«Presented by the Carolina Environmental Program Morehead City Field Site Students: Joseph Hester, Alison Kitto, Elizabeth Newland, Erika Poarch, ...»
Figure II-9: Average Stem Height (cm) for Spartina in 3 transects for 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.
Discussion Conventional scientific wisdom has assumed that bulkheads adversely affect coastal productivity and ecosystem health. In highly developed areas, the amount of bulkheaded shoreline present and visible from waterways is often assumed to have replaced what was once salt marsh. Our observational survey concluded that approximately 64% of bulkheads on Bogue Sound support marsh. This suggests limited negative effects of bulkheads on marsh. The exclusion of bulkheaded sites adjacent to creeks and channels provides a reasonable basis for a sound analysis, since the effect of boat wakes on these areas would have been exacerbated. Our survey sites are relatively isolated from such impacts and should be expected to accurately reflect current trends.
Based on analysis of all the metrics that were included in our study, none of the biological parameters were significantly different between bulkheaded and natural marshes at any of our four sample sites. The only statistical difference we found indicated a very high siteto-site variability, suggesting that complex local environmental conditions influence marsh productivity more so than do shoreline alterations. Such an effect was unexpected, given the fact that all sample sites were within several kilometers of each other. Seasonal wind patterns and tides, dramatically influence hydrodynamics along the waterfront edge of marsh areas. A deep boat channel and added boat traffic, approximately 500 meters off the shore, could create a different wave dynamic at our 3 mainland sites as compared to site PKS. The scouring by wave action increases the erosion on the seaward marsh edges. This could serve to flush benthic microalgae, sediment organic matter, and vegetation from the marsh. Three of the sites were on the south-facing shore. These sites may receive more photosynthetically available radiation than the other side. Adjacent to site AB, upland construction may have significantly altered the composition of stormwater runoff that enters the salt marshes in this area. These are just a few of the factors that could have lead to the dramatic local variation seen in our four sites, thereby preventing a sound comparison of the data.
With respect to entire coastal ecosystems, the fact that we found such high site variability within a study area of 13 square kilometers indicates that future studies must utilize locally stratified sampling regimes in order to accurately compare different locations. In order to develop a significant data set, more replicated samples must be collected. Our analysis has shown that site differences must be controlled before definitive conclusions can be drawn.
Based on our results, we suspect that at least 10 natural-bulkhead marsh pairs, stratified by local conditions, are needed for a more thorough investigation.
Though none of our metrics were significant within a 95% confidence interval, some data were statistically significant at a 90% confidence level, suggesting important trends that should be considered; with additional sampling these differences would likely become statistically significant. At this expanded interval, site MC measurements indicate lower sediment organic matter content in front of the bulkhead as compared to the natural marsh. In addition, vegetation height measurements at bulkheads are significantly lower across all sites. Thus, there is reason to suspect that vegetation heights may be suppressed due to the presence of a bulkhead. Since bulkheads reflect wave energies, it is possible that these reflected waves are scouring finer sediments. The presence of high organic matter concentrations in landward samples as compared to seaward samples tends to support such a situation. Organic matter concentrations seem to depend on proximity to water, leading us to conclude significant environmental interactions have greater influence on marsh health than does the presence of a shoreline hardening structure.
Chl a measurements collected at all four sites, though not statistically significant, fell within the expected range of values for estuarine environments in coastal North Carolina (Mike Piehler: personal communication). This suggests that our values may be representative of real biological trends in bulkheaded marshes, but further replication may reduce variability. Based on our data, there is no definitive effect of bulkhead presence on the biomass of benthic microalgae in these marshes.
Living and dead Spartina stem count differences were insignificant between bulkheaded and natural marshes for all sites. When individual sites were isolated, we found significant differences between bulkheaded and control marshes- different sites reacted in different ways to the presence of a bulkhead. Site MV showed a much greater number of living Spartina plants in the natural marsh, but the same natural marsh maintained fewer dead stems than its bulkheaded counterpart. At site PKS, the natural marsh supported much fewer living and dead stems. Sites MC and AB showed no differences. Even if a discernible pattern had emerged, conclusions about relative marsh health would be difficult to ascertain from alive-dead ratios as indicators. It is usually assumed that a larger proportion of living plants correlates with a higher degree of marsh health. However, standing dead stems contribute to primary productivity, and the overall health of a salt marsh ecosystem (Currin et al. 2006). By serving as important structure for benthic microalgae, these stems provide a great deal of surface area for trophic energy transfer and nutrient cycling. Moreover, the increased structural complexity provided by standing dead stems can serve to increase current baffling throughout the marsh. This hinders sediment scouring by waves, and allows for the deposition of sediments. Standing dead stems facilitate accretion of the marsh, protect fragile seedlings, and recruit new vegetation. The lack of a definitive pattern suggests that the marsh vegetation in front of bulkheads is as healthy as vegetation in adjacent areas free from bulkhead influence.
Despite limited sampling breadth, our data indicates that bulkheads do not negatively impact marsh health, as defined by our parameters. However, our results do not exclude the possibility of negative impacts occurring at and after a certain threshold value for wave action.
We suggest that under moderate stress levels caused by usual tides, boat wakes, and wind-waves, bulkheads do not harm marsh health, but we cannot conclude anything about marsh health during periods of high stress, potentially caused by large storm events, exacerbated wave action, or sea level rise and subsequent inundation. The degree to which sediment deposition can sustain these marshes in the face of rising water levels is uncertain. We cannot predict marsh-bulkhead behavior in this environment, and further investigation is recommended.
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