Identification of processes responsible for egg exhumation and transport in the swash zone is paramount to conservation of species that use the intertidal foreshore. One example where the understanding of these processes is critical to egg exhumation, transport and deposition is in Delaware Bay, USA. Beaches in Delaware Bay provide foraging grounds to many shorebird species that migrate thousands of miles from Central and South America to feed on nutrient rich horseshoe crab eggs during their peak spawning season. Eggs laid at depth by horseshoe crabs are exhumed and transported by bioturbation, wave and swash processes and made available to foraging shorebirds. The objectives of this dissertation are to: (1) compare differences in the significance of wave and swash processes to horseshoe crab egg exhumation and transport on the mid - foreshore relative to the upper foreshore in the absence of spawning; (2) compare how horseshoe crab eggs are mobilized relative to sediment; and (3) evaluate the processes responsible for textural changes of sediment in transport. The study was conducted on a steep, predominantly sandy foreshore on the New Jersey shoreline of Delaware Bay.
Instrumented wave (height and period) and swash (depth, duration, velocity, width) measurements were gathered on October 12 and October 14, 2007 during spring tidal conditions. Dyed horseshoe crab eggs and sediment were injected at a location on the mid-foreshore that were influenced by wave breaking and swash flows and at a location on the upper foreshore that was influenced by swash flows alone. Total load traps were used to measure textural changes in sediment and quantities of egg and sediment tracer transported over individual swash events during the rising, high and falling tide.
Sediment on the foreshore comprised medium to coarse sands with a gravel fraction of granules and pebbles. The proportion of gravel within the foreshore prior to trapping was low. An increase in the percent gravel transported was observed in the swash approaching the time of high water. Results suggest that as the energy under incident waves increase with tidal rise, the quantities of gravel mined out from the bed also increase and are incorporated into the beach step. Plunging waves breaking over the step suspend gravel and transport it up the foreshore in the swash. The lag in the rate of step migration relative to migration of the breaker zone during the falling tide increases the likelihood of mining gravel from the step and transporting it downslope in the backwash.
Results from the tracer experiments reveal that wave breaking is the primary mechanism that accounts for the greater quantities of sediment and eggs trapped from the mid-foreshore in the uprush relative to the backwash despite offshore directed flows during the tidal cycle when wave heights ranged from 0.43-0.64 m. Low quantities of eggs and sediment entrained from the upper foreshore are a function of the decreasing swash depths and flow velocities approaching the uprush limit. Waves did not activate sediment on the mid-foreshore to depths where crabs would generally lay eggs and subsequent spawning would be required to make these eggs available to shorebirds. Data reveal that higher wave heights ranging from 0.65-1.1 m representing storm conditions resulted in accretion across the foreshore, and no eggs were released from the mid or upper foreshore.
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