Anoxia begets anoxia: A positive feedback to the deoxygenation of temperate lakes

Abstract

Declining oxygen concentrations in the deep waters of lakes worldwide pose a pressing environmental and societal challenge. Existing theory suggests that low deep‐water dissolved oxygen (DO) concentrations could trigger a positive feedback through which anoxia (i.e., very low DO) during a given summer begets increasingly severe occurrences of anoxia in following summers. Specifically, anoxic conditions can promote nutrient release from sediments, thereby stimulating phytoplankton growth, and subsequent phytoplankton decomposition can fuel heterotrophic respiration, resulting in increased spatial extent and duration of anoxia. However, while the individual relationships in this feedback are well established, to our knowledge, there has not been a systematic analysis within or across lakes that simultaneously demonstrates all of the mechanisms necessary to produce a positive feedback that reinforces anoxia. Here, we compiled data from 656 widespread temperate lakes and reservoirs to analyze the proposed anoxia begets anoxia feedback. Lakes in the dataset span a broad range of surface area (1–126,909 ha), maximum depth (6–370 m), and morphometry, with a median time‐series duration of 30 years at each lake. Using linear mixed models, we found support for each of the positive feedback relationships between anoxia, phosphorus concentrations, chlorophyll a concentrations, and oxygen demand across the 656‐lake dataset. Likewise, we found further support for these relationships by analyzing time‐series data from individual lakes. Our results indicate that the strength of these feedback relationships may vary with lake‐specific characteristics: For example, we found that surface phosphorus concentrations were more positively associated with chlorophyll a in high‐phosphorus lakes, and oxygen demand had a stronger influence on the extent of anoxia in deep lakes. Taken together, these results support the existence of a positive feedback that could magnify the effects of climate change and other anthropogenic pressures driving the development of anoxia in lakes around the world.

Reservoir drawdown highlights the emergent effects of water level change on reservoir physics, chemistry, and biology

“…Conversely, increased buoyancy frequency was likely more relevant in controlling biogeochemical dynamics during the summer stratified period (Lewis et al, 2024a). Ultimately, our work highlights the importance of calculating multiple metrics of thermal stratification to understand the complex effects of drawdown on reservoir physical processes.…”

Ecological memory of spring air temperature drives summer water quality dynamics in temperate lakes

“…To explore associations between seasonal air temperatures and bottom-water dynamics, we analyzed a database of temperature and DO profiles from 615 widespread lakes, all of which exhibit stable summer thermal stratification (Lewis et al 2023(Lewis et al , 2024. This database includes lakes across 18 countries and 5 continents (Figure 1), with a median depth of 14 m (Z max ; range: 6-370 m), median surface area of 100 ha (range: 1-126,909 ha), and median time series duration of 30 years.…”

“…This database includes lakes across 18 countries and 5 continents (Figure 1), with a median depth of 14 m (Z max ; range: 6-370 m), median surface area of 100 ha (range: 1-126,909 ha), and median time series duration of 30 years. Detailed data descriptions are provided by Lewis et al (2023Lewis et al ( , 2024. All data analyses were performed in R, version 4.3.2 (R Core Team, 2023), and analysis code is archived on Zenodo (Lewis 2024).…”

“…We calculated mean summer water temperature and DO in surface and bottom waters for each lake-year following Lewis et al (2024). Briefly, we interpolated summer temperature and DO profiles to a 1-m depth resolution, then averaged all depth-interpolated data from the epilimnion and hypolimnion to generate surface-and bottom-water means for each summer, respectively (Winslow et al 2019).…”

“…While surface-water temperatures are generally increasing, summer bottom-water temperature trends are highly variable among lakes and show little to no correlation with summer or mean-annual air temperature trends (e.g., Richardson et al 2017;Pilla et al 2020;Jane et al 2021;Zhou et al 2023). As summer bottom-water conditions play a critical role in regulating internal nutrient loading, habitat availability, and greenhouse gas dynamics (e.g., Hounshell et al 2021;Carey et al 2022;Jane et al 2024;Lewis et al 2024), characterizing drivers of summer bottom-water temperature and DO is necessary to anticipate future lake ecosystem function.…”