Assessing the influence of upstream drainage lakes on fluvial organic carbon in a wetland-rich region

The role of aquatic ecosystems in regional and global carbon cycles is becoming increasingly apparent, and lakes and reservoirs may be particularly important to the retention and processing of organic carbon. If this is the case, then lakes and reservoirs may act as control points that decrease OC concentrations and fluxes in downstream aquatic ecosystems. We tested this hypothesis at a regional scale by comparing dissolved organic carbon (DOC) concentrations and fluxes in 52 randomly selected streams and rivers with and without upstream lakes in the water‐rich Northern Highlands Lake District (NHLD), Wisconsin, USA. DOC concentrations were significantly higher (p < 0.01) in drainage networks that did not contain lakes (25.02 mg/L) than they were in networks with upstream lakes (10.38 mg/L). However, when accounting for differences in wetland extent between watersheds, we were unable to detect a lake effect on downstream DOC concentrations (p > 0.49). Likewise, there were no significant differences in DOC:DON or DOC:DOP ratios, or in yields from watersheds with and without upstream lakes after compensating for wetland influences. We suggest that lake OC storage or processing may be limited by high hydrologic flushing in lakes with stream outlets and overwhelmed by larger scale influences of landscape composition in the NHLD. Consequently, drainage lakes in carbon‐rich regions like the NHLD may have limited influence on terrigenous carbon exports to the ocean. Key Points Drainage lakes do not decrease baseflow organic carbon concentration and fluxes Failure to account for wetland effects resulted in erroneous lake effects Drainage lakes may only minimally reduce terrigenous carbon exports to the ocean