Effects of the American oyster Crassostrea virginica on growth rates of the seagrass Halodule wrightii

American oysters Crassostrea virginica have significant effects on ecosystem structure and function and may potentially determine whether coastal systems are dominated by either benthic or pelagic primary production. Field experiments were used to determine the effect of oyster density on the abundance and growth rate of shoalgrass Halodule wrightii in Mississippi Sound, Alabama, USA. Oysters were deployed at densities of 0, 15, 75, and 150 ind. m(-2) in 1 m(2) Plots Of H. wrightii in September 2004. Changes in shoalgrass shoot density, growth rate, total suspended solids (TSS), porewater nutrients, light transmittance, and epiphyte loads were all measured from October-November 2004, and June-July 2005. Results showed no significant relationship between oyster density, light transmittance, TSS, or epiphyte loading. However, there was a significant positive relationship between oyster density and porewater NH4 and PO4, Seagrass growth rates were highest at intermediate oyster densities, while high oyster densities negatively affected growth rates, shoot density, and plant biomass.