Previous estimates of suspended food resources have not incorporated the hydrodynamics of species-specific food entrainment. To assess food resources available to the siphonate bivalve Mercenarja mercenarra, we developed a n in situ sampling protocol utilizing realistic incurrent siphon characteristics (e.g diameter, sampling height and pumping rate) with simultaneous measurements of the current veloclty profile. Microalgal food resources, a s estimated by clam-blased chlorophyll a measurements, varied 10-fold over both temporal (hourly and seasonally) and spatial (centimeter and meter) scales in estuarine vegetated and adjacent unvegetated habitats. For the near bottom (1 cm height) samples, chlorophyll a concentrations and variations were greater in a seagrass bed than those measured in the adjoining bare sand habitat. Samples collected at 5, 15, and 45 cm above the bottom did not exhibit any habitat-specific pattern. U p to 90 ' ?L of the near-bottom microalgal count in the seagrass bed was pennate diatoms suggesting that much of the chlorophyll a is derived from the bed. Current speeds measured just above the bottom in the seagrass bed were considerably lower ( c 1 cm S -' ) than elsewhere. These results suggest a relatively concentrated, locally-generated near-bottom food resource for M. mercenaria within the vegetated habitat. Therefore, resource depletion (i.e. suspension feeders reducing the food level for others downstream) may not be relevant to hard clam populat~ons at this site.
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