Biotic Segregation in an Upper Mesotidal Dissipative Ridge and Runnel Succession, West Salish Sea, Vancouver Island, British Columbia

Ridge and runnel systems develop on low-gradient beaches under limited fetch conditions and moderate to high tidal ranges. The influence of beach morphology in ridge and runnel systems on biotic distribution is not well understood. This study focuses on infaunal population trends within a laterally extensive, shore parallel ridge and runnel system at Craig Bay, British Columbia, Canada. Within the study area, dense populations of Dendraster excentricus dominate lower intertidal runnels. These echinoids are both epifaunal and shallow infaunal. Other macroscopic infaunal organisms are absent within the runnel systems, with the exception of scattered, solitary anemones (Anthopleura artemisia). During low-tide intervals, D. excentricus in subacrially exposed runnels burrow beneath a veneer of sand and A. artemisia retract into their burrows. Abundant infaunal bivalves (including Macoma nasuta, M. balthica, M. secta, Tresus capax, Clinocardium nuttallii, Protothaca staminea, and Venerupis philippinarum), tube-dwelling polychaetes, and threadworms characterize ridge faunas. Relatively few D. excentricus occur on the ridges. Carnivorous polychaetes (Nereis sp. and Nephtys sp.) occur in both ridges and runnels. Exclusion of vertical faunal components in the runnels is attributed to the feeding activities and population density of D. excentricus, which prefer moister sediment. Preservation of this type of ridge and runnel system in the rock record would result in a succession with zones dominated by horizontal trace fossils (e. g., Beaconites/Scolicia), interbedded with zones characterized by abundant vertical forms (e. g., Siphonichnus, Skolithos, and Trichichnus). The resultant trace fossil succession would have no bathymetric implications, but would reflect the influence of beach morphology on infaunal populations.