Large subsurface deposit feeders are common and abundant in near shore and shelf benthic communities. The ability of these animals to successfully exploit an environment may depend on their ability to modify that environment. Bioturbation by a deposit feeder can have a profound effect on the rates of production, cycling, and flux of materials through benthic ecosystems. This may be particularly important in coarse grained, organic poor sediments where there is a tighter coupling between the hydrodynamic regime, production and supply of food resources, (inorganic nutrients and reactive organic matter), and the activities of the local benthic community. This study examines the role of population level control on sediment structure and subsequent impacts on resource availability, in a dense assemblage of the deep head down deposit-feeding polychaete Clymenella torquata.

Clymenella torquata occurs in large, spatially distinct populations in high energy, intertidal and subtidal sandy sediments. It is a prolific tube builder with tubes being 2-3 times more abundant than worms. C. torquata feeds selectively on fine grained material 15 -20 cm below the sediment-water interface (SWI) and egests it at the sediment surface, resulting in a coarsening of material at it's feeding depth and the formation of a shell lag layer at its base. These changes in sediment structure coupled with physical and biological irrigation of the sediment effectively create a confined biological micro-aquifer at C. torquata's feeding depth, as indicated by the presence of a laterally extensive, deep oxidized layer.

Results from particle mixing studies show that empty C.torquata tubes act as conduits through which reactive surface material is rapidly transported to C. torquata's feeding depth. Enhanced vertical transport of particles and water down tubes coupled with lateral transport through the deep micro-aquifter produced subsurface peaks in oxygen and elevated nitrate production in sediments dominated by C. torquata. Integrated ammonia production flux, as an indicator of organic matter decomposition, was 4 times higher in the C. torquata patch compared to adjacent nonpatch sediment. The quality of this organic matter was assessed by measuring C. torquata's carbon absorption efficiency. C. torquata absorbed ~ 70% of the carbon in sediment from its feeding depth but only 6% of the carbon from sediment collected from the equivalent depth horizon outside the patch.

Sediment bioengineering, (production and maintenance of multiple tubes and a subsurface micro-aquifer) by Clymenella torquata, coupled with physical and biological irrigation of the sediment, led to clear differences in transport processes across the SWI and the subsurface cycling of particles and solutes within bioturbated sediment. Broad scale alteration of sediment structure may be an effective strategy by which deep head down deposit feeders like C. torquata exploit low carbon sandy environments and is likely to play an important role in determining the structure, stability and spatial distribution of benthic communities in sandy sediments by creating areas of intense transport or biogeochemical 'HOT SPOTS'.