Influence of configuration of bulkheads on use of estuarine beaches by horseshoe crabs and foraging shorebirds

Construction of bulkheads is a common response to erosion of estuarine shorelines. Bulkheads are usually built incrementally, resulting in wider sandy beaches remaining as enclaves between bulkhead segments. This paper measures the characteristics of bulkheads and enclaves and evaluates (1) whether horseshoe crabs (Limulus polyphemus) utilize enclaves for spawning when they provide partial sheltering during periods of high wave energies; and (2) whether eggs become trapped in the enclaves and are available to shorebirds when the beaches in front of bulkheads are inundated. The characteristics of bulkheads and beach enclaves were identified in five developed reaches in Delaware Bay, USA. Counts of horseshoe crabs were made in enclaves in two of these reaches during times of high wave energies and compared to counts in nearby unarmored segments. Egg tracer and trapping experiments were conducted at one of the enclaves to assess egg movement. Results indicate that the percent of bulkheads intersecting the beach below mid-foreshore varies from 10 to 50 %. Spawning densities were greater in enclaves than on longer unarmored segments on some days. Enclaves serve as a sink for eggs moving along the base of the bulkheads. Most birds feeding on horseshoe crab eggs preferred sites outside the enclaves and bulkhead segments. Any advantage of bulkheads creating enclaves and sinks for eggs moving alongshore is likely overridden by their disruption to natural process and habitats, but having unarmored enclaves between bulkhead segments may be preferable to one continuous bulkhead, based on environmental benefits.