Selection by a deposit-feeding polychaete, Polygordius jouinae, for sands with relatively high organic content

Field observations and flume experiments investigated whether subsurface migration and selection by adult Polygordius jouinae for sediment enriched with organic flocs could help explain small-scale patchiness of this deposit-feeding polychaete in rippled, sandy sediments on the inner continental shelf off New Jersey. Paired ripple crest–trough samples taken from stations located kilometers apart in several months in 1994, 1995, and in May 2006 found higher but more variable densities of P. jouinae in troughs compared with crests. Sedimentary particulate organic matter was also patchily distributed at similar spatial scales. In a racetrack flume under realistic flow (shear velocity 5 0.32 cm s21) and flatbed conditions, arrays of alternating, fresh ambient sediment (including natural levels of particulate organic material) and freshly sieved sediment (reduced in particulate organic content) showed significant subsurface movement of P. jouinae to sediment patches containing higher amounts of particulate organic matter in 48 h. Subsequent experiments showed that locating organic patches was not the consequence of a directed search. However, worms that located richer patches remained there and fed. Rate of movement in sediments indicated that P. jouniae could potentially travel the wavelength of a typical ripple (14–30 cm) at the Long-term Ecosystem Observatory in 15-m water depth in 35–75 min. Thus in a dynamic environment where food concentrations are low and patchy, the affinity of P. jouinae for particulate organic matter and its undirected, high rate of subsurface movement is a plausible mechanism to account for the similar spatial distributions of P. jouinae and its food resources in continental shelf sediments. Sedimentary landscapes in high-energy, physically active continental shelf environments encompass a rich array of habitats or patch structures that appear significant to benthic organisms (e.g., Zajac et al. 2003; Barros et al. 2004). Small-scale patch dynamics and dispersal of infaunal organisms in these environments are poorly understood, even though such knowledge is critical to predicting ecological responses to long-term habitat changes on continental shelves. It is well established that infauna are patchily distributed on many spatial scales, and population and community patterns are created and maintained by complex interactions among a host of biological and environmental factors (reviewed by Ólafsson et al. 1994; Snelgrove and Butman 1994). Moreover, ecological patterns and processes operating at one spatial scale may differ from those at another scale (e.g., Dayton and Oliver 1980) or in another environment. At larger spatial scales, primary factors responsible for patterns of distribution of infauna may include sediment grain size, larval dispersal, and habitat selection (reviews by Butman 1987; Snelgrove and Butman 1994). At relatively smaller scales, dispersal and habitat selection by mobile, postlarval juvenile and adult stages seeking food may play a central role. Availability of food can stimulate the migration of organisms to areas where food resources are more abundant (Hughes 1993; Nilsson et al. 2000), and information on the patchiness of species and resources is used in many ecological models and in generating ecological theory. Sedimentary particulate organic matter (derived from organic-rich flocs containing phytodetritus, decomposing macroalgae, fecal pellets, microrganisms, and to a lesser extent seagrass and land-derived detritus) is an important component of the diet of many infaunal organisms. Particulate organic matter concentrations decrease with increasing grain size of the sediment. Thus its relative abundance in the coarse, sandy sediments that cover ,70% of continental shelves may be especially important in influencing patterns of distribution of infauna. Although poor in organic matter, highly permeable sands allow for concentration mechanisms such as pore-water advection, a process not possible in finer sediments. This advection can enhance the levels of fresh particulate organic matter in the 1 Corresponding author ([email protected]).