The inventory and assessment of tidal restrictions and the Phragmites mapping indicate that several hot spots of ecological concern are found in the marsh. Two tidal restrictions, the Eastern Road (Restriction # 15, Figure 3) and Route 1 (Restriction #4, Figure 3) were selected for further analysis. The Eastern Road was selected because the tide gate that was located there for approximately 60 years seems to have affected the plant community now found on a significant portion of the marsh inland of the road. Marsh peat frequently decays due to lower salinity levels caused by a tidal restriction, resulting in a lower marsh surface elevation inland of a restriction. This in turn often results in a different plant community than found seaward of the restriction. We suspected the marsh surface may have subsided inland of the Eastern Road. The second location was Route 1, which was selected because almost half of the Phragmites found in the Scarborough Marsh is located in this area.

The objectives of this portion of the study were to:

1. estimate the impact that these tidal restrictions have on tidal elevations (depth of flooding over the marsh), and
2. determine if tidal restrictions had caused the marsh surface to subside, and
3. look for differences in plant communities on either side of restrictions.

Four pairs of transects were established on either side of the Eastern Road and Route 1. Each member of a transect pair was located on opposite sides of the restriction and in similar topographic positions (generally on high marsh). Transects RR 1-2 and RR 3A-3B are located on the Eastern Road; Transects 4 and 5 are located on opposite sides of Route 1 just west of the Dunstan River; and Transects 6 and 6A are located about 1,000 feet east of the Dunstan River on opposite sides of Route 1 (Figure 4).

A surveyorís transit was used as a level to measure elevations at representative points along each transect. The percent coverage of plant species was estimated on a 10-ft. diameter plot surrounding each elevation data point. One tidal monitoring gauge was located on each transect to record maximum tidal elevation at spring tides. Salinity was recorded with a refractometer from water samples taken from the high marsh surface at low tide. The measurements reflect the salinity of the top 1-2 cm of the marsh peat. Tidal elevations and salinity were measured on two to three spring tides per transect from September to December 1998.

The impact of the Route 1 tidal restriction is shown in Table 3. During spring tides the high tide level averaged 2.8 inches lower on the inland side of Route 1. Flooding depth ranged from 10 inches to 15 inches over the marsh surface, which is similar to depths found seaward of the Eastern Road where there are fewer tidal restrictions. On Transect pair 6-6A flooding depths were greater on the inland side of the highway, which apparently is due to the marsh surface having subsided over 5 inches on the inland side. The data indicate possible subsidence of the inland side of Transect pair 4-5, but the estimate is less reliable due to the more variable terrain in this area. Salinity measurements (parts per thousand) indicate that there is little difference on either side of the highway near the Dunstan River (Transects 4 and 5). However, salinity appeared to be lower on the inland side of the highway at transect pair 6-6A where a large freshwater wetland combined with the dam effect created by Route 1 is apparently affecting the salinity of this part of the marsh. More salinity measurements should be taken to confirm this apparent trend.

The differences in tidal depth, marsh surface subsidence, and salinity are reflected in the plant communities observed along the transects. The plants observed along Transects 4-5 were generally similar on either side of the highway. The predominant plants, in order of relative abundance, are listed in Table 4. The mix of species is typical of the inland reaches of a tidal marsh where freshwater is beginning to influence the plant community. The Transect passes through a small stand of Phragmites, on the seaward side of Route 1. Phragmites generally is found on the inland side of tidal restrictions or on the marsh fringe. This stand is surrounded by apparently healthy high marsh with patches of salt meadow hay and bulrush, and there are no significant physical features (marsh elevation, disturbance, or ditches) that might explain its presence. It is clear from this stand that Phragmites is capable of spreading seaward of Route 1 in areas of healthy high marsh.

The differences in the plant communities at Transects 6-6A are more typical of a restricted marsh and reflect the differences in salinity and marsh surface elevation. Typical high marsh vegetation is found seaward of Route 1, whereas a mixture of salt marsh, brackish marsh, and invasive species is found immediately across the highway on the inland side (Table 5).

Overall, it appears that the Route 1 tidal restriction is a relatively minor influence on tide heights. However, observed subsidence of the marsh surface and prevalence of Phragmites on the inland side away from the influence of the Dunstan River indicates that the impact of Route 1 as a restriction is significant where freshwater influence is strongest. This suggests that attempts to slow the spread of Phragmites should focus first on reducing freshwater influence near the marsh perimeter, possibly through a system of ditches. This potential mitigating measure is supported by observations of plant communities on either side of the Dunstan River inland of Route 1. Phragmites is almost absent and typical salt marsh vegetation is more dominant west of the river where there are ditches extending from the river toward the upland. On the opposite side of the river there are very few ditches and Phragmites is much more aggressive.

The impact of the Eastern Road tidal restriction is shown in Table 6. The high tide level was 4.2 to 7.1 inches lower (average of 5 inches lower for 8 observations) on the inland side of the road during spring tides. Flooding depth ranged from 14.2 inches over the marsh surface on the seaward side of the road to 15.1 inches on the inland side. (Because Transect pair RR 1-2 consisted of high marsh on the seaward side and mostly low marsh on the inland side, depth of flooding and subsidence could not be compared at that location.) The survey found that the high marsh surface was 6 inches lower on the inland side of the road at Transect pair RR 3A-3B, where the marsh surface was otherwise similar on both sides of the road. The subsidence of the marsh surface accounts for the greater depth of water over the marsh surface on the inland side of RR 3A-3B, despite a slightly lower water level on that side of the road. Measurements indicate that there is little difference in salinity on either side of the Eastern Road. Salinity was slightly higher on the seaward side on two occasions. More salinity measurements should be taken to confirm this apparent trend.

Transect RR 3A-3B provided the best comparison of plant communities on either side of the Eastern Road. The mix of plants on the seaward side of the road is typical of a healthy high marsh, with Black Grass and Salt Meadow Grass predominating (Table 7). The vegetation is noticeably different on the inland side of the road, where Black Grass is entirely absent and the short form of Smooth Cordgrass is the most abundant plant. The short form of Smooth Cordgrass, commonly found where high marsh has decayed or become less well drained, is characteristic of the lower marsh elevation on the inland side. The differences in the plant communities on opposite sides of the Eastern Road are quite apparent even to a casual observer standing on the road.

Table 7. Eastern Road, Transect RR 3A-3B; Predominant plants in order of relative abundance.

The marsh surface subsidence and subsequent change in plant community are the result of the tide gate that was located in the Eastern Road for approximately 60 years (see Section III for a history of tidal restrictions on the Eastern Road). Dead birch trees at the end of Transect 3A, located approximately 500 feet seaward of the current edge of the marsh, attest to the severity of the restriction. Because birch trees cannot survive in salt-saturated soils, the dead trees verify that there was no salt water influence inland of the Eastern Road for a significant time. Although tidal flows are much improved due to the removal of the tide gate and the new channel that cut through the road during the 1950ís, the legacy of past tidal restrictions remains today.