Sediment reworking and tube irrigation by the deposit-feeding polychaete, Clymenella torquata, reduced the quantity of particulate organic matter (POM) at the sediment-water interface and increased concentrations of dissolved nutrients (nitrite, ammonia, silicate, phosphate) in the water overlying laboratory microcosms. To determine the effect of these changes on growth of the filter-feeding bivalve, Gemma gemma, clams of the same initial size were grown under different laboratory conditions, representing all combinations of the following three treatments: (a) presence or absence of light, (b) presence or absence of C. torquata, and (c) proximity to C. torquata (clams living in sediment with worms or in sediment-filled microcosms positioned 1 cm above the worms). Clams grown in sediment with C. torquata, and in light, experienced less POM at the sediment surface than clams elevated in the water column, due to the burial of POM by accumulations of worm-defecated mineral particles. Among clams grown in the same tank, those in sediment with worms grew faster (p < 0.1) than those elevated above worms, indicating that worm-induced POM reduction at the sediment surface, or factors correlated with it, were responsible for increased clam growth. Clams elevated above worms (in light) were exposed to higher concentrations of dissolved nutrients and microflora than control populations in another tank which did not contain C. torquata. Elevated clams grew faster (p < 0.1) when worms were present in the tank than when worms were absent. Worm activity transported nutrient rich sedimentary pore water into the overlying water, apparently stimulating microfloral populations which supported improved clam growth. The effects of C. torquata on G. gemma growth may be one mechanism contributing to the common association of these species in New England sand flats.
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