The influence of seagrass cover on population structure and individual growth rate of a suspension-feeding bivalve, Mercenaria mercenaria

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Abstract
The average density of M. mercenaria in 216 1/4-m2 samples taken in spring 1980 from an eelgrass (Zostera marina) bed in Back Sound, North Carolina [USA], was 9.0 m-2, more than 5 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 2 collections. The average sizes of 0-, 1- and 2-yr-class Mercenaria were significantly greater in the seagrass collection. 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 lays 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.