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Nancy L Jackson and others Department of Chemistry and Environmental Science New Jersey Institute of Technology Newark, New Jersey 07102, USA Abstract: This field study evaluates the effect of nourishing an estuarine beach with gravel to enhance spawning rates by horseshoe crabs. A total of 564 m3 of coarse sand and gravel were emplaced in two 90 m-long treatment areas at Bowers Beach, Delaware, USA from 9 to 11 April 2001. Field data were gathered between 6 April and 24 May 2002 to characterize the two fill areas and the un-nourished area between them as well as a control location on the adjacent Ted Harvey Beach. Sediment samples were taken from the foreshore surface and at depth before and after the nourishment. Bay water levels, wave heights, and beach groundwater characteristics were monitored over a 12-hour tidal cycle at one of the nourished and the unnourished segment at Bowers Beach and at Ted Harvey Beach using pressure transducers and piezometers. The beaches were cored to estimate the density of horseshoe crab eggs deposited during the spawning season. Horseshoe crab eggs were buried in pouches at 0.15 to 0.20 m depth for 30 to 40 days to evaluate their viability in developing into embryo or larval stage. Bulk sediment samples were taken to evaluate moisture characteristics next to locations where egg pouches were buried. Density of spawning females at Bowers Beach was 1.04 m2-1 in 2001 and 1.20 m2-1 in 2002. These rates are lower than at Ted Harvey Beach but reveal an increase in spawning while Ted Harvey Beach underwent a considerable decrease (2.63 m2-1 to 1.35 m2-1). Sediments low on the foreshore remained nearly saturated throughout the tidal cycle on all beaches. The average hydraulic conductivity on the upper foreshore at Bowers Beach (0.19 cm s-1) was less than at Ted Harvey Beach (0.27 cm s-1), and the finer, better sorted sediments at depth on Bowers Beach resulted in a higher porosity, creating greater moisture retention potential. Egg development was greatest at the middle elevation of the intertidal foreshore at all sites. Eggs low on the foreshore remained viable, but did not develop to the embryo stage. Between-beach differences were 1 limited to high elevations where higher mortality occurred on Ted Harvey Beach due to desiccation. Adding small amounts of gravel to a sand beach may change the appearance of the surface but may not appreciably increase mean grain size and sorting at depth or the hydraulic conductivity over the spawning season. The pebble fraction may be important for site selection, but finer sizes may be more important for egg viability because of moisture retention. |
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