Strong Habitat Compression by Extreme Shoaling Events of Hypoxic Waters in the Eastern Pacific

Abstract

Ocean deoxygenation has led to a measurable shoaling of subsurface hypoxic waters ([O2] < 60 mmol m−3) in the tropical Pacific and elsewhere. This shoaling compresses the vertical habitat of heterotrophic organisms between the well oxygenated surface and the hypoxic interface. Superimposed on this long-term trend, natural variability can cause shorter-term, extreme habitat compression events, which we term Transient Habitat Reduction Extreme Events (THREEs). Here, we investigate THREEs in the Eastern Pacific (EP) between 1979 and 2016 using output from a high-resolution hindcast simulation with the coupled physical-biogeochemical ocean model ROMS-BEC. We identify THREEs on the basis of the hypoxic interface being shallower than the 99th percentile of its variability and requiring a minimum duration of five days. We find that THREEs compress the aerobic habitat by up to 50%–70% (locally exceeding 80%) in the subtropical and tropical EP and reach horizontal scales exceeding 300 km in the tropical EP. There, THREEs are pre-conditioned by La Niña conditions and occur primarily in boreal winter and spring. In the subtropical EP, THREEs are associated with mesoscale eddies and occur independently of the season. 71% (32%) of all THREEs are characterized by unusually high acidity (cold) conditions (i.e., below the first percentile), thus constituting compound events. The strong habitat compression during THREEs is bound to lead to substantial changes in water column biogeochemistry, in food-web interactions, and in ecosystem structure. Yet, so far, very little is known about the impact of these transient phenomena.