ABSTRACT: This is a progress report on the
restoration of Mosquito Beach salt marsh, Block Island, Rhode
Island. Mosquito Beach marsh is a small, narrow marsh that had
a severe tidal restriction thereby producing large areas suitable
for mosquito production. Restoration of the marsh involved open
marsh water management (OMWM) techniques that restored tidal flushing
and provided adequate habitat for fish. In this report the specific
details of the OMWM construction are detailed.
INTRODUCTION
Mosquito Beach salt marsh is a 800 ft long by 50 ft wide and located
on the eastern shore of the Great Salt Pond on Block Island, Rhode
Island. In the 1940's mosquito ditches were excavated in the marsh
and a breach was dug through the dune to drain the marsh. The
breach has since filled, resulting in a severe restriction to
tidal flushing. This restriction traps tidal water behind the
dune impounding an area of approximately one acre. Conditions
in the impounded area are conducive to larval mosquito development
but do not support predatory fish populations. This situation
produces excessive populations of salt water mosquitoes following
full moon high tides. Mosquito populations
originating from the marsh have been so large that not only has
the marsh been named for them, but it has also had a negative
economic influence on the neighboring bed and breakfast establishments.
Local and state agencies agreed that a long term solution to the
excessive mosquito production of Mosquito Beach was required and
that this marsh was an excellent candidate for open marsh water
management (OMWM). Restoration of the marsh was completed in the
spring of 1998.
METHODS
Prior to the restoration of Mosquito Beach salt marsh, baseline
information was collected to assist in the OMWM design. A 10 ft
incremental grid pattern was laid out over the marsh. Dominant
vegetation zones were mapped and elevation points were taken with
a laser level accurate to 0.01 ft. A contour map of the marsh
surface was generated from 3,030 elevation points collected. The
contour map allowed for spatial analysis of the existing water
regime and the prediction of water regimes that would result from
varying the depth of the dune breach. Pre-restoration surveillance
of mosquito larvae indicated that particularly excessive production
was located in the extreme northern and southern ends of the marsh.
Dip counts of 50 to over 100 mosquito larvae per dip were common
in these areas. The OMWM restoration included breaching the dune,
creating two new channels to improve fish access to northern and
southern ends of the marsh, cleaning out clogged portions of existing
mosquito ditches, deepening and damming the existing fish reservoir
and filling one unnecessary mosquito ditch.
RESULTS AND DISCUSSION
The dominant vegetation on the marsh is saltgrass, Distichlis spicata. The common reed, Phragmites australis, was also present and was slowly advancing into the marsh (Fig. 1a). The contour map (Fig 1b.) allowed for the prediction of the water's maximal extent after the dune was breached. Breaching of the dune should substantially reduced the area where water was retained, thereby eliminating mosquito production from the vast majority of the marsh and also would enhance tidal exchange. Several scenarios of breach depth were compared and it was established that a depth of 8.4 inches would considerably reduce the habitat available for mosquito production. The predicted water regime for the 8.4-inch breach indicated that water would be confined to ditches and the existing fish reservoir (Fig 1c). To insure adequate tidal exchange of the marsh and to compensate for sedimentation, it was decided to dig the breach to a 12-inch depth. The breach was lined with cobblestone for stability. The cobblestone was collected from the town beach with a special permit. The spoil (approximately 7.5 cubic yards) generated from the breach was used to make a sill dam to retain the current depth of the water reservoir. A new center channel was created in the northern portion of the marsh under the existing boardwalk and also in the southern end of the marsh. Due to sand over wash, a 40 ft x 2 ft pipe was installed in a portion of the northern channel. In addition, clogged sections of the center ditch were cleared to permit tidal exchange throughout the marsh and improve fish access. Approximately 21.5 cubic yards of spoil was generated from the ditch excavation. Some of the spoil was used to fill in smaller clogged ditches located in the upper edges of the marsh and the remainder was deposited at the town's transfer station. In order to minimize damage to the marsh surface, all ditches were dug by hand and a wheelbarrow specially fitted with low ground pressure tires was used to transport spoil across the marsh surface. An excavator that remained on the beach dug the breach.
Post-restoration monitoring will include surveillance for mosquito
larvae, fish sampling in the new channels and mapping of the water's
maximal extent to determine the relative success of the OMWM restoration.
ACKNOWLEDGEMENTS
This project could not have been completed without the help of Alan Gettman, Sean Healy, Mark Egan, Daniel Markowski, Matt Nicholson, Carol Baker and Dave Holt. Rhode Island Department of Environmental Management's Office of Mosquito Abatement Coordination and Narragansett Bay Estuary Program, United States Fish and Wildlife Service, and the Town of New Shoreham provided funding for the OMWM restoration of Mosquito Beach.