TIDAL REORIENTATION AND TRANSPORT OF RECENT BIVALVES ON A TEMPERATE TIDAL FLAT, NORTHWESTERN UNITED-STATES

The effects of waves and tides on the behavior of single and articulat ed dead shells of Macoma nasuta and Clinocardium sp. were studied in F alse Bay (San Juan Island, Washington). Most of the bay is subaerially exposed during spring low tides (-0.8 to 2.6 m), whereas waves enter from the Straits of San Juan de Fuca during periods of submersion. The se species were the most numerous and conspicuous skeletal elements pr esent on the tidal flats. Transects of natural death assemblages at th e surface indicate that shell abundance was significantly greater clos er to shore and decreased toward the bay center. The majority of these values were oriented in the convex-up attitude. To determine the effe cts of waves and tides, marked shells were placed both in a channel an d on a sand bar at low tide and left for 1-2 tidal cycles. Regardless of initial attitude, the majority of shells were later found convex-up ; those initially convex-down were reoriented to convex-up position (7 5%-86%), while convex-up valves remained unchanged (90%-98%). Distance of shell transport depended upon initial orientation and location. Va lves within the tidal channel moved < 1.0 m while those on the sand ba r showed either negligible transport (less-than-or-equal-to 2.0 m) or significant transport (10- greater-than-or-equal-to 119 m). Subaeriall y exposed, convex-down shells were transported the greatest distance v ia flotation on the incoming tide. Shell flotation was not correlated to valve size, handedness, nor species of bivalve tested. Flotation ma y be an overlooked fair weather transport process and could account fo r the greater concentration of shells on the landward portions of Fals e Bay sand bars.