PRIMARY PRODUCTION OF THE SAND MICROFLORA IN SEAGRASS BEDS OF MISSISSIPPI SOUND

The sandy sediments beneath beds of the seagrass Halodule wrightii Ascherson in Mississippi Sound support a diverse and productive benthic microalgal assemblage. Over an annual cycle (June 1989-June 1990), virtually all algal cells comprising the sand microflora were small araphid, monoraphid, and biraphid pennate diatoms < 15-mu-m in length. Chlorophyll a concentrations in the sediments beneath Halodule ranged from 14 to 125 mg m-2 for individual beds with an annual mean of 44 mg m-2. Stepwise multiple regression identified water temperature and the density of Halodule blades as the best predictors of chl a concentration; however, R2 was only 0.38. Hourly production rates ranged from 9 to 276 mg C m-2 h-1 for individual beds with an annual mean of 80 mg C m-2 h-1. Stepwise multiple regression identified water temperature, tidal range, and light energy as the best environmental predictors of hourly production (R2 = 0.72). Annual production of the sand microflora was estimated as 339 g C m-2, which represented 23% of total benthic and 17% of total system production. The magnitude of its production and the dominance of the flora by diverse diatom taxa suggest the sand microflora may be trophically important in some seagrass beds.