Response of plant and invertebrate communities to pothole blasting in a giant cutgrass marsh

Thousands of hectares of tidally-influenced, forested wetlands were cleared in the South Atlantic Coastal Zone and put into rice production during the eighteenth and nineteenth centuries. Many of these ricefields were abandoned in the late 1800s and were not maintained thereafter; hence, they no longer have functional dikes and provide poor habitat for waterfowl and wading birds due to colonization by dense stands of giant cutgrass (Zizaniopsis miliacea). Because efforts to open these extensive stands with herbicides and fire have been largely unsuccessful, in April 1997 we used an ammonium nitrate gel to blast a cluster of five potholes in a 162-ha abandoned ricefield system in Georgetown County, South Carolina. Potholes ranged from 100-175 m(2) and cost ranged form US$700 to $1,000 per pothole. Our objectives were to estimate and compare plant community characteristics and invertebrate biomass in the pothole cluster and control sites. We evaluated coverage of giant cutgrass and other vegetation 6 and 18 months post-treatment and estimated invertebrate biomass in November and March, one and two years after blasting. Giant cutgrass coverage was greatest along the edge and decreased in coverage from the edge to the middle of potholes. Giant cutgrass coverage increased in potholes during the 2-year period, but remained less dense than in other control sites. Biomass of isopods (O. Isopoda), amphipods (O. Amphipoda), and leeches (Cl. Hirudinea) increased from fall (November) to spring (March) in control and pothole sites. Amphipod biomass was greater in pothole than control sites each spring (P < 0.05). Blasted potholes in abandoned ricefields reduced emergent vegetation cover and provided open water for at least two years. In historically altered ricefield marshes with broken dikes and water control structures that cannot be repaired, blasting potholes is an option for improving the diversity of waterfowl and wading bird habitat in extensive giant cutgrass stands. However, more data from larger-scaled studies are needed to determine if open water areas will persist longer than two years, provide winter foraging and loafing sites, as well as refuge from disturbance.