Taunton River Salt Marsh Assessment Results from 2014 season December, 2014 During the late summer and fall of 2014, Save The Bay evaluated salt marshes in Assonet Bay, Freetown and Broad Cove in Dighton and Somerset, using the Rhode Island Salt Marsh Assessment monitoring protocol. This assessment has been used during the past three years in marshes within Narragansett Bay. The Broad Cove marsh was the northernmost marsh surveyed within the Narragansett Bay watershed to date. We also assessed a section of marsh in Assonet Bay near Payne Cove in the fall of 2012. The results of these assessments were compared with the overall data for all Bay marshes. 1
Locations of Assonet Marsh Transects, all located around Assonet Bay Shores in Freetown Payne Cove Marsh, completed in 2012 Shepherds Cove 2
Wescott Island In general, these areas of salt marsh are isolated within small coves and are bordered by steep uplands surrounded by residential development. It is important to note that Assonet Bay Shores relies on City of Fall River water and septic systems, so residential water is discharged to the ground, resulting in high water tables in the surrounding salt marsh areas. 3
Locations of Transects in Broad Cove East side, Dighton East side, Somerset 4
West Side, Dighton The marshes around Broad Cove are bordered by gentle uplands that are in use as pasture or residential landscape and open woods. This area is sewered and is served by public water. The marsh on the west side is bordered by Route 138 and on the east side by Pleasant Street and the Taunton River. Marsh Zonation It is generally recognized that the salt marshes in the Taunton River and in the Assonet River in particular are some of the healthiest in Narragansett Bay. The Assonet Bay has the largest areas of contiguous marsh in the Narragansett Bay watershed. In general, the marshes in the Taunton River have a higher elevation in relation to their tidal range. This higher elevation makes these marshes less susceptible to sea level rise as documented by the large percentage of the marsh that is dominated by high marsh vegetation. Other marshes in Narragansett Bay are dominated by Spartina alterniflora and show larger transitional zones and pannes. The tidal range at Fall River is about one foot higher than at Newport. The marshes in Broad Cove in particular have more space to migrate inland across a low coastal plain that is currently in agricultural use. Marsh migration is occurring in a mowed field on the 5
eastern side of Broad Cove as documented by Phragmites and upper salt marsh vegetation growing in a field adjacent to the marsh. Many of the marshes in Narragansett Bay have begun to show a change in plant zonation due to frequent flooding and increasingly high tides. Typical marsh zonation outlined below In marshes that are transitioning due to sea level rise and other flooding factors, areas of high marsh shrink to the outer and upper edges of the marsh that are higher elevation and can drain. Short-form Spartina alterniflora (a low marsh grass that can tolerate more frequent flooding and standing water to a degree) becomes dominant in a larger percentage of the marsh due to increased salt water inundation and flooding. In transitional marsh areas, high marsh vegetation and short-form Spartina alterniflora grow together. Over time high marsh vegetation is replaced by short-form Spartina alterniflora or bare areas based upon the amount of flooding or inundation. 6
Transitioning marsh outlined below Salt marshes in Assonet and Broad Cove exhibit a more traditional zonation and a higher percentage of high marsh than other Narragansett Bay and coastal Rhode Island marshes. They have a much smaller percentage of short form Spartina alterniflora and transitional marsh where low marsh and high marsh plants grow together. When Broad Cove and Assonet are compared, Broad Cove exhibited a smaller percentage of low marsh because of high rates of erosion along the marsh edge. Large numbers of fiddler crabs were observed during sampling and in some areas low marsh on the edge was almost entirely eroded away with high marsh growing right to the marsh edge. Broad Cove also showed a large terrestrial border when compared to Assonet which is bordered by a steep upland. Broad Cove has large areas of Phragmites that are expanding into what were once active fields. 7
Plant species The biggest contrast between Taunton watershed marshes and those in the rest of the study area was shown with percentage cover of Spartina alterniflora. There is less coverage of Spartina alterniflora in the Taunton River marshes, little or no short-form Spartina alterniflora or transitional marsh and no unvegetated areas. In the Taunton, Spartina alterniflora is found on the outer edge of the marsh. There is a greater percentage of high marsh grasses including Spartina patens, Juncus gerardi and Distichlis spicata in marshes of the Taunton River in comparison to marshes in the rest of Narragansett Bay and the south shore of Rhode Island. There is little or no transitional marsh in the Taunton River where high marsh plants and shortform Spartina alterniflora grow together in comparison to the rest of Narragansett Bay. Additionally, there are no bare or die off areas, where no vegetation is growing. These bare areas were documented in most marshes in Narragansett Bay and the coastal ponds. Marshes in the Taunton River, in particular within Broad Cove, are rapidly eroding along their tidal edge, but are retaining their interior elevation. The marsh interior shows higher percentages of salt marsh grasses and rushes such as Juncus (black rush) and Distichlis (spike grass). Tall form Spartina alterniflora along the outer edge of the salt marsh has largely been eroded away, and there are few if any transitional zones. The cause of this marsh edge erosion is unclear and may be related to higher tides that cause erosion of soil particles within the marsh peat or it could be caused by herbivory of the marsh plants due to higher numbers of fiddler crabs and purple marsh crabs. The rapid erosion of the marsh edge and widening of creeks is being observed in marshes throughout the upper Narragansett Bay. 8
Plant Species/Cover Type Soil bearing capacity Soil bearing capacity also presented a large contrast between Taunton River marshes and those in the rest of the study area. Soil bearing capacity measures the ability for the soil to support a load applied to the surface. It shows the integrity of the marsh peat which holds plant roots and soil. In the study area, the bare areas and the areas of short-form Spartina alterniflora had low bearing capacity (high penetration depth), which means that their internal structure is weak. Broad Cove exhibited low bearing capacity in the low marsh with rapid erosion and slumping of the marsh edge and more stable high marsh in comparison to all marshes. Assonet Bay showed a very soft high marsh, with a more stable low marsh. The integrity of the high marsh in this area may be compromised by nutrient rich groundwater due to septic system discharges. It has been shown that in eutrophic areas (areas with high human input of nutrients), salt marsh peat tends to be weaker, more decomposed and less resistant to erosion. In addition, increased inundation due to sea level rise causes declines in plant productivity and accumulation of below ground biomass. These results show that these marshes are beginning to show early signs of stress and may begin to experience die back in the future. 9
Penetration Depth by Marsh Zone Conclusions While the Assonet and Broad Cove marshes seem to be healthy by looking at them, they are highly susceptible to erosion and are beginning to show signs of stress related to high nutrient loads, sea level rise and crab predation. Because of their higher elevation, the traditional marsh zonation with large areas of high marsh remains in this area. The marsh surface, however, shows signs of pock marks and hummocks, and bearing capacity shows that the internal marsh structure is failing in the Assonet marshes. A recent study has shown that marshes may be swelling due to being waterlogged. Below ground growth of organic material is the primary way that marshes keep up with sea level rise, because our watersheds are generally sediment poor. Without this growth of marsh peat, continued inundation will cause further marsh subsidence. This is being seen first in marshes throughout Narragansett Bay, and evidence is beginning to show signs of marsh degradation in the Taunton River as well. Continued monitoring and evaluation will need to be conducted to see if this process continues. 10
The Taunton River presents an opportunity to support marsh migration through the continued protection of land adjacent to salt marshes. While those marshes that are bordered by steep upland, such as those near Assonet Bay Shores, may not be able to migrate and may drown in place, others that are bordered by low lying agricultural fields may be able to migrate inland. It will also be interesting to look at brackish marshes higher up in the watershed to watch their transition as the salt wedge moves northward with sea level rise. Additionally, there are sites on Broad Cove where creek excavation could occur to drain newly forming impounded water areas. Photos Broad Cove, September 2014. Photo shows crab holes and edge erosion. Note the lack of Spartina alterniflora and high marsh extending to marsh edge. 11
West Side of Broad Cove, September 2014. Large areas of Spartina patens with some standing water in un-vegetated pannes. Assonet, Shepherds Cove, October, 2014. This photo shows the pock marked nature of the high marsh where hummocks of Spartina patens grow with areas of bare ground between. These areas of high marsh showed relatively low bearing capacity compared with high marsh in other areas. 12
East Side of Broad Cove, October 2014. High marsh dominated by Juncus gerardii and Distichlis spicata. This marsh zone was very stable and solid. This zone type is much more dominant in the Taunton River than in other marshes in Narragansett Bay and the Rhode Island south shore where transitional areas dominate. 13