Anoxia decreases the magnitude of the carbon, nitrogen, and phosphorus sink in freshwaters

Abstract

Oxygen availability is decreasing in many lakes and reservoirs worldwide, raising the urgency for understanding how anoxia (low oxygen) affects coupled biogeochemical cycling, which has major implications for water quality, food webs, and ecosystem functioning. Although the increasing magnitude and prevalence of anoxia has been documented in freshwaters globally, the challenges of disentangling oxygen and temperature responses have hindered assessment of the effects of anoxia on carbon, nitrogen, and phosphorus concentrations, stoichiometry (chemical ratios), and retention in freshwaters. The consequences of anoxia are likely severe and may be irreversible, necessitating ecosystem‐scale experimental investigation of decreasing freshwater oxygen availability. To address this gap, we devised and conducted REDOX (the Reservoir Ecosystem Dynamic Oxygenation eXperiment), an unprecedented, 7‐year experiment in which we manipulated and modeled bottom‐water (hypolimnetic) oxygen availability at the whole‐ecosystem scale in a eutrophic reservoir. Seven years of data reveal that anoxia significantly increased hypolimnetic carbon, nitrogen, and phosphorus concentrations and altered elemental stoichiometry by factors of 2–5× relative to oxic periods. Importantly, prolonged summer anoxia increased nitrogen export from the reservoir by six‐fold and changed the reservoir from a net sink to a net source of phosphorus and organic carbon downstream. While low oxygen in freshwaters is thought of as a response to land use and climate change, results from REDOX demonstrate that low oxygen can also be a driver of major changes to freshwater biogeochemical cycling, which may serve as an intensifying feedback that increases anoxia in downstream waterbodies. Consequently, as climate and land use change continue to increase the prevalence of anoxia in lakes and reservoirs globally, it is likely that anoxia will have major effects on freshwater carbon, nitrogen, and phosphorus budgets as well as water quality and ecosystem functioning.

Conceptual models of dissolved carbon fluxes considering interannual typhoon responses under extreme climates in a two-layer stratified lake

“…a concentrations differ significantly between upper and lower layers in the typhoon years (Figure 3) due to thermal stratification (Chiu et al, 2020;Lin et al, 2022;Tsai et al, 2008;Tsai et al, 2011). In addition, the abundance of organisms leads to intensive respirations in the lower layers during the non-typhoon period; for example, an anoxic condition at the hypolimnion may affect C mineralization and remineralization rates in non-typhoon years (Carey et al, 2022;Chiu et al, 2020;Lin et al, 2022;Shade et al, 2010;Shade et al, 2011). Therefore, these physical and biogeochemical processes might describe different patterns between the upper and lower layers, as revealed by Pearson correlations (Figure 5).…”

“…Moreover, without the typhoon-induced mixing and refreshing of the water column, anoxic conditions may occur (Carey et al, 2022;Vachon et al, 2021), which could result in negative remineralization rates in non-typhoon years. Thus, the conceptual two-layer C models well 3), but SD was higher in the typhoon years as well (Figure 10).…”

Effects of Hypoxia on Coupled Carbon and Iron Cycling Differ Between Weekly and Multiannual Timescales in Two Freshwater Reservoirs

“…Freshwater lakes and reservoirs are increasingly recognized as hotspots in the global carbon cycle (Bastviken et al., 2011; Battin et al., 2009; Carey, Hanson, et al., 2022; Raymond et al., 2013; Tranvik et al., 2018). Due to high organic carbon (OC) loading from the surrounding watershed, lakes and reservoirs likely bury more OC than coastal sediments each year (Dean & Gorham, 1998; Knoll et al., 2013; Mendonça et al., 2017; Pacheco et al., 2014; USGCRP, 2018).…”

“…Periods of hypoxia have the potential to decrease OC sequestration through reductive dissolution of Fe(III) in Fe‐OC complexes (Chen et al., 2020; Huang et al., 2021; Patzner et al., 2020). However, hypoxia also has the potential to increase OC sequestration by decreasing the rate of OC respiration (Carey, Hanson, et al., 2022; Carey et al., 2018; Hargrave, 1969; Peter et al., 2017; Sobek et al., 2009; Walker & Snodgrass, 1986), particularly if Fe‐OC complexes are resistant to, or protected from, changes in oxygen concentrations in overlying water. Decreased OC respiration rates under hypoxic conditions is thought to occur primarily because respiration is less thermodynamically favorable in the absence of oxygen (e.g., Arndt et al., 2013; LaRowe & Van Cappellen, 2011).…”

Nitrogen and Phosphorous Retention in Tropical Eutrophic Reservoirs with Water Level Fluctuations: A Case Study Using Mass Balances on a Long-Term Series

“…The increasing availability of P and biologically reactive N in aquatic ecosystems has led to their ecological deterioration globally [4,5] and has compromised their water quality [6,7]. Anoxic conditions are among the most critical effects of cultural eutrophication because they can affect the budgets of N, P and C, and in particular, they can further significantly modify the N transformations and dynamics [8].…”