Historic Ditching Effects on Salt Marsh Structure

by Doug Lashley | November 15, 2013

Salt marshes on a global scale are highly productive transition zones between marine and terrestrial habitats.  Salt marshes are often called the “nurseries of life,” providing sanctuary for thousands of aquatic, avian and terrestrial animals.  Migratory birds use these resources as resting, foraging and nesting sites year after year.  One third of our nation’s endangered species call these resources their permanent homes, while half utilize them some point in their life.  Salt marshes in particular contribute to the national economy by producing resources and commodities and providing other societal benefits. Salt marshes Yield fish for the nation, Provide recreational opportunities, Improve and Maintain water quality, Help control floods, Buffer storm damage and Maintain/support both recreation and commercial fishery based economies.

Salt marshes in an historical context, within the New England States, were highly valued by early colonists for salt hay and aesthetics. In time these values diminished with the introduction of European grasses and industrial development.  Many marshes were eventually filled to accommodate development, maintain navigation channels and heavily ditched to promote agriculture and control mosquito populations. Following the great depression, rational methods, such as seasonal surveys and breeding connectivity studies were abandoned in favor of large scale parallel ditching that today dominates the landscape of our Atlantic Coast marshes. Unskilled laborers dug ditches in a grid pattern to effectively drain the marsh. Aside from being unaesthetic, these practices are detrimental to marsh structure and function.  Additionally, many marshes that were ditched never supported breeding populations of mosquitoes. Widespread use of this ditching method, whether to control mosquitoes, facilitate agricultural expansion or create developable land has resulted in a damaging domino effect, initially altering the natural salt marsh hydro-period (depth and duration of inundation) and causing major losses of important associated functions. Hydrology is the keystone in the arch of wetland functionality, defining character, composition, structure, function and value. When natural hydrology is altered, many other important functions are impaired.  One can note today in these ditched systems the prevalence of invasive/exotic vegetative communities, indicating a visible expression that hydrologic alterations have impacted the marshes chemical, physical and biological structure. Extensively ditched salt marsh systems drain to quickly result in prolonged oxidized conditions which negatively effects organic matter accumulation, sulfide production, and pH. This results in well drained oxygen rich soils that facilitate the establishment and growth of certain invasive species like common reed (Phragmites australis) along with attendant impacts on the sediment and nutrient cycles.

Further understanding of these historic ditching practices has revealed that; 1) Altered hydrology significantly impacts physical and chemical processes due to changes in the patterns of reduction and oxidization such as marsh building or the accretion of  marsh peat which influences vegetative composition, biological usage and nutrient cycling and the ability of marsh to self-maintain, to name a few. The geometry of a typical ditch versus natural tidal creeks presents very little wildlife habitat for prey or predators, limiting accessibility and foraging opportunities.  For example, ditching restricts the movement and minimum habitat configuration of killifish (Fundulus spp.) a keystone prey species.  The absence of this species results in unchecked population growth in invertebrate species such as mosquitos and eliminates foraging opportunities for wading birds such as egrets and great blue herons.

To reverse this historic trend and restore systemic function and value, GV has engaged in several hydraulic and hydrologic (H&H) restorations of ditched salt marsh systems. This H&H restoration re-establishes the historic patterns of tidal ebb and flow and thereby nominal chemical and physical processes. The restored tidal ebb and flow results in greater periods of reduction on the marsh plain and increased salinities on the surface of the marsh and within soil pore water. Resetting these parameters re-establishes associated chemical, physical and biological processes, thus resetting the systems functionality. A surficial sign of system health is the return of native plant assemblages and habitat structure. When the habitat structure is restored along with other key processes, wildlife usage of the marsh rebounds and closes the “biotic feedback loop” supporting a self-sustaining natural cycle of life. Overall, this restoration has a positive net effect on water quality, nutrient transformations/cycling, detrital generation and export, sediment import/export, and wildlife habitat.

GreenVest (GV), LLC has internally focused on identifying and restoring important coastal resources in New Jersey. These coastal resources help build community resiliency by absorbing storm energy and buffering against flood damage.  Projects such as our Atlantic Coastal Mitigation Bank will reverse historic impairments and restore habitat for various endemic, transient and endangered aquatic, avian and terrestrial fauna. This project will also contribute to local community resiliency in the Pleasantville/Atlantic City area, as a first line of defense for deflecting storm energy/damage and absorbing flood flows. This project will be preserved in perpetuity, in a restored condition, becoming part of New Jersey’s protected natural lands as a component of the Absecon Wildlife Management Area.   Implementing important coastal restoration projects using cutting edge techniques with private sector funding, GreenVest remains one step ahead.

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