Sporadic hotspots for physico-chemical retention of aquatic organic carbon: from peatland headwater source to sea

Few studies have quantified the role of in-stream processes on net dissolved and particulate organic carbon (DOC and POC, respectively) export from peatland catchments, and those that have offer conflicting evidence. In this study, we evaluated evidence for active organic matter processing under field conditions, via a coordinated campaign across four UK catchments with peatland headwaters, targeted on potential 'hotspots' and 'hot moments' of physico-chemical carbon cycling. We hypothesised that specific hotspots and hot moments would occur where waters enriched with DOC and POC sourced from headwaters are exposed to: (1) mixing with freshwaters of different pH, conductivity and metal concentrations; and (2) mixing with seawater during autumn when DOC concentrations were at their highest. We observed instances of POC removal in headwaters, and potential for rapid conversion between dissolved and particulate carbon forms and for net removal of peat-derived carbon at confluences further downstream (where observed, on the order of 52-75 % for POC, and 5-44 % for DOC). Estuary transect surveys indicated that up to 30 % of fluvial DOC can be removed under high flow conditions. However, in the majority of cases concentrations remained within the range that would be expected based on conservative transport. These findings indicate that rapid (e.g. solubility-related) processes within the river system may be important but sporadic, thus are unlikely to provide major removal pathways for peat-derived organic carbon.