The fate of phytoplankton detritus in the muddy sediments of shallow marine ecosystems was studied by labelling the water column of a 13 m3 microcosm with radiocarbon bicarbonate from January to July. By the end of the study, more than 9% of the original inorganic label was found as organic carbon in the top 10 cm of sediment. The accumulation of labelled organic carbon in the sediment totalled 14.5 gC/m2. We estimate that this amount represented 15% of daytime net primary production and roughly half of the labelled organic carbon that was deposited on the sediment. The finding of sedimentary carbon accumulation directly demonstrated that time lags on the order of months can exist between the deposition and mineralization of phytoplankton detritus in nature. Observed time lags may have occurred because heterotrophic activity was minimize by low winter and spring temperatures, pelagic and benthic grazing was minimal, and bioturbation rates were high. Mineralization of organic carbon may have been retarded by conditions in subsurface sediments. Detritus buried in the sediment during the winter and spring would have been available to the subsurface feeding benthos during the summer, when maximum metabolic demand occurs.