«Presented by the Carolina Environmental Program Morehead City Field Site Students: Joseph Hester, Alison Kitto, Elizabeth Newland, Erika Poarch, ...»
Theoretically, a bulkhead dissipates wave energy by forcing the waves to break against the wall, but studies have shown that the displaced energy may increase scour and erosion at the toe of the wall and around the edges (Segar 1998). Since bulkheads have low permeability, they can reduce freshwater inputs through groundwater exchange. This assists the invasion of salttolerant species of plants, as well as disrupting biogeochemical cycling (Brinson et al. 1995).
These physical changes impact biological processes that allow diverse communities of plants and benthic invertebrates to survive. Scour and erosion can selectively remove fine sediments and nutrients, effectively lowering the growth rates of marsh grasses (Keddy 1985).
The increased sediment suspension may inhibit seedling emergence or result in the destabilization, uprooting or burial of vegetation (Kennedy and Bruno 2000). The presence of a bulkhead has been shown to correlate with reductions in fish biodiversity (Peterson et al. 2000;
Hendon et al. 2000). In contrast, other studies assert that bulkheads can serve as viable habitats for a range of primarily immobile species (Chapman 2003). These contrary findings indicate the current uncertainty of the degree to which marsh ecosystems are affected by bulkheads and other SHS.
Some believe bulkheads negatively impact marsh health and inhibit natural ecosystem functioning. This study attempted to quantify the ecological condition of marshes seaward of bulkheads relative to natural marshes. Relative condition was determined by comparing Spartina spp. stem counts and heights, chlorophyll a (chl a) concentrations, and sediment organic matter at four selected sites in eastern NC. An observational survey was also conducted to determine the number of bulkheads with marshes in relation to the total number of bulkheads along the shorelines of Bogue Sound, NC.
Methods and Materials Study Sites Located south-west of Cape Lookout lighthouse in the Outer Banks barrier islands of North Carolina are several shallow coastal water bodies that can be divided into two sections: the Newport River Estuary and Bogue Sound. The Newport River Estuary is a relatively small estuary of approximately 163 km2 and a watershed draining area of 595 km2 (Kirby-Smith and Costlow 1989). This study focuses on Bogue Sound (Figure II-1). Bogue Sound is a brackish lagoon separating Morehead City on the mainland side from the barrier island Bogue Banks, where the towns of Pine Knoll Shores and Atlantic Beach are found. Bogue Sound serves many commercial and residential uses and has salt marsh along its shoreline.
Figure II-1: The Southeast Region of the US showing Bogue Sound, NC (NOAA) Four study sites were selected in Bogue Sound, three on the northern bank and one on the southern bank. The first study site was located on the north side of Bogue Sound, in Mitchell Village (MV), at 34042N, 76048W. The second site was on the south side of Bogue Sound in the town of Pine Knoll Shores (PKS) at 34043N, 76046W, the third (MC) and the fourth (AB) sites were located on the north side at 34043N, 76049W, and 34043N, 76048W, respectively (Figure II-2).
Figure II-2: Location of our four study sites (MV, PKS, MC, and AB) (Google Earth) Study sites were selected as bulkhead, no-bulkhead pairs (Figure II-3). The study sites were selected based upon the presence of the following criteria: a shoreline that had a section of bulkhead with marsh in front of it, adjacent to a salt marsh with no bulkhead. Therefore, the marsh with no bulkhead served as the healthy marsh control in order to properly assess the bulkhead’s effect on marsh health.
Boat Survey In order to determine the number of bulkheads with and without marsh on Bogue Banks, a boat survey was conducted on November 1, 2006, at low tide. Bulkheads adjacent to channels and rivers were not included because they are subjected to different flow regimes.
Field Sampling During sampling the control (natural marsh) and bulkhead marshes were each divided into three transects, making a total of six vertical transects per site. These transects were further divided horizontally. Samples were taken approximately every 3-5m, depending on the length of the marsh. Sampling only occurred on one side of the transect to minimize disturbance to the marsh and our samples (Figure II-4). Measurements along the transect were taken at the first occurrence of Spartina spp..
All sampling occurred during low tide. At each of the sample areas, a 0.5m by 0.5m quadrant was sectioned off for assessment of the vegetation. Within this quadrant the following measurements were taken: percent coverage, type of dominant vegetation, Spartina spp. stem counts, and average plant height. Spartina spp. was counted because it is a dominant salt marsh species and an indicator of ecosystem health (Odum 1988). Sample metrics along each transect were divided in half, with the portion closest to the land designated as landward and the other portion seaward.
Cores for organic matter were taken twice per transect, once at the beginning (landward) and once at the end (seaward). Cores were 4cm long and had a diameter of 2.20cm. These cores were divided into surface (0-2cm) and depth (2-4cm). A subsample was taken to determine organic matter content which was estimated by weight lost on ignition at 525oC (Byers et al. 1978). Sediment sections taken from the sub-samples were dried at 1050C for 3h and weighed (A). The samples were then placed into a terminal muffle oven at 525oC for 3h and weighed again (B). Percent organic matter was calculated using Equation 1. Percent organic matter is used to determine the organic content of the sediment, an indicator of sediment quality (Schlesinger 1991). A higher percentage of organic matter in the sediment correlates to a greater amount of vegetation
The final parameter used for the determination of marsh health and quality was a measure of chl a per square meter, indicating the amount of benthic microalgae, which correlates to the amount of primary production. Benthic microalgae are a food source for many marsh organisms and regulate nutrient exchange within the ecosystem. Two cores, 1.0 cm in length with a diameter of 0.8 cm, were collected for chl a beginning at the first quadrant where Spartina spp. was present.
Table 1 shows the number of samples taken for each metric.
Table 1: Table indicating the sample size (n) for the field study metrics for each site.
Chl a was extracted from the sediment by sonicating the sample over ice in 10mL of 45:45:10 methanol: acetone: water mixture for 30 seconds, and then chilled for twenty four hours to allow for self-extraction. After extraction, 4ml of the sample was filtered through a Whitman CCF glass fiber filter (25mm) and analyzed (Stickland and Parsons 1972). The concentration of chl a was determined using a fluorometer (Triology Flourometer, Turner Designs) using chl a NA method, measuring raw fluorescence in RFU). For raw fluorescence over 6000 RFU, samples were analyzed in a spectrophotometer for absorbance at wavelengths of 750, 664, 647, and 630 nm (Tri-Chromatic detection using Mini 1240 UV-Vis spectrophotometer (Shimadzu)).
Statistical Analysis All data collected was analyzed for statistical significance. SigmaStat (SPSS 2003) software was used to perform ANOVA relationships. Kruskal-Wallis One Way Analysis of Variance on Ranks was used to evaluate one-way variance. Student-Newman-Keuls Method was used for pair-wise multiple comparisons, and two-way analysis. For the running of statistical tests, outliers were removed from the dataset (i.e. negative values for organic matter measurements).
Results Our field study, conducted at four sound-side locations in eastern North Carolina, examined the effects of bulkhead presence on marshes. We studied two different types of environments: marshes in front of bulkheads and natural marshes, defined as those without bulkheads. Marsh health was assessed using the following metrics: percent organic matter in sediment cores, chl a concentration, and vegetative characteristics (stem heights and live and dead standing stem counts). For all parameters, we found significant site variability, but overall, bulkhead presence did not have a significant effect. Significance was determined by a confidence interval of 95% (p0.050). A site survey of Bogue Sound revealed the interesting observation that bulkheads with marsh present on the seaward side were more common than bulkheads with no marsh present (Figure II-5).
Total without Marsh Total with Marsh Figure II-5: Numbers of bulkheads on Bogue Sound with and without marsh Organic Matter Percent organic matter at the natural and bulkhead marshes was plotted at each site for depth (2-4cm) and surface (0-2cm). The average difference in organic matter between the bulkheaded and natural marshes at each of the four field sites is portrayed in Figure II-6 and II-7 for landward and seaward margins, respectively. Standard error of means was used to determine the error associated with the measurements.
There was no significant relationship between samples from marshes with bulkheads compared to natural marshes (p=0.110), and no significant difference between surface and depth samples. There was high site variability (p=0.039), and a significant difference between landward and seaward samples across all sites (p0.001). By expanding the confidence interval to 90%, landward organic matter was significantly influenced by the presence of a bulkhead (p=0.072).
Samples were divided into two locations based on their location in the marsh: landward and seaward. Samples were also divided based on their core depth, where surface samples represented 0-2cm and depth samples represented 2-4 cm. Landward samples showed no significant bulkhead effect, site effect, or depth effect. Seaward samples also showed no significance with relation to bulkhead presence, site selection, or core depth. Both depth and surface samples showed a significant seaward/ landward dependency, p=0.001 and p=0.007, respectively. With a 90% confidence interval, the presence of a bulkhead significantly affected the organic matter at depth (p=0.084).
Site specific variation was also examined. Organic matter at sites AB and PKS varied significantly with respect to landward and seaward samples (p=0.001 in both cases). This factor was insignificant at sites MC and MV. Neither bulkhead nor depth was significant at any of the four sites. However, the bulkhead had a significant relationship with organic matter at site MC (p=0.073), if the definition of significance was expanded to p 0.10.
Figure II-7: Average percent organic matter for the seaward side of each site from both the cores taken at depth and surface. Sites include: Atlantic Beach (AB), Morehead City (MC), Mitchell Village (MV), and Pine Knoll Shores (PKS). Error indicated as the standard error of means.
Figure II-8: Average chlorophyll-a concentration (mg/m2) for three transects in marsh with and without bulkhead.
Sites include: Atlantic Beach (AB), Morehead City (MC), Mitchell Village (MV), and Pine Knoll Shores (PKS).
Error indicated as the standard error of means.
Vegetative Characteristics Spartina stem height (cm) was averaged for landward and seaward locations in each transect and plotted for each site (Figure II-9). Figure II-10 shows the total average Spartina stem density per 0.25m2, and the proportion of live and dead stems, for transects in natural marsh and marsh in front of a bulkhead at each site. Error associated with these measurements was assessed using standard error of means.
All vegetative characteristics exhibited significant variation by site: average stem height (p0.001), average number of live stems standing per m2 (p=0.023), and average number of dead stems standing per m2 (p=0.006). Neither live nor dead stem counts were significantly impacted by bulkhead presence (p=0.744, p=0.511), though there was a significant interaction between the site and bulkhead presence (p=0.014). The effect of the bulkhead on average vegetation height was not significant by the standard 95% confidence interval used in our analysis, however there was a significant effect at a 90% confidence interval (p=0.069).