Enhanced dissolved organic carbon production in aquatic ecosystems in response to elevated atmospheric CO2

Although aquatic ecosystems are a major carbon reservoir, how their carbon dynamics will respond to increasing concentrations of atmospheric CO 2 is not well understood. The availability of essential nutrients has the potential to modify carbon fluxes under elevated CO 2 by altering carbon processing and storage in the biota. Here, we describe a semi-continuous culture experiment with natural phytoplankton and bacteria assemblages designed to investigate (1) how carbon dynamics in aquatic ecosystems respond to continuously elevated atmospheric CO 2 , and (2) whether carbon fluxes resulting from elevated CO 2 are modified by changes in inorganic nitrogen and phosphorus availability. Our results showed that elevated CO 2 led to significant increases in photosynthetic carbon uptake, despite a decrease in the algal chlorophyll a concentrations and no significant change in total algal biovolume. This enhancement of inorganic carbon uptake was accompanied by a significant increase in dissolved organic carbon (DOC) production. Concurrent increases in the C/N and C/P ratios of dissolved organic matter also suggested that DOC stability increased. Nutrient availability, especially nitrogen availability, had strong effects on inorganic carbon uptake and biomass carbon pools. In contrast, CO 2 -enhanced DOC production was not significantly affected by varying concentrations of inorganic nitrogen and phosphorus. Our study underscores the importance of DOC as a potential carbon sink in aquatic ecosystems. The observed responses to changes in CO 2 and nutrient availability could have important implications for long-term carbon cycling in aquatic ecosystems, and highlight the potential buffering capacity of aquatic ecosystems to future environmental change.