Variability of natural hypoxia and methane in a coastal upwelling system: Oceanic physics or shelf biology?

Emerging understanding of the variability of natural coastal hypoxia is divided between two main hypotheses: the biogeochemical oxygen demand linked to locally‐driven organic matter decay or to supply of low‐oxygen waters by physical processes. The precise role of either mechanism in triggering hypoxia has remained elusive. A combined methane and oxygen high resolution year‐long hourly data time series in a coastal upwelling system suggests that these systems may be responding to a complex interaction between the two. The data show how anoxia is initially triggered by remote equatorial hypoxic waters after which it can be sustained by a local biogeochemical flux of exported production. Crucially, without a remote trigger the local forcing could not develop anoxic conditions because the physical flux of oxygen would be too high. The robustness of this interpretation is tested using a 10‐year data set (1994–2003) with seasonal and interannual scales of variability.