Restoring Coastal Habitat Using Marsh Terracing: Does Cell Size Matter?
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PRESENTATION
L. P. Rozas1and T. J. Minello2
NOAA Fisheries Service/Southeast Fisheries Science Center
1Estuarine Habitats and Coastal Fisheries Center
646 Cajundome Boulevard
Lafayette, LA 70506
2Galveston Laboratory
4700 Avenue U
Galveston, TX 77551
Abstract
Marsh terracing is used to restore habitat by converting shallow sub-tidal non-vegetated bottom (SNB) to inter-tidal wetlands. Terraces are constructed from excavated bottom sediments and arranged in some pattern to form a terrace field. A common arrangement is a checkerboard pattern of square cells with open corners. The size of terrace cells can vary, but habitat-related information that could be used to select a cost-effective cell size for marsh terracing projects is lacking. In 1999, terrace cells of three sizes (large=122-m sides with 1.30-ha ponds, medium=61-m sides, 0.29-ha ponds, small=30-m sides, 0.06-ha ponds) were incorporated into a restoration project constructed at Galveston Island State Park (GISP). This terracing project provided a rare opportunity to assess the effect of cell size on habitat quality and to assess the general efficacy of the terracing technique for restoring nursery habitats of fishery species. Our study objectives were to evaluate marsh terracing as a method for restoring estuarine habitat and fishery production in Galveston Bay, test whether cell size affects the fishery value of habitat created by marsh terracing, and conduct a benefit-cost analysis on terracing that contrasts fishery benefits with construction costs and estimates optimal cell size. We compared the habitat value of terraced areas 2-3 yr after construction with the area before terraces were constructed, with nearby sub-tidal non-vegetated bottom similar to that replaced by the terrace fields, and with natural marsh habitat. Habitat use of marsh and open water habitat types also was compared among the three cell sizes in the GISP terrace fields. Our sampling indicated that animal abundance and biomass increased substantially in the project area following restoration by marsh terracing. An analysis of post-construction samples detected few statistically significant differences in animal density and biomass among cell sizes, but density, biomass, and species richness varied significantly by habitat type (i.e., higher in marsh vegetation than over SNB). Most taxa were generally not abundant over SNB, and within terrace cells, densities of most fishery species were similar between deep borrow and shallow pond areas.
Using these post-construction density and biomass data; GIS to estimate the percentage of marsh, borrow area, and shallow pond area in terrace cells; and population models for three fishery species (brown shrimp, white shrimp, blue crab), we examined how populations of these fishery species and cost effectiveness of terracing projects would vary with cell size. Based on our modeling estimates, populations of these fishery species increase as cell size decreases, but the cost of terrace construction increases much faster than population size as cell size decreases. Our analysis shows that terrace fields constructed of medium cells would be more cost effective than terrace fields composed of either small or large cells.
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