Delivery and movement of horseshoe crab eggs (Limulus polyphemus) in the breaking waves and swash uprush of an estuarine foreshore

Knowledge of conditions for entrainment of eggs and larvae from beach spawning sites is critical in determining the likelihood that these food sources will be delivered to shorebirds. The exhumation of horseshoe crab eggs buried within the beach foreshore is a function of changes in wave and current velocities over a tidal cycle. This field study was conducted to evaluate the temporal constraints to egg release at two different locations across the foreshore of a sandy estuarine beach in Delaware Bay, New Jersey. Wind, wave, current characteristics, beach change and eggs in transport were sampled over two tidal cycles. Egg counts were determined using four colors of dyed-egg tracers placed as point sources 5 and 10 m bayward of the expected upper limit of swash. Tracer recovery varied from 1% to 18% of the tracer eggs buried. Wave heights were relatively low (instrumented H _s = 0.09-0.17 m) but with long periods (5.6-8.8 s). Results reveal that egg release is discontinuous and occurs in pulses when the swash and breakers migrate across the injection points on the rising tide and when the breakers migrate past the injection points during the falling tide. A lack of tracer at the upper (5 m) injection point after early stages of high tide is likely due to the maximum disturbance depth being achieved early, leaving no eggs within the swash boundary layer. Peaks in recovery of tracer initially buried at the lower (10 m) location after the breakers migrated up the foreshore well past the injection location are likely from eggs deposited in the wrack on the upper beach or reburied in the sediment near the upper limit of swash and remobilized by high swash uprushes. These results using point sources of tracer eggs suggest that the contribution of eggs from within the beach matrix is temporally variable across the foreshore. Most eggs are released during rising tide, but additional eggs can be released during falling tide if the depth of activation of the bed is greater than on the rise due to net erosion or change to higher wave-energy conditions. Successful shorebird foraging at other times in the falling tide appears due to transport of eggs released earlier in the tidal cycle and delivered alongshore and not due to eggs exhumed in-situ at that time.