The objective of this research was to compare and contrast C dynamics within plots occupied by Phragmites australis, Typha spp., and Sagittaria latifolia in a Lake Erie coastal wetland (Ohio, USA). The effect of each species on above- and belowground biomass, soil C pools, soil labile C, litter decomposition rates, and microbial catabolic response profiles were analyzed. Phragmites australis and Typha spp. produced significantly more aboveground biomass (1,522 ± 464 and 1,177 ± 164 g DM m⁻², respectively) than S. latifolia (500 ± 80 g DM m⁻²), although no difference was observed in terms of belowground biomass. After 208 days in the field, litter of S. latifolia had lost 72% of its initial mass while only 47% of the litter of Typha spp. and P. australis had decomposed. This coupled process of high primary production and slow litter decomposition within P. australis and Typha spp. communities did not translate into greater accumulation of C in the soil. In fact, we observed lower rates of C mineralization and greater biomass of methanogens in the S. latifolia plots. Despite similar water level, soil conditions in the S. latifolia community was more saturated, which might have limited availability of C for microbial consumption in these plots. Microbial catabolic responses to 24 substrates demonstrated distinct differences in the respiration responses of the soil microbial communities of the three macrophyte species. The microbial community found in the rhizosphere of P. australis was particularly responsive to phenolic acids. Few differences in C fluxes and pools were observed between plots occupied by P. australis and Typha spp., but the replacement of S. latifolia by one of the two other species could have a significant effect on the C cycle in the Great Lakes coastal wetlands.