THE EFFECT OF DISTURBANCE BY LIMULUS POLYPHEMUS ON THE DISPERSAL OF INFAUNA AT AN INTERTIDAL SAND FLAT IN DELAWARE BAY

Abstract

An emerging model of infaunal dispersal views passive bedload transport of infauna as an essential part of dispersal in soft bottom systems and bioturbation of sediment as enhancing near-bed flux of animals and sediment. This model was examined by exploring the relationship between bioturbation by the horseshoe crab, Limulus polyphemus, and the dispersal of infauna at an intertidal flat in Delaware Bay, a soft bottom system with extensive patch formation by foraging and mating Limulus. The following hypotheses were tested: bioturbation by the horseshoe crab causes a reduction in infaunal density and an increase in near-bed sediment flux and infaunal dispersal rates, and infauna are dispersed passively as part of near-bed flux. Exclosure cages were used to prevent Limulus from bioturbating caged sediments. Sediment traps and core samples were taken within cages and controls at three times during the summer of 1991: in May before Limulus invaded the flat, in June during the invasion, and in August after the invasion. Infaunal densities declined in both caged and uncaged sites throughout the summer but significantly more so in the uncaged sites (caged: 12.95 +/- 0.81 (SE) animals per core in May versus 7.65 +/- 0.57 animals in June and 3.65 +/0.37 animals in August; uncaged: 13.42 +/-0.69 animals versus 3.73 +/-0.36 animals and 1.30 +/- 0.20 animals). These findings suggest that Limulus is just one of a number of factors influencing infaunal abundance on the flat. Results also indicate that Limulus bioturbation causes a significant increase in near-bed sediment flux. In May before the invasion, sediment volume per trap was 21.63 +/1.31 cm³ in cages and 26.48 +/- 1.32 cm³ in controls. In June during the invasion it was 19.84 +/- 1.90 cm³ in cages and 34.92 +/- 0.37 cm³ in controls. In August after the invasion the two treatments had virtually identical sediment volumes of 52.66 +/-3.08 cm3 in cages and 51.27 +/-2.37 cm³ in controls. The increase in near bed sediment flux in the June controls did not enhance infaunal dispersal. In May there was a statistically significant regression (r²=.19 in cages and r²=.13 in controls) between sediment volume and infauna collected in sediment traps. This relationship did not exist during or after the Limulus invasion. These results indicate that in polychaete-dominated assemblages such as Cape Henlopen, Limulus bioturbation may play an important role in regulating dispersal by reducing densities of juvenile and adult infauna, rather than enhancing passive bedload transport as has been demonstrated in some bivalve-dominated systems.