BIOTIC SEGREGATION IN A MACROTIDAL DISSIPATIVE RIDGE AND RUNNEL SUCCESSION

Ridge & runnel systems develop on low gradient beaches under limited fetch conditions and moderate to high tidal ranges. Although the physical development of these beaches is well understood, the influence of beach morphology in ridge & runnel systems on biotic distribution has not been addressed. This study focuses on trends in infaunal populations in a laterally extensive (~1100 m), long wave length (40-80 m), low amplitude (0.2 to 0.5 m), shore parallel ridge & runnel system at Craig Bay, Vancouver Island, British Columbia, Canada. The lower to middle intertidal portion of the Craig Bay ridge & runnel system shows clear segregation of extant biota and thus the biogenic sedimentary structures they created.

Within the study area dense populations of Dendraster excentricus dominate the runnels. These flattened, mobile echinoids live on the sediment surface and within the upper decimeter of the sand-dominated substrate. With the exception of scattered, solitary anemones, other macroscopic infaunal organisms are absent within runnel systems. During low-tide intervals Dendraster in subaerially exposed runnels burrow beneath a veneer of sand and anemones retract into their burrows.

Abundant tube-dwelling polychaetes (Heteromastus) sp. and infaunal bivalves (including Macoma nasuta, M. bathica, M. secta, Tresus capax, Protothaca staminea, Tapes japonica) characterize ridge faunas. Relatively few Dendraster 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 the Dendraster, which prefer the moister sediment within the runnel areas. Preservation in the rock record of his type of ridge & runnel system would result in a succession with zones dominated by horizontal trace fossils (Beaconites / Scolicia) and rare plug-shaped traces (Conichnus / Berguaria) interbedded with zones characterized by abundant vertical forms (Siphonichnus, Skolithos and Trichichnus). The resultant trace fossil succession would have no bathymetric implications but rather would reflect the influence of original beach morphology on infaunal populations.