Oxygen depletion and sediment respiration in ice‐covered arctic lakes

Abstract

Processes regulating the rate of oxygen depletion determine whether hypoxia occurs and the extent to which greenhouse gases accumulate in seasonally ice-covered lakes. Here, we investigate the oxygen budget of four arctic lakes using high-frequency data during two winters in three shallow lakes (9-13 m maximal depth) and four winters in 24 m deep main basin of Toolik Lake. Incubation experiments measured sediment metabolism. Volume-averaged oxygen depletion measured in situ was independent of water temperature and duration of the ice-covered period. Average rates were between 0.2 and 0.39 g O 2 m À2 d À1 in the shallow lakes and between 0.03 and 0.14 g O 2 m À2 d À1 in Toolik Lake, with higher rates in smaller lakes with their larger sediment area to volume ratio. Rates decreased to $ 20%-50% of initial values in late winter in the shallow lakes but less or not at all in Toolik. The lack of a decline in Toolik Lake points to continued oxygen transport to the sediment-water interface where oxygen consumption occurs. In all lakes, lower in situ oxygen depletion than in incubation measurements points toward increasing anoxia in the lower water column depressing loss rates. In Toolik, oxygen loss during early winter was less in years with minimal snow cover. Penetrative convection occurred, which could mix downwards oxygen produced by photosynthesis or excluded during ice formation. Estimates of these terms exceeded photosynthesis measured in sediment incubations. Modeling under ice-oxygen dynamics requires consideration of optical properties and biological and transport processes that modify oxygen concentrations and distributions.

Regime Shifts in Lake Oxygen and Temperature in the Rapidly Warming High Arctic

“…2FB Lake underwent unusually rapid oxygen depletion, with bottom‐water anoxia developing only 4 and 11 days after freeze‐up in 2019 and 2020, respectively. Arctic lakes of similar depth have either not developed winter deepwater anoxia (Michaud & Apollonio, 2022) or only experienced near‐bottom depletion weeks to months after freeze‐up (Bégin et al., 2021; Palmer et al., 2019; Schwefel et al., 2023) with similar observations at lower latitudes (Couture et al., 2015; Robarts et al., 2005; Terzhevik et al., 2010). Deepwater oxygen depletion rates in 2FB Lake (0.8–2.6 mg L −1 day −1 ) greatly exceeded those of Alaskan lakes, which ranged from 0.1 to 0.3 mg L −1 day −1 (Schwefel et al., 2023).…”

“…Major ion concentrations were key drivers of the density gradients and water column stability of 2FB Lake. Solute gradients in cold lakes affect mixing depths prior to and following ice‐off (Cortés & MacIntyre, 2020; Schwefel et al., 2023), and many features of 2FB Lake's temperature stratification indicated that such gradients influenced the unusual under‐ice and summer thermal structures. Higher solute concentrations at depth allowed three unusual thermal structures: stable stratification despite surface temperatures of ∼4°C; inverse temperature stratification in summer 2021 despite water temperatures >6°C; and the mid‐depth temperature maxima.…”

“…With warmer summers, shorter ice‐cover durations, and increased exposure to solar heating (Dibike et al., 2011), high Arctic ecosystems such as 2FB Lake may undergo more frequent mixing and shorter periods with anoxia during winter, with consequences for biogeochemical processes and biological communities (Klanten et al., 2023). Earlier ice‐off, longer open‐water seasons and increased thermal stability will cause shifts in phenology and rates of primary productivity, potentially resulting in higher labile DOC sustaining respiration and intensifying winter oxygen depletion (Schwefel et al., 2023; Smejkalova et al., 2016). Newly ice‐free lakes will also be increasingly exposed to wind events that may cause mixing‐induced oxygenation of water columns despite stronger thermal gradients.…”

“…Arctic lakes of similar depth have either not developed winter deepwater anoxia (Michaud & Apollonio, 2022) or only experienced near-bottom depletion weeks to months after freeze-up (Bégin et al, 2021;Palmer et al, 2019;Schwefel et al, 2023) with similar observations at lower latitudes (Couture et al, 2015;Robarts et al, 2005;Terzhevik et al, 2010). Deepwater oxygen depletion rates in 2FB Lake (0.8-2.6 mg L 1 day 1 ) greatly exceeded those of Alaskan lakes, which ranged from 0.1 to 0.3 mg L 1 day 1 (Schwefel et al, 2023). Diel cycles following ice-on indicated a precarious balance between oxygen production and consumption.…”

“…Climate‐mediated shifts in oxygen concentrations can therefore impact ecological services vital to northern Indigenous communities, including the provision of drinking water and the survival of harvested fish species (Preece et al., 2019). Our understanding of temperature and oxygen dynamics in low Arctic lakes has seen substantial progress (Deshpande et al., 2015; MacIntyre et al., 2018; Schwefel et al., 2023), but the High Arctic is subject to a considerably different climate, with lake ice persisting for most or all the year. Moreover, circumpolar syntheses have shown that more northerly lakes have amplified ecological responses to warming relative to lower latitude Arctic and Subarctic systems (Smol et al., 2005).…”