Do terraces and coconut mats affect seeds and submerged aquatic vegetation at Sabine National Wildlife Refuge?

Terraces are a widely used wetland restoration tools in coastal Louisiana, yet the benefits of terraces are poorly documented. Like terraces, coconut mats also may increase abundance of submerged aquatic vegetation (SAV), but their benefits are undocumented. I compared SAV and seed abundance in a marsh pond among three treatments: terrace, coconut mat, and marsh. I evaluated terraces constructed in 1999 and coconut mats installed in 2001 in portions of unit 7 of Sabine National Wildlife Refuge that converted from emergent marsh to open water between 1956 and 1978. I randomly selected 3 terrace, marsh, and coconut mat stations. I evaluated transects 0 meters, 5 meters, and 50 meters (here after open water) from emergent vegetation at each station. Submerged aquatic vegetation was evaluated on the terrace and marsh by harvesting SAV from 10-cm diameter cores, harvesting SAV from 1-m2 plots, and raking: only 1-m2 plot sampling was conducted on coconut mat treatments. I evaluated seed abundance on terrace and marsh transects with 10-cm diameter cores, which could not be used on the coconut mats. Submerged aquatic vegetation biomass differed between treatments and sampling dates. Biomass of SAV, as estimated by the cores, was greater on the marsh transects than the terrace transects. The SAV biomass, as estimated by 1-m2 plots, was greatest during September 2002. Biomass of SAV, as estimated by 1-m2 plots, was greater on coconut mats than terrace or marsh transects. Raking indicated that in September 2002, the percent occurrence of SAV was greater on the marsh transects than on the terrace transects. Seed biomass was greater adjacent to marsh than to terraces, which was similar to that in open water. Seeds of sawgrass (Cladium jamaicense Crantz) accounted for 87% of the seed biomass but did not germinate in a greenhouse. My results indicated that terraces failed to increase SAV abundance above levels found in open water as was predicted. I concluded that restoration planners should no longer assume that terraces increase SAV abundance. Coconut mats increased SAV abundance. Additional studies on a variety of areas and configurations are needed to determine if my observations are typical