the Gulf of Maine (Dionne 1999, Dionne and Orringer unpublished data). Softshell clam is also abundant in tidal marsh intertidal flats. Eelgrass systems are likely to support similar finfish diversity and are also important habitat for lobster (Davis 2000).

Climate Mediated Changes in Coastal Environments

Increasingly rapid sea level rise is the most dramatic coastal environmental change predicted as a result of global climate change. Estimates of sea level rise over the next century range from 9 cm to 86 cm (0.9 to 8.6 mm yr^-1 respectively) (IPCC 1998). Global mean sea level has risen by about 0.1-0.2 mm yr^-1 over the past 3,000 years and by 1-2 mm yr^-1 during the last century. Tidal wetlands have had the ability to accrete peat and increase elevation in response to the rates of sea level rise experienced in the Gulf of Maine over the past 3,500 years (Kelley et al. 1995. Gehrels 1996). However, the current rate of sea level rise may be near the threshold beyond which these wetlands will begin to drown, resulting in partial or total habitat loss if rates were to reach 10 mm yr^-1 (Leatherman 2000). Other habitat effects of sea level rise include: erosion of beaches and bluffs, increased flooding and storm damage, salt intrusion into aquifers and surface waters, and higher water tables. Increased temperatures and shifts in precipitation and ice cap melting will alter estuarine and coastal salinity and temperature regimes. Declines in latitudinal temperature differences may restrict coastal ocean circulation, vertical mixing and upwelling, hydrographic processes that are responsible for the high productivity of coastal waters (Armentano et al. 1988). This loss of water column productivity would increase the dependence of the coastal ecosystem on coastal wetland productivity. Increased frequency and intensity of storm and precipitation events will lead to aberrant patterns of erosion, sedimentation and turbidity in coastal habitats, altering the light regime and consequently primary productivity by eelgrass, macroalgae, and phytoplankton.

Human Mediated Changes in Coastal Habitats

The challenges faced by coastal habitats due to climate change are magnified by the fact that they share the coastal margin with a burgeoning human population. On a global scale more than half the people on earth live within 60 km of the coastline. Up to 60% of the nation’s superfund sites are located adjacent to estuarine systems. Along the Gulf of Maine, humans alter the coastal habitats through deforestation, agriculture, development (residential, commercial and recreational), roads and other pavement, sea walls and bulkheads, freshwater impoundment, diversion, and withdrawal, tidal restriction through tide gates and culverts, point and non-point source pollution, nutrient enrichment from runoff and atmospheric inputs and consequent dissolved oxygen depression, and sewage treatment plant effluent. These uses are intensifying significantly in the short term. When these local human-mediated environmental changes combine with the increased effects of global climate change, the stresses faced by coastal habitats intensify. It is very likely that these combined effects are synergistic, leading to greater negative effects than would be predicted from simply adding together the individual effects.

Habitat Response to Climate Mediated Environmental Change

Through the process of peat accretion, salt marsh systems have been migrating landward along the Gulf of Maine for several millennia, and should continue to do so until such time as the rate of sea level rise becomes too great. Using earlier predicted sea level rise rates, Alexander (1986) predicted that if sea level were to rise by 1 m (10 mm yr^-1) over the next century, the rate of wetlands loss through drowning would exceed that acquired through accretion and migration, resulting in a 60% net habitat loss. Using the full range of sea level rise scenarios at the time, coastal wetlands loss could range from 47% - 82% nationwide. A more recent study focused specifically on the question of tidal wetland loss in Maine in response to sea level rise (Dodson 2000) predicted a 51% loss when sea level change was 20 – 49 cm 100 yr^-1, and a loss of 79% if the rate increased to 86 cm 100 yr^-1. Dodson’s predictions took into account such factors as sedimentation rate, upland slope, and the presence of development, which essentially prevents wetland migration. As we continue to develop and protect the marsh-upland perimeter throughout the Gulf of Maine, we create conditions for even greater loss of wetlands (up to 50% more) as sea level rises. Tomorrow’s habitats will be created from today’s undeveloped coastal buffer. Unimpeded retreat is the best mechanism by which to offset loss of existing wetlands with the creation of new ones.

In a recent review, Short and Neckles (1999) suggest a number of effects of global climate change on the world’s seagrasses. They determined that increased atmospheric temperature will alter growth rates and other aspects of these species’ physiology. Increased temperature stress will cause shifts in seagrass distribution and will alter the patterns of reproduction. Increased water depths, changes in tidal amplitude and tidal prism, will increase seawater intrusion into estuaries, leading to a redistribution of estuarine and brackish habitats and associated seagrass species. Note that within the Gulf of Maine, there is only one seagrass species, known commonly as eelgrass, and a related submergent plant species known as widgeon grass. Species-specific changes in seed germination, vegetative reproduction, and photosynthesis in response to increased salinity drive climate-induced changes in seagrass distribution. Conversely, eelgrass may decline due to the greater impact of wasting disease at higher salinities. Indirectly, increased water temperatures, depth and turbidity (from increased tidal circulation) may increase water column productivity to the point where phytoplankton and epiphytes shade out the eelgrass. Increased CO_c will alter competitive ranks among seagrasses and between seagrasses and macoralgae. Short and Neckles conclude there is every reason to expect the response of seagrasses to global climate change will be great, as has been predicted more widely for upland land vegetation.

Faunal Response to Climate Mediated Coastal Habitat Change

The loss of tidal wetlands will have a direct impact on those species of fish that depend on estuarine waters and wetlands for feeding, refuge or spawning during some stage of their life cycle. To put this in perspective, Bigford (1991) estimated that 32% of the Northeast harvest (ME to CT) was comprised of estuarine dependent species. From a model based on Louisiana tidal wetlands, a 58% reduction in marsh primary productivity translates into a 15-20% drop in estuarine-dependent fish harvest (Dow et al. 1987).

In areas where coastal habitats do migrate landward, fish and the food webs they depend upon may experience an initial expansion of habitat, but will also be subject to the stresses associated with greatly altered habitat structure and value, and new dynamics in competitive and predator-prey relationships. A species’s behavioral, developmental, and genetic responses will need to be swift if that species population is to persist in the new environment. Processes of habitat formation that once took a century now take place on a decadal time scale. Slower rates of change allow species to more successfully adapt or migrate to areas with more favorable conditions. Acute environmental changes, such as those created by storms (which are increasing in frequency and severity), are more stressful. The types of environmental disturbance resulting from sea level rise will greatly increase the vulnerability of the Gulf of Maine to invasive marine invertebrates from the world over. Such invasions have already begun (Harris and Tyrell 2001).

Human Response to All of Above

"To ensure its own future, the fishing industry can ill afford to ignore the same potential implications of higher waters that have spurred other coastal residents to contemplate their future along the shoreline."

"Ecologists documenting environmental change must coordinate with engineers and economists to seek realistic solutions that are cost-effective over the span of decades"

Bigford (1991)

Paradoxically, the need for the fishing industry to have ready access to the shore results in an existing infrastructure that is very vulnerable to sea level rise. If fishing as we know it today is to persist into the next century, it will be necessary to plan an infrastructure retreat (either vertical or horizontal) from today’s piers and pilings in many of the nation’s ports. Eleven of the nation’s top 20 ports in terms of fish landings are at risk in the face of rising sea level, including Portland, Gloucester and New Bedford (O’Bannon 1990). Power, road access, storage and boat access will all need to be relocated. In his 1991 paper entitled Sea-Level Rise, Nearshore Fisheries, and the Fishing Industry, Tom Bigford (then with the National Marine Fisheries Service in Gloucester and now a NMFS Division Chief at headquarters) made the following recommendations:

1) Analysis of sea level rise impacts should consider the physical, ecological and economic effects on fish habitat and fishing industry infrastructure.

2) All levels of government should consider sea level rise in the award of permits and other support for coastline development

3) The fishing industry needs to be informed of likely economic losses that will result from sea level rise

4) Government agencies at the federal, state and municipal levels must study the implications of sea level rise on ports and coastal habitats

Even though these recommendations were published a decade ago, a review of the literature shows little evidence that attention is being paid to the response of coastal fish, their habitats, or the fishing industry to global climate change. In fact, there was no mention made of these issues in recent major volumes on the subjects of fish, fish habitat, and fisheries sustainability published by the American Fisheries Society (Fish Habitat:essential fish habitat and rehabilitation 1999), Kluwer Academic Publishers (Concepts and Controversies in Tidal Marsh Ecology 2000), and the National Research Council (Sustaining Marine Fisheries 1999).