Effect of wind and tidal advection on distribution patterns of rock crab Cancer irroratus megalopae in Block Island Sound, Rhode Island

Abstract

The planktonic period of benthic marine invertebrates can significantly affect distribution patterns of benthic juveniles. In this paper we address the relationship between advection and the subsequent abundance of planktonic megalopae of the rock crab Cancer irroratus in Block Island Sound, Rhode Island (USA), over an 8 yr period. At small scales (several meters distance with samples taken simultaneously), megalopae were found to be similarly distributed; at larger temporal (tens of minutes) and spatial scales (hundreds of meters) megalopae were very patchy, which indicates a complex, highly variable pattern of abundance typical of planktonic systems. Using the receptor-mode trajectory capability of OILMAP, a numerical hydrodynamic model, we detected a significant relationship between the direction of transport prior to collection (as predicted by the model) and the subsequent catch of megalopae. We argue that rock crab megalopae are often advected tens of kilometers over short time spans and are concentrated on south-facing shores in Block Island Sound. Further, enhanced planktonic delivery to our study area results in large pulses of individuals to the benthos. Directional transport would be an effective larval delivery strategy even if rock crab megalopae were subject to lower advection, perhaps owing to a deep vertical distribution; a significant relationship between transport direction and collection date was detected even under a lower advective regime.