We investigated seasonal changes in the production of extracellular polymeric substances (EPS) and short-chain organic acids (SCOA) exuded by benthic diatoms, and the use of these exudates as a carbon source by heterotrophic bacteria. An in situ ¹³C pulse-chase method was used to follow the fate of EPS for 5 consecutive days. These experiments were done at 2 mo intervals for 1 yr. The EPS were recovered from the sediment as 2 operationally defined fractions (i.e. water-extractable and EDTA-extractable EPS). Seasonal differences in EPS production correlated to light intensity and temperature. From February until June the biomass and production of diatoms and bacteria were closely coupled. It was concluded that SCOA were the most important substrates for the bacteria. Sulfate-reducing bacteria (SRB) in particular benefited from SCOA released by diatoms. From August on, the coupling of biomass and production of diatoms and bacteria weakened and was almost lost in December. During the period from August to December, EPS produced by diatoms promoted the growth of other bacterial taxa rather than SRB, and the production of SCOA was low. Thus, it appears that the seasonal variation in exudates produced by diatoms plays an important role in shaping community composition and maintaining the diversity of the associated bacteria.