Guidelines for “Open Marsh Water Management” in Delaware’s salt marshes—Objectives, system designs, and installation procedures

Abstract

Open Marsh Water Management (OMWM) is a method for controlling salt-marsh mosquitoes using physical alterations of marsh habitat. Ponds and ditches are selectively excavated in order to create unsuitable environs for mosquito eggs and larvae while creating suitable habitat for larvivorous fishes. Based on environmental effects observed at two experimental sites, plus operational experiences in Delaware and adjacent states, guidelines are presented for designing and installing OMWM systems in Delaware. These guidelines should be applicable to other salt marshes from New England to Florida that have similar environmental characteristics.

The guidelines are intended to produce OMWM systems that will control mosquitoes while minimizing long-term ecological disruptions of the marsh community. They emphasize the following topics, including the environmental or economic reasons for why particular approaches were chosen: 1) use and location of open tidal ditches; 2) interspersion of open tidal ditches with closed, non-tidal ponds and pond radial ditches; 3) use of semi-tidal sill ditches and ponds; 4) incorporation of OMWM systems into previously parallel-grid-ditched marshes; 5) permissible lowering of the water table elevation in relation to local marsh surface, as caused by spoil deposition on the marsh surface and/or drainage from open tidal ditching, but not to such an extent that the original vegetation is replaced by other species during vegetative recovery; 6) protocols for designing, demarcating and installing OMWM systems; 7) density, depth and surface areas of ponds; 8) geometric vs. naturalistic excavations; 9) habitat enhancement for waterfowl use; 10) water quality, fish kills and dependable mosquito control; 11) OMWM alterations under special situations, such as upland border marshes with excessive freshwater runoff or marshes with intensive muskrat burrowing activity; 12) blending of OMWM with other marsh management goals.