Climate-Forced Variability of Ocean Hypoxia

Abstract

Oxygen (O(2)) is a critical constraint on marine ecosystems. As oceanic O(2) falls to hypoxic concentrations, habitability for aerobic organisms decreases rapidly. We show that the spatial extent of hypoxia is highly sensitive to small changes in the ocean's O(2) content, with maximum responses at suboxic concentrations where anaerobic metabolisms predominate. In model-based reconstructions of historical oxygen changes, the world's largest suboxic zone, in the Pacific Ocean, varies in size by a factor of 2. This is attributable to climate-driven changes in the depth of the tropical and subtropical thermocline that have multiplicative effects on respiration rates in low-O(2) water. The same mechanism yields even larger fluctuations in the rate of nitrogen removal by denitrification, creating a link between decadal climate oscillations and the nutrient limitation of marine photosynthesis.

Deoxygenation alters bacterial diversity and community composition in the ocean’s largest oxygen minimum zone

“…For all of these reasons, the aggregate effects of deoxygenation are complex, yet our gradient approach and use of MSDMs represent a first step to understanding these effects. Microorganisms are ultimately responsible for the decrease in DO with depth in the ocean 1,4 , biogeochemical models indicate that microbial responses are more important than physical factors in driving ocean deoxygenation 7 , and diverse bacterial communities found at the top of the OMZ likely have important roles in S oxidation 5 and coupled anaerobic and aerobic N cycling 4,9 . Changes in the functioning of these communities may have far-reaching effects on ocean biogeochemistry.…”

“…These low DO concentrations 2 delineate oxygen minimum zones (OMZs) found in the Arabian Sea, the Eastern tropical South Pacific Ocean (ETSP), and the Eastern tropical North Pacific Ocean (ETNP). Large aerobic organisms are mostly absent from OMZs 3 , but anaerobic microbial processes are active, and OMZs consequently have an important role in global biogeochemical cycles [4][5][6][7] . However, both the volume of OMZs and the biogeochemical processes occurring within them are highly sensitive to anthropogenic climate change: OMZs are warming, expanding and being further depleted of oxygen 1,3,8 , while model results indicate that anaerobic nitrogen (N) loss from the ETNP has varied fourfold in response to twofold changes in suboxic water volume during the past 50 years 7 .…”

“…How deoxygenation will affect OMZ microbial communities and processes is therefore poorly understood. Biogeochemically, OMZ expansion may increase global N loss 4,7 , but our understanding of N and sulphur (S) cycling in OMZs is still evolving [4][5][6] . At the community level, microbial diversity in the ETSP was originally thought to increase as oxygen decreased with depth 12 , but more recent data display an opposing pattern 10 .…”

Effects of hypoxia in the gills of the Manila clam Ruditapes philippinarum using NMR-based metabolomics

“…Hypoxification of the regional marine environment not only triggers a habitat loss for local marine organisms, reducing marine biological population, and altering the structure and function of marine ecosystem, but also interferes with the global biogeochemical cycling process (Deutsch et al, 2011). Impacts of marine hypoxia start from the sediment-water interface, causing a mass mortality of macrobenthos (Danise et al, 2013;Montagna & Ritter, 2006).…”

Nitrite oxidation in the upper water column and oxygen minimum zone of the eastern tropical North Pacific Ocean

“…Adding to this complexity, OMZs are expanding as a consequence of climate change (Stramma et al, 2008, Keeling et al, 2010, potentially altering the rates and distribution of N transformations in the water column (Gilly et al, 2013). In the ETNP, N loss rates have varied severalfold over the past few decades in response to variations in the DO (Deutsch et al, 2011). Placing quantitative bounds on microbially driven biogeochemical processes occurring in OMZs-and specifically the ETNP-is therefore essential.…”