Ecological impacts

Coastal marsh loss and its effects

Nicholls et al (1999), using estimates for global sea level rise based on the Hadley Centre climate model, predicted that existing coastal marshes worldwide could be inundated by 2080 AD. For Chesapeake Bay, like many microtidal coasts (i.e., 0—2 m mean tidal range), this dire prediction could come true much earlier. Marsh loss has been occurring in the Bay since at least the 1920s (Stevenson et al, 1985), and by 1993 nearly 50 per cent of the marshes in the estuary had become degraded, being unable to keep pace with the past rate of sea level rise (Kearney et al, 2002). This situation parallels that which has characterized the brackish marshes in the Mississippi River Delta (see Boesch et al, 1994). Typically, the degradation is associated with the growth of large open water areas, or ponds in interior marsh sites ('interior ponding'), with erosion of shoreline areas being a considerably less significant process (Stevenson et al, 1985; Stevenson and Kearney, 1996). Mendelssohn et al (1981) described the underlying mechanism

Figure 7.6 Large waves demolishing docks and boathouses at the Virginia Institute of Marine Sciences during Hurricane Isabel in 2003

Source: Virginia Institute of Marine Science.

as involving depletion of dissolved oxygen in waterlogged surface marsh sediments (anoxia) as the upward growth of marsh surface is outpaced by rising sea levels. The resulting plant dieback leads to break up of the marsh from collapsing of the root mass. The process may be triggered by events like storms, after which an increasing number of small open water areas (ponds) coalesce into larger ponds. Eventually, the formation of ever-larger ponds reaches a point where pond size (about 2.5 hectares) is large enough that wave erosion of the pond edge accelerates pond enlargement (Kearney et al, 1988; Stevenson and Kearney, 1996).

Compounding the problem of accelerated marsh loss is the fact that some of the biggest and ecologically most important marshes in the Chesapeake Bay may be especially predisposed to rapid loss. For biogeochemical reasons that are not fully understood, older organic materials buried below the root zone can undergo further (so-called 'refractory') degradation. By this process, larger structures like roots and rhizomes that keep the sediments together are broken down to a finely divided, relatively loose organic mass (Stevenson et al, 1985), resulting in a sequence of materials that is readily eroded if the marsh peat becomes fragmented. The loose materials below the shallow surface peat make them quake when walked over, and rise and fall slightly with the tide. They are uniquely fragile and, unfortunately, marshes of this type are probably more common in the Chesapeake Bay than is realized. An even more sobering thought is that presently, stable tidal freshwater marshes and very slightly brackish marshes (salinities of around 5 parts per 1000 salt) could be transformed by rising sea level into these highly vulnerable semi-quaking marshes. It is believed that sulfates from seawater are the agent behind the biogeochemical processes that cause the older peat to become essentially ooze. A rapid rise in sea level, moreover, could catch salt intolerant plants in these marshes before they were able to migrate naturally inland, with a massive die-off within a few years. Such a scenario has been occurring in freshwater parts of the Blackwater Wildlife Refuge, where a natural marsh 'dike' of brackish plants historically kept the high salinity waters of this part of the Bay from reaching a large freshwater cattail marsh more landward. In the late 1990s, the marsh dike broke, probably as a consequence of sea level rise, and very brackish water began pouring into the cattail marsh, killing the plants within a few years (see Figure 7.7, Plate 8).

Recent studies (Kearney et al, 2002) of marsh 'health' in the Chesapeake Bay have indicated that they may be poised for a rapid dieback as a result of not keeping pace for most of the 20th century with sea level rise. The trigger for such a catastrophe could be sea level rise itself, especially an episode like the decade long 'ramp up' in rate that occurred in the 1990s, which saw dramatic increases in rates of loss. Or, it could be a very hot summer, which would exacerbate the low dissolved oxygen problem further — or both factors combined with a drought, which causes inadequate flushing of sulfides that hinder the necessary uptake of nitrogen by the plants.

However, whatever the precise trigger that exploits conditions promoted by decades of rapid sea level rise, it will likely happen when least expected and sooner rather than later. The consequences will be losses and severe degradation of fundamental coastal ecosystems that sustain animals ranging from waterbirds and crabs to finfish, as well as providing a buffer to storm surges and waves in many areas. Unfortunately, these impacts, though having the potential to change fundamentally the Chesapeake's ecology and ecosystem services, may not be the only ones. Wicks (2005) found that eroding marshes leave a sea bottom not conducive to the establishment of sea grasses, which often are found just seaward of marshes. As sea level rises, and the sea grasses must migrate to shallower water to survive, in many cases there may be no place for them to go.

Impacts of increasing water temperatures and salinity

The struggle to control the pernicious effects of excess nutrients (nitrogen and phosphorus) flooding into the Chesapeake Bay has held center stage among the environmental concerns of the region for more than a quarter of a century.

Thematic class change: 1988 -2001

Figure 7.7 Marsh loss at Blackwater Wildlife Refuge on the Maryland Eastern Shore from 1988-2001 as derived from satellite data (see Plate 8 for color version)

Note: This marsh system, once the largest in the Chesapeake Bay, has lost more than 5000 acres of marsh since the late 1930s, with much of the marsh area that is left in severely degraded condition. It can be seen in the images that losses between 1999 and 2001 were greater than those between 1993 and 1999, showing the increasing effects of a sea level rise of over 1cm per year for most of the 1990s on the Chesapeake Bay region.

Some successes in mitigating the effects of this problem have been achieved, most notably in the return of sea grasses to many areas of the Bay (though this may indeed be offset by marsh loss as described above). Burgeoning population growth in the Chesapeake Bay watershed still remains the major threat to the Bay's ecology, with all the pollutants (such as nutrients) that the development produces. Nevertheless, future warming of the Bay waters during the summer as a result of global warming, coupled with more rainfall in the Susquehanna watershed (Fisher, 2000), could contribute to curtailing any further remediation. Warmer water has less capacity to hold dissolved oxygen, and the flushing of increasing amounts of nutrients from future development during spring floods into the Bay's tributaries could dramatically forestall any scenario for successful management of the Bay's living resources. At the very least, expectations for improvement in overall estuarine health may have to be revised, or even more stringent controls for nutrient influxes put into effect.

A greater unknown is the long-term impact of increasing salinity as ocean waters intrude into the system. An estuary represents a delicate balance between the mixing of freshwater from a river (in the case of the Bay, principally the Susquehanna) and saltwater from the ocean. Organisms correspondingly adjust to local variations in this balance, and even short-term large shifts in salinity up and down the system (i.e., longitudinal changes) can exact major repercussions. The intrusion of oyster parasites such as Haplosporidium nelsoni (which causes Multinucleated Sphere X (MSX) disease) into the middle and upper Bay, which are generally held in check by lower salinities, is a recent example. Salinities in the Chesapeake Bay have, of course, changed over time, as sea level rose and ocean waters intrude further inland. But whether organisms, especially those that tolerate only a narrow range of brackish water (i.e., salinities less than those of ocean water), can adjust to rapid, large-scale and persistent salinity changes is an open question. Aspects of competition for space in an ever-narrowing zone of appropriate salinity for bottom dwellers (benthic species) have yet to be examined in detail. Nonetheless, this could be the wild card in the mix of ecological changes that arise from sea level rise and global warming, with effects that could prove more lasting than any other.

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