The oyster pond under study has a natural sediment bottom and water is changed only once or twice every month depending on tidal height. Between successive water renewals, the pond is a closed system, equivalent to a batch culture in which sinking and biodeposition of particulate matter are the only sources of organic matter to the bottom. Particulate organic matter enrichment of the sediment in winter is followed by intensive ammonification in summer with very limited nitrification. Seasonal nitrogen budgets involving particulate and dissolved phases show that in summer during batch mode period, ammonium enrichment from the sediment to the overlying water is an order of magnitude higher than the initial nutrient supply in the feed water. This allows a large increase of the phytoplankton biomass. We introduce the term 'deferred' primary production for this phenomenon because of the seasonal lag between particulate nitrogen deposition and dissolved nitrogen fluxes from the sediment.