WATER-FLOW OVER SUBTIDAL ROCK WALLS - RELATION TO DISTRIBUTIONS AND GROWTH-RATES OF SESSILE SUSPENSION FEEDERS IN THE GULF-OF-MAINE - WATER-FLOW AND GROWTH-RATES

Moving water delivers food resources to sessile suspension feeding inv ertebrates and the interaction between water motion and substrate topo graphy may dramatically influence the ecology of these animals. Growth rates of two species of sessile suspension feeders were compared betw een positions within individual 1.5-m high rock walls at an offshore s ite at 30-m depth in the Gulf of Maine, USA. The site is characterized by strong, highly variable water flow. Growth rates for the active su spension feeding mussel Mytilus edulis (Linnaeus), were greater at upp er than at lower wall positions, while growth rates for the facultativ ely-active suspension feeding sponge Halichondria panicea (Pallas) sho wed the opposite pattern, with higher rates at lower than upper wall p ositions. Mussel and sponge growth rates were related both to patterns of bulk fluid flux across the walls and to naturally occurring distri butions of sessile suspension feeders. Dissolution of alabaster and pl aster solids indicated that upper wall positions experienced significa ntly higher bulk fluid flux than lower wall positions. Distribution da ta show non-random suspension feeder abundances across a series of sma ll rock walls at two offshore sites. The abundance of passive suspensi on feeders was greatest near the top edges, while active and facultati vely-active suspension feeders occur in greatest densities at lower wa ll positions. The results show that the interaction of small scale, ab rupt, topographic features with variable local flow conditions can lea d to physical gradients within rock walls. These gradients significant ly affect the growth of suspension feeders, and may influence the stru cture sessile of invertebrate communities on subtidal rock walls. Copy right (C) 1997 Elsevier Science B.V.