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, seven-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.