Biomass and Salinity Tolerance of Shoalgrass and Manateegrass in Lower Laguna Madre, Texas

Abstract

Three different shoalgrass (Diplanthera wrightii) stands and a manateegrass (Syringodium filiforme) community in Lower Laguna Madre, Texas, were sampled for their plant biomass. The winter standing crop of all (wet) shoalgrass herbage and roots is estimated to be 317,345 tons, a yield of 4,656 lb/acre. The standing crop of (wet) manateegrass is estimated to be 48,214 tons, a yield of 5,795 lb/acre. Shoalgrass exhibits a seasonal abundance, the smallest standing crop occurring in late winter and spring. Manateegrass is not seasonal in abundance. Shoalgrass sprigs planted in culture vessels lived in salinities ranging from 9.0 to 52.5 parts per thousand (ppt), but died in salinities of 3.5 ppt, and 70.0 ppt, or more. Manateegrass rhizomes planted in culture vessels survived best in a sa- linity of 35.0 ppt, while those planted in a vessel containing 52.5 ppt died. Shoalgrass is an important food of waterfowl wintering in Lower Laguna Madre, and provides spawning and nursery grounds for fish and shrimp. Manateegrass appears to be of minimal value to fish and waterfowl. If the usual sa- linity of Laguna Madre is permanently decreased by construction of fish passes from the Gulf of Mexico, or by drainages from river diversion projects, the shoalgrass biomass is likely to be reduced and the production promoted of manateegrass or other marine spermatophytes intolerant of hypersaline water. In terms of fish and wildlife, the Lower Laguna Madre is probably the most im- portant bay along the Texas coast. The lagoon has become the leading producer of important bay fishes with the general de- cline of commercial production in other bays (Breuer 1962:153). The Lower La- guna Madre also winters about one-half million waterfowl annually, including ap- proximately 78 percent of the existing red- head duck (Aythya americana) population (Weller 1964:64). One reason for the exceptional abundance of fish and waterfowl in Lower Laguna Madre is dense stands of shoalgrass, a sub- mergent marine spermatophyte which pro- vides cover and spawning grounds for fish and shrimp. Shoalgrass also is the chief plant food of most ducks wintering in South Texas bays (Singleton 1953:46). Manatee- grass, another submergent spermatophyte, extends in a near homogeneous stand over a large area of the southern part of Lower 1 This paper is a contribution from the Pittman-