Seasonal variation and importance of catchment area composition for transport of bioavailable carbon to the Baltic Sea

Transport of terrestrial carbon through riverine systems to coastal water has a negative impact on oxygen concentration in coastal areas. However, information on seasonal variation and the impact of catchment composition on the bioavailability of allochthonous carbon is lacking. In this project we address this knowledge gap by investigating the reactivity of dissolved organic carbon (DOC) at river mouths which originate from agricultural or forested dominated catchments over a year. Using a high-capacity oxygen sensing system biological oxygen demand (BOD) was measured and converted to carbon utilization rate. This allowed a spatial and temporal resolution necessary to understand how concentrations of total nitrogen, total phosphorus, DOC, as well as carbon composition influence carbon bioavailability. Seasonality and the differing catchment compositions yielded variable results about which factors were significantly contributing to reactivity. In addition, we found that carbon utilization rate was highest during April, June, and October for most rivers. The bioavailable fraction (BFc) was significantly higher in rivers with agricultural compared to forest dominated catchments during January, April, June, and October. However, rivers with agricultural dominated catchment had a significantly higher carbon utilization rate in August. This indicate that rivers dominated by forest transport larger and more refractory pools of carbon, while rivers with agricultural dominated catchments have a higher percentage of BFc. Based on these results we suggest that management efforts, to reduce the transport of bioavailable carbon, would be most efficient during spring and autumn with equal importance on rivers with catchment areas dominated by agriculture and forest.