Individual study: Vegetation change in created emergent wetlands near Hartford, Connecticut, USA
Moore H.H., Niering W.A., Marsicano L.J. & Dowdell M. (1999) Vegetation change in created emergent wetlands (1988-1996) in Connecticut (USA). Wetlands Ecology and Management, 7, 1988-1996
To compensate in part for wetland loss due to road construction, the Connecticut Department of Transportation created five small wetlands in the Connecticut Lowlands in 1983–84. In 1988 and again in 1996, changes in hydrology, water quality and vegetation were evaluated in four palustrine emergent wetland pairs, each including created and reference sites, to assess how closely they resembled the natural wetlands.
In 1983–84, five basins (<1 ha each) were created in the form of shallow depressions (to facilitate natural establishment of emergent wetland vegetation) and were additionally designed for sediment and erosion control in order to retain sediments during road construction and subsequent road runoff. They are located along interstate highways in the Connecticut River Lowlands to the east and southwest of Hartford.
Changes in hydrology, water quality and vegetation were evaluated in 1988, and again in 1996 in four of these wetlands, each paired with a natural reference site. The one created wetland not resurveyed had become a deeply flooded impoundment, and likewise one reference wetland that had been disturbed by road construction.
Overall, the artificial wetlands showed decreases in open water and water depth between 1989 and 1996 compared to more stable conditions in reference sites. Total nitrogen and specific conductivity (chloride levels above 800 mg/L attributed to road salt) was also generally higher compared to reference sites. Emergent plant cover increased from 30 to 39% at three created sites, but decreased at the fourth; reference sites remained relatively stable. Wetland plant species richness increased from 31 to 39 species at created sites and 35 to 42 species at reference wetlands. By 1996 there was an increase in invasive species, particularly common reed Phragmites australis (from <1 to 15% at created sites) and non-native purple loosestrife Lythrum salicaria (increasing at one reference site from <1 to 16%).
Common cattailTypha latifolia, dominant in the created wetlands in 1988, decreased from 16 to 5% while narrow-leaved cattail Typha angustifolia increased from 2 to 10%. At two created sites with high sedimentation, Phragmites had become dominant or co-dominant with Typha spp., whereas tussock sedge Carex stricta and T.latifolia remained dominant at reference sites.
These created wetlands were considered to have fulfilled the services of sediment retention, flood storage, and provided wildlife habitat, but a system to remove excess sediments and nutrients upstream of the wetland designed to compensate for wetland loss would be beneficial.
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