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

Abstract

Moving water delivers food resources to sessile suspension feeding invertebrates and the interaction between water motion and substrate topography may dramatically influence the ecology of these animals. Growth rates of two species of sessile suspension feeders were compared between positions within individual 1.5-m high rock walls at an offshore site 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 suspension feeding mussel Mytilus edulis (Linnaeus), were greater at upper than at lower wall positions, while growth rates for the facultatively-active suspension feeding sponge Halichondria panicea (Pallas) showed the opposite pattern, with higher rates at lower than upper wall positions. Mussel and sponge growth rates were related both to patterns of bulk fluid flux across the walls and to naturally occurring distributions of sessile suspension feeders. Dissolution of alabaster and plaster solids indicated that upper wall positions experienced significantly higher bulk fluid flux than lower wall positions. Distribution data show non-random suspension feeder abundances across a series of small rock walls at two offshore sites. The abundance of passive suspension feeders was greatest near the top edges, while active and facultatively-active suspension feeders occur in greatest densities at lower wall positions. The results show that the interaction of small scale, abrupt, topographic features with variable local flow conditions can lead to physical gradients within rock walls. These gradients significantly affect the growth of suspension feeders, and may influence the structure sessile of invertebrate communities on subtidal rock walls.