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 wholeecosystem 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.

Classifying Mixing Regimes in Ponds and Shallow Lakes

“…A corollary is that stratification may reduce emissions, as seen in farm ponds that consume nitrous oxide (Webb et al., 2019). Mixing regime may also influence the balance of carbon storage versus emissions as increased stratification can promote carbon burial in lakes through cooler bottom water (Bartosiewicz et al., 2019) and anoxia (Carey et al., 2022). As mixing regimes may regulate ecology and biogeochemistry, monitoring mixing patterns may be necessary to predict greenhouse gas budgets and accurately model ecosystem dynamics.…”

Anoxic age as a new tool to predict biogeochemical consequences of oxygen depletion in lakes

“…We also found good, positive relationships with SRP and NH 4 + , two compounds mostly related to sediment processes (Carey et al 2022). The SRP increase likely reflected the initial phase of organic phosphorus diagenetic release, unobstructed by SRP-capturing iron oxides at the sediment surface (Hupfer and Lewandowski 2008).…”

Causes and consequences of changing oxygen availability in lakes

“…2a). While sediment fluxes of DOC, NH4 + , and SRP are 2-4× greater in anoxic than oxic lake conditions, denitrification, an important process removing nitrate (NO3 -) from freshwaters, can be 4× greater in anoxic than oxic conditions (Carey et al 2022a).…”