The average density of Mercenaria mercenaria in 216 '/4_m2 samples taken in spring 1980
from an eelgrass (Zostera marina) bed in Back Sound, North Carolina, was 9.0 m-2, more than
five times the average density (1.6 m-2) in 216 1/4-m2 samples from a nearby sand flat.
Size-frequency distributions differed between environments, with the sand flat containing a
larger fraction of its Mercenaria in the smallest size class (0-1 cm). Use of internal growth lines
to age all Mercenaria collected revealed that age-frequency distributions also differed between
environments but that average Mercenaria age was identical in the two collections. The average
sizes of 0-, 1-, and 2-year-c1ass Mercenaria were significantly greater in the seagrass collection.
Furthermore, the logarithmic growth curve fit through the mean sizes of each year class for the
seagrass collection fell significantly above the analogous sand-flat curve for all ages, implying
higher growth rates inside the seagrass environment.
The seagrass environment contained a higher proportion of finer sediments, more silts and
clays, and higher organic content both in surface (0-2 cm) and-in deep (0-20 cm) cores. Current
velocities measured by dye release in the field demonstrated a substantial baffling effect by the
seagrass, with average surface velocities above the blades about 3-5 x average velocities at
depths within the seagrass canopy. This baffling by seagrass reduced currents near the bottom,
where Mercenaria feeds, to levels 50% lower than those measured simultaneously on the sand
flat. The paradoxically higher growth rate of the filter-feeding Mercenaria in the lower current
regime inside the seagrass bed may be a consequence of higher particulate food concentrations
produced by the hydrodynamic baffling of the emergent vegetation.