Macrophyte canopy structure and the success of an invasive marine bivalve

In both terrestrial and aquatic environments introductions of non-indigenou s species are continuing and represent one important component of global ch ange. Negative biotic interactions by resident species may prevent successf ul invaders from becoming pests. Few experimental data are available on the presence and significance of such biotic resistance other than predation o r competition. This study addresses the role of habitat structure provided by a native eelgrass (Zostera marina) canopy on growth and survival of the non-indigenous mussel Musculista senhousia, a habitat-modifying gregarious suspension feeder with strong effects on native infauna and eelgrass. In 2 southern California bays, a series of transplantation experiments using tag ged mussels revealed that inside an eelgrass canopy, Musculista growth rate s were reduced by more than half in 3 of 4 experiments compared to adjacent unvegetated areas. Musculista survival also decreased inside the vegetatio n in a 4-mo experiment. As one element of habitat structure, we tested the effects of eelgrass parch size, using natural (1 site) and planted (1 site) eelgrass patches of defined sizes. Growth rates of Musculista were highest outside the veneration and decreased as eelgrass patch size increased. As a potential mechanism for the canopy effects, we suggest that Musculista re ceives less food inside the vegetation. In the experimental plots, the pres ence and spatial extent of the macrophyte canopy strongly affected near bot tom (10 cm) horizontal water flow assessed with a direct dye tracking metho d. Reduced mussel growth rates were linearly associated with lower water fl ow, and presumably, food flux. Over a period of 7 mo, food resources (parti culate chlorophyll a) were consistently lower 1 and 5 cm above the sea floo r inside eelgrass patches compared to the sand Aat. The reduction in food a vailability matched the growth reduction of Musculista. Also, mussel condit ion (dry flesh mass/shell mass) was worse in individuals growing in eelgras s than in the sand Aat. Previous experiments revealed that dense beds of Mu sculista impede the rhizome growth and vegetative propagation of eelgrass, yet mussels attain abundances sufficient for interference only if eelgrass beds are patchy. Thus, anthropogenic disturbances on eelgrass beds, which o ften result in meadow fragmentation, and the proliferation of Musculista ma y have synergistic negative effects on the persistence of eelgrass beds.