Exploring Stream-Lake Interaction Impacts on the Biogeochemistry of a Lake-Dominated Arctic Watershed

Abstract:

The Arctic is warming at twice the rate of the rest of the planet, releasing terrestrially-stored carbon and nutrients that were previously frozen in permafrost soils. The enhanced release of nutrients and carbon from land to water is evidenced by increasing fluxes of biogeochemical solutes have been documented at the outlets of most large Arctic rivers. Additionally, there is emerging indirect evidence that stream-lake interactions may modify or confound the biogeochemical signals observed at river outlets. Here, we directly explored how stream-lake interactions alter biogeochemical signals in a permafrost-dominated stream network by analyzing water chemistry from repeated synoptic sampling campaigns that occurred twice a year during June and August across three years (2016-2018). We focused on a subset of samples taken above and below three lakes in a lake-dominated watershed of Oksrukuyik Creek on the North Slope of Alaska. To determine how lakes affect solute concentrations, we calculated pairwise (above vs below lake) and seasonal (early vs late) differences in water chemistry for a suite of biologically reactive solutes (e.g., Carbon as DOC; Nitrogen as NO₃^-, NH₄⁺, and TDN; and phosphorous as SRP and TDP). We also estimated the mean residence time of these lakes and found that they varied between 17 days and 4 years. We found that some solute concentrations were different above and below lakes, though the direction of change was solute-dependent. More importantly, above/below solute differences became more pronounced in the late season. For example, DOC increased while NO₃^- and TDP decreased at the lake outlet, likely the result of increased terrestrial input and aquatic biological processing as the lake warms. Additionally, we found a relationship between lake residence time and the change in above/below concentrations, suggesting that there is a role of water residence in biogeochemical processing in this watershed. Our analysis directly reinforces that stream-lake interactions play an important role in modulating biogeochemical signals created by the release of terrestrially-stored carbon and nutrients in a rapidly changing Arctic.