Threat: Sea Level Rise

Sea level rise in Wellfleet, MA is predominantly a combination of rising ocean waters and land subsidence. Changes in sediment transport within the harbor may also affect relative sea level rise, by changing the amount of sand deposited on the bottom in different locations. An additional factor that may increase sea level rise in the northeastern US compared to other parts of the globe is the slowing of ocean currents.

In the northeastern US the combination of rising ocean waters and land subsidence is leading to higher relative sea level rise compared to many other coastlines in the US and elsewhere. Since 1900 the rate of sea level rise in the northeast has exceeded the global average by approximately 4 inches to about 1 foot in the Northeast versus about 8 inches globally over the past 110 years. The most recent assessments project a global sea level rise of 1-4 feet by 2100, with the northeast continuing to exceed global averages.

Sea Level Rise Impacts Relevant to Shellfishing

1. Increasing salinity in estuaries by extending saltwater penetration upstream. Increased salinity has been linked to higher QPX mortality in clams and MSX infections in oysters.
2. Loss of habitat, including important foraging and nursery habitat for some species. Loss of habitat is due in part to submergence of coastal areas and increased erosion of coastlines in areas where the habitat and coastline cannot migrate landward like they would in a natural system. This is especially important in terms of the loss of intertidal habitat. Loss of habitat will be exacerbated as sea level rises against armored shorelines that prevent the landward migration of intertidal flats and wetlands.
3. An increase in the proportion of freshwater runoff into the Harbor from direct stream discharge, versus groundwater discharge, as coastal aquifers rise along with mean sea level. There is an increased risk of septic leachfields become inundated by rising sea and groundwater levels. A larger fraction of freshwater discharge will no longer pass through highly organic, anaerobic and, thus, denitrifying coastal sediments; in this way more nitrogen may be delivered to coastal waters to fuel eutrophication.

Implications for Shellfishing

Rising sea levels in Wellfleet Harbor may :

1. Change the proportion of inter-tidal and sub-tidal habitat areas, impacting the areas and techniques of growing both oysters and quahogs. An oyster that is continually submerged has the opportunity to feed longer than an oyster exposed at low tide but that oyster under water is also increasingly subject to sedimentation and burial as well as longer exposure to diseases, pests, and predators. There is some evidence that the food supply varies at different depths. At deeper depths food supply can be very restricted. Fishermen may need to change (and buy) gear to adapt to changing conditions.
2. Alter species’ location and composition, including predators of shellfish as habitat type and availability shifts.
3. Degrade water quality in Wellfleet Harbor. Septic systems may fail as groundwater level rises to flood septic leachfields further inland. Furthermore, the route of freshwater discharge into Wellfleet Harbor will be altered with an increased surface flow into the Harbor. Consequently, less freshwater will flow through highly organic wetland, and intertidal and subtidal sediments, leading to less denitrification and bacterial removal. Because of development, marshes may not be able to migrate landward as sea levels rise. The consequent loss of marshes, which help to filter water, can exacerbate water quality problems in the Harbor.
4. Change how shellfish can be grown and harvested. Rising sea levels will mean more grants are in deep water areas, such as those in Provincetown and Truro, thus requiring different methods of growing and harvesting over time.
5. Disrupt access to grants and lead to inadequacy of infrastructure. Some shellfishermen in Wellfleet are already experiencing difficulties driving to their grants because of erosion, shoreline retreat, and access constraints. Rising sea levels (and coastal erosion associated with sea level rise) will exacerbate this problem. More moorings may be required if more shellfishermen need boats to work in deeper waters.
6. Lead to resource use conflicts. Some gear used for deep water growing may lead to new conflicts among different users of the Harbor (e.g., with recreational boaters . Use of gear in deeper water has the potential to cause entanglements of marine mammals and sea turtles.