Priming effect is widely reported in studies of carbon dynamics in upland soils but there is limited information regarding this phenomenon in organic C-rich systems such as freshwater sediments. Several recent studies propose an increase in microbial biomass as the underlying mechanism. It is hypothesized that the degree of priming is related to the availability of labile C in existing organic matter and that, in systems where the labile C pool is limiting, addition of complex organic substrates could elicit a priming effect through increased microbial diversity and enhanced production of hydrolytic enzymes. To evaluate the merit of these hypotheses, fresh and aged sediment were amended with ¹³C-labeled algae and algal cellulose and incubated for 25 days. The aged sediment was pre-incubated for 150 days and was thus considered depleted in labile C fractions. Priming effect was assessed as CO₂ evolved from mineralization of existing organic matter. During the first week of the incubation, results showed that 80-88% of evolved CO₂ originated from sediment C, but as the incubation progressed, a decrease in both the priming effect intensity and the diversity of the microbial community (Shannon's index) was observed. β-glucosidase activity exhibited no specific trend with respect to the source of organic added but showed a strong temporal decline in the aged-sediment samples. Consistent with observed trends in priming effect, phenol-oxidase activity and diversity index were consistently higher in the algae than in the cellulose-amended sediment samples. These results suggest that microbial diversity and the dynamics of hydrolytic enzymes are additional factors that could help elucidate the mechanism of the priming effect in freshwater sediment.