We used the protobranch bivalve, Yoldia limatula, in a series of incubation experiments to test whether activities of deposit-feeding macrofauna cause differences in lipid degradation processes relative to controls lacking macrofauna. Uniformly 13C-labeled algae (13C > 98%) were used as a source of fresh planktonic lipids, easily distinguished from bulk sedimentary lipids by GC/MS. Variations in concentration of major lipid components ([16:1(ω7), 16:0 and 18:1(ω9)] fatty acids and phytol) were followed as a function of time in incubations with and without Yoldia. Results showed that Yoldia can significantly enhance the degradation of planktonic lipids in sediments. Net degradation rate constants of lipids such as fatty acids and phytol in surface sediments were linearly correlated with abundance of Yoldia (in the range 230-1160 animals/m2). Yoldia altered the sediment and decomposition regime in several ways: (1) lipid-containing particles were ingested and then ejected into the oxic overlying water, with selective ingestion and digestion of different components; (2) particles were moved into subsurface regions by bioturbation; (3) sediment resuspension occurred, porosity increased, and dissolved oxygen and suboxic conditions penetrated deeper in the presence of Yoldia; (4) Yoldia grazed bacteria, influencing net degradation pathways of algal material, as indicated by higher accumulation of a bacteria-specific branched fatty acid and an intermediate C16 alcohol in the absence of Yoldia than in presence of Yoldia when plankton material was introduced as a pulse); (5) the activities of Yoldia enhanced solute exchange and altered the spatial and temporal patterns of redox reactions, as indicated by time-dependent depth distributions of Br (introduced tracer), ΣCO2 and NH4+ in the microcosms.