The Importance of Aquatic Carbon Fluxes in Net Ecosystem Carbon Budgets: A Catchment-Scale Review

The growing importance of resolving ecosystem carbon budgets has resulted in more studies integrating terrestrial and aquatic carbon fluxes. Although recent estimates highlight the importance of inland waters in global carbon budgets, the extent to which aquatic pathways contribute to the net ecosystem carbon budget (NECB) of different ecosystems remains poorly understood. Here, we provide a cross-ecosystem review of annual carbon budgets integrating terrestrial and aquatic fluxes. Large variability in the proportion of aquatic carbon offset to terrestrial net ecosystem productivity (NEP) was observed, with aquatic offsets ranging from <1% in a boreal forest to 590% in a freshwater marsh. The total aquatic carbon flux was positively correlated with terrestrial NEP, suggesting highly productive ecosystems will have greater aquatic carbon offsets. However, due to an order of magnitude difference in the range of terrestrial NEP (similar to 1000gCm(-2)y(-1)) compared to aquatic fluxes (similar to 100gCm(-2)y(-1)), ecosystems with small NEP's had greater relative aquatic carbon offsets overall in their NECB's. Northern hemisphere peatlands and forests represented 54% of all integrated carbon budget studies collected, indicating a severe ecosystem and spatial bias. Mangroves, agricultural, and disturbed ecosystems were the most underrepresented, yet had extreme ranges in terrestrial NEP and NECB (-638 to 1170gCm(-2)y(-1)). To improve our mechanistic understanding of the role of aquatic pathways in NECB's, more site-specific integrative studies need to be undertaken across a broader range of climatic regions and ecosystem types.