Tree line advance reduces mixing and oxygen concentrations in arctic–alpine lakes through wind sheltering and organic carbon supply

Abstract

Oxygen depletion in lake bottom waters has adverse impacts on ecosystem health including decreased water quality from release of nutrients and reduced substances from sediments, and the reduction of fish growth and reproduction. Depletion occurs when oxygen is consumed during decomposition of organic matter, and oxygen replenishment is limited by water column stratification. Arctic–alpine lakes are often well mixed and oxygenated, but rapid climate change in these regions is an important driver of shifts in catchment vegetation that could affect the mixing and oxygen dynamics of lakes. Here, we analyze high‐resolution time series of dissolved oxygen concentration and temperature profiles in 40 Swedish arctic–alpine lakes across the tree line ecotone. The lakes stratified for 1−125 days, and during stratification, near‐bottom dissolved oxygen concentrations changed by −0.20 to +0.15 mg L−1 day−1, resulting in final concentrations of 1.1−15.5 mg L−1 at the end of the longest stratification period. Structural equation modeling revealed that lakes with taller shoreline vegetation relative to lake area had higher dissolved organic carbon concentrations and oxygen consumption rates, but also lower wind speeds and longer stratification periods, and ultimately, lower near‐bottom dissolved oxygen concentrations. We use an index of shoreline canopy height and lake area to predict variations among our study lakes in near‐bottom dissolved oxygen concentrations at the end of the longest stratification period (R2 = 0.41). Upscaling this relationship to 8392 Swedish arctic–alpine lakes revealed that near‐bottom dissolved oxygen concentrations drop below 3, 5, and 7 mg L−1 in 15%, 32%, and 53% of the lakes and that this proportion is sensitive (5%−22%, 13%−45%, and 29%−69%) to hypothetical tree line shifts observed in the past century or reconstructed for the Holocene (±200 m elevation; ±0.5° latitude). Assuming space‐for‐time substitution, we predict that tree line advance will decrease near‐bottom dissolved oxygen concentrations in many arctic–alpine lakes.

The Scaling Relationship for the Length of Tributaries to Lakes

“…deterministic calculations. We expected high junction angles (i.e., close to 90°) based on maps, satellite imagery, and our field experience in the study region (e.g., Klaus et al, 2021;Norman et al, 2022;Riley & Seekell, 2021;Seekell, Cael, Norman, et al, 2021). This is not a trivial expectation because qualitative evidence indicates that there is a substantial range of junction angles, including very low junction angles in some regions (Supporting Information S1).…”

Predicting arctic‐alpine lake dissolved oxygen responses to future tree line advance at the Swedish forest‐tundra transition zone

“…Transition of current tundra lakes to forested is likely to alter the assemblage of species in these lakes, favoring those tolerant of lower DO concentrations, and threatening their potential role as climate refugia, despite the presence of cold deep‐water temperatures. Although Klaus et al (2021) find that such forest advance is not likely to change many of these lakes to completely anoxic, it is likely that DO levels favorable for methane production and other biogeochemical changes may become more prevalent. Increased DOM in many of these lakes may favor increased release of not only methane, but also CO 2 as higher DOM concentrations will likely favor increased heterotrophy (Hanson et al, 2003).…”

“…Northern lakes at the transition between forested and tundra environments tend to be relatively pristine by comparison. The findings of Klaus et al (2021) imply that many lakes in this region are not likely to remain immune to such anthropogenic alteration as warming continues. Currently, these northern lakes represent a refuge for many cold‐water species (Jeppesen et al, 2010), many of which have lost habitat in more southern locations due to deep‐water DO losses and warming of surface waters (Carpenter et al, 2011).…”

“…In addition, arctic tundra environments play an important role in global carbon cycling (Kuhn & Butman, 2021), and conversion of watersheds from tundra to forest dominated implies substantial alterations to regional carbon cycling. Klaus et al (2021) are among the first to demonstrate the potential changes to lake DO in this context through a combination of space for time substitution surveys and structural equation modeling. The investigators monitor deep‐water DO in 40 Swedish arctic‐alpine lakes at this critical transition zone, ranging from unforested tundra lakes to fully forested lakes.…”

“…While nominally a terrestrial ecosystem change, this advance may dramatically alter aquatic ecosystems, with lakes in particular being highly responsive to events that occur in the surrounding landscape. Klaus et al (2021) address this important issue by using a space‐for‐time substitution across 40 Swedish arctic‐alpine lakes to understand how a transition from tundra to forested watersheds will influence lake dissolved oxygen (DO).…”