«Presented by the Carolina Environmental Program Morehead City Field Site Students: Joseph Hester, Alison Kitto, Elizabeth Newland, Erika Poarch, ...»
Landowners are often only concerned with erosion on their shoreline, ignoring implications for neighboring properties. Bulkheads lead to changes in water flow and sediment dynamics that affect the entire estuary. The construction and implementation of a bulkhead reduces the volume of sand that is available for shoreline transport, causing adjacent sites to become starved of sediment (Lee et al. 1998). To remedy this situation, a neighborhood bulkhead effort, involving contractors and government officials who understand the potential cumulative impacts, is critical (NRC 2006).
The biological impacts associated with SHS range from immediate impacts caused by bulkhead construction, to post-construction changes in biodiversity, community structure, and ecosystem function in tidal marsh and other intertidal habitats adjacent to hardening structures.
The construction process for all SHS causes some degree of immediate, short-term ecosystem disturbance. Hard-bottom structures, such as rock jetties, groins, and revetments, initially eliminate benthic and infaunal organisms at the construction site. The intrusion of heavy equipment may also elicit behavioral responses from mobile marine organisms and nesting seabirds that cause them to relocate (Feist et al. 1996; Mulvihill et al. 1980). Over the long-term, SHS perpendicular to the shore may indirectly alter neighboring soft-bottom benthic communities by modifying the transport of sediment and organic matter by longshore currents.
In addition, maintenance of jetties may involve repeated dredging and therefore continual disturbance of benthic and infaunal communities (Williams and Thom 2001).
The construction of SHS parallel to the shore, such as bulkheads or rip-rap, is likely to cause direct physical damage to fringing salt marsh vegetation which often characterizes the estuarine water line. If this removed native vegetation, which includes marsh grasses Spartina spp. and Juncus roemerianus, is replaced by non-native lawn grasses, the biological functions of the marsh, including erosion control and filtering of stormwater by nutrient transformation, may be diminished (Street et al. 2005; Watts 1987). Furthermore, the vegetation in the upper marsh, or land-sea transition zone, is more likely to be destroyed by the construction process. This upper marsh vegetation is typically more diverse than that found in the lower marsh, possibly because the milder environment allows a greater range of species to survive here.
A study in the Great Bay Estuary, New Hampshire of five paired salt marsh sites, each with a bulkhead adjacent to natural shoreline, found complete loss of the transition community in bulkheaded sites. The result of this loss is decreased biodiversity in marshes adjacent to bulkheads, with potential implications for productivity and the community structure supported by the habitat. The between-site variation in plant species composition was higher for the transition zone compared to lower marsh (Bozek and Burdick 2005). In addition to vegetation loss, the disturbance of soil along the shoreline during construction can cause increased sediment loads to enter the water, resulting in higher turbidity in the water column that may reduce primary productivity by light attenuation, interfere with visual feeding by fish, and smother benthic organisms (Watts 1987).
There is evidence that the ecological consequences of bulkhead construction for the salt marsh in front of these structures are not limited to initial construction-related disturbances, but persist as a result of the physical presence of the bulkhead. Whereas fringing salt marsh would normally migrate further upshore to avoid being permanently submerged by rising sea level, the presence of a bulkhead prevents this from occurring, potentially resulting in a reduction in total marsh area over time (Titus 1988). Any marsh that might have existed prior to bulkhead construction will be permanently flooded, leading to an increase in turbulence and scouring which prevents vegetated communities from re-establishing (Watts 1987). In 1973, Garbisch et al. found that Spartina seeds planted in front of a bulkhead experienced 63% mortality after 2.5 months while the seeds along the natural shoreline had a mortality rate of 12%. This likely resulted from the increase in erosion or re-suspension of sediment caused by the bulkhead.
There is additional evidence that the placement of the bulkhead can influence the magnitude of biological impacts. A field survey of Raritan Bay, New Jersey found lower meiofaunal abundance and increased eroded sediment in estuarine sandy beach foreshore associated with bulkheads constructed lower on the intertidal profile, while the sites with a bulkhead built high on the profile had meifoaunal abundances comparable to sites with no bulkhead present (Spalding and Jackson 2001).
As previously mentioned, scouring at the base of the bulkhead can result in long-term increases in mean suspended sediment in the water column, causing chronic increases in turbidity (Miles et al. 2001). Impacts on marsh vegetation from sediment removal may include decreased growth from the removal of nutrients, burial by erosion, destabilization, or complete removal of plants (Kennedy and Bruno 2000). The species composition and ecosystem function of marsh vegetation can also be altered if tidal influences are reduced as a result of SHS. For instance, as marsh salinity decreases, it provides more favorable habitat for the invasive marsh grass Phragmites, which may out-compete native species (NRC 2006).
Estuarine salt marsh provides habitat for a number of resident finfish, shellfish, and crustaceans. Transient nekton also use the marsh edges for seasonal spawning and nursery grounds. Among these organisms are economically important fisheries species such as spot, croaker, red drum, and penaid shrimp (Street et al. 2005). Some studies have indicated decreased nekton abundances and lower diversity of taxa found in salt marshes in front of bulkheads. For example, in Juncus/Spartina marshes along the Gulf Coast, bulkhead presence corresponded with lower abundances of both demersal resident species and transient species, as well as lower overall species diversity (Peterson et al. 2000). Hendon et. al (2000) found significantly fewer larval naked gobies, a common benthic fish, in bulkheaded marshes compared to natural marsh.
These studies suggest that the value of salt marsh as a nursery habitat may be reduced by the presence of a bulkhead.
However, some researchers argue that bulkheads offer a unique habitat for colonization.
Studies have shown that algae and invertebrates settle on the artificial structures, though the recruitment and settlement rates may differ from natural assemblages (Bulleri 2005). Bulkheads also appear to supply suitable habitat for mollusks, although the lack of spatial heterogeneity may exclude rare taxa (Chapman 2006). As mentioned above, mobile species appear to be less common in bulkheaded areas than natural areas (Chapman 2003). Chapman asserts that the potential value of bulkheads as viable habitats will be dependent on their ability to support a full range of species, including rare taxa (2003). No conclusive evidence exists that the artificial bulkhead environment can successfully mirror, or take the place of, the natural habitat that would exist without the bulkhead.
Other SHS, particularly those that are not vertical, may provide habitat for a greater variety of species, including mobile organisms. Rubble structures, such as jetties and groins, provide hard substrate for colonization by macroalgae and sessile invertebrates such as oysters, barnacles, and mussels. Macroalgae and the epiphytic algal species they host provide food for fish, small crustaceans, and other benthic grazers. The most abundant resident fish documented at jetties in the South Atlantic Bight, are pinfish, spottail pinfish, black sea bass, and pigfish, as well as blennies and gobies. These smaller fish in turn attract larger piscivorous fish species and transient fish that migrate seasonally (Hay and Sutherland 1988). Jetties often extend from the intertidal into the subtidal zone, which accounts for their ability to support higher trophic levels compared to vertical shoreline structures (Williams and Thom 2001). In general, greater structural complexity accommodates a higher diversity of species due to the presence of additional microhabitats, such as protective crevices and gaps.
Rip-rap, like jetties and groins, supports more diverse communities than its vertical counterparts. A comparison between natural marsh and marsh in front of rip-rap and bulkheads in two tributaries of the Chesapeake Bay found higher bivalve densities and benthic species diversity in natural marsh; however, in the more pristine system, overall benthic diversity and abundance in marsh adjacent to rip-rap more closely paralleled those of the natural marsh. This suggests that adjacent marsh may be less affected by rip-rap when a smaller percentage of the total shoreline is developed (Seitz et al. 2005). While species richness is typically greater in natural shoreline compared to hardened sites, rip-rap in San Diego Bay provides a unique habitat, supporting species that thrive in an environment with less wave energy and higher turbidity.
This SHS also supports open-coast species that favor hard-bottom substrate. Organisms that colonize the rock surfaces can also provide food for highly motile fish at high tide, and foraging shorebirds at low tide (Davis et al. 2002).
All of the previously cited impacts of SHS on biodiversity, recruitment, and community structure in intertidal environments have implications for the overall productivity of these habitats. Detrital material is a major component of most aquatic food webs and influences secondary production. When marsh vegetation is destroyed it reduces organic matter as well as production and export of detritus, leading to a decrease in productivity of the shoreline. The amplification of waves and currents caused by SHS removes detritus, further decreasing productivity (Brinson et. al 1995). Retention of nutrients and benthic primary productivity are reduced as a result of higher energy shorelines. Loss of vegetated habitat will affect nutrient exchange at the sediment-water interface.
Since the passage of the Rivers and Harbors Act in 1899, the US Army Corps of Engineers has maintained jurisdiction over all permitting for coastal projects which may affect the navigability of public waters. General permits, which landowners need not apply for, are granted for activities that the Corps has deemed acceptable and even beneficial on a nationwide scale. These permits require no review process or approval by the USACE and include actions such as the restoration of degraded wetland areas. However, one such general permit includes permission to construct shoreline protection structures shorter than 500 feet in order to prevent erosion losses. This general permit enabled anyone to construct a small bulkhead in any location they deemed appropriate. In contrast, individual permits are required for any activity that involves sills or breakwaters since the proposed structures would be located offshore, within navigable waters (NRC 2006). Since United States public waters are so tightly regulated, this has led many landowners to avoid the expensive and time-consuming individual permitting processes with a stronger inclination toward bulkheads landward of the mean high tide line.
Therefore, the general permit process has effectively created an incentive for landowners to only concern themselves with terrestrial sediment protection and abandon the protection of eroding marshes.