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

Abstract

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.

Methane production induced by dimethylsulfide in surface water of an upwelling ecosystem

“…These effluxes were relatively high with respect to other coastal areas (see Bange, 1994) and comparable with CH 4 emissions from other coastal upwelling areas (Monteiro et al, 2006;Kock et al, 2008). CH 4 emission has been indirectly linked to high primary productivity, which favors CH 4 formation in sinking organic particles or in sediments (Owens et al, 1991;Sansone et al, 2001;Monteiro et al, 2006;Reeburgh, 2007).…”

“…CH 4 emission has been indirectly linked to high primary productivity, which favors CH 4 formation in sinking organic particles or in sediments (Owens et al, 1991;Sansone et al, 2001;Monteiro et al, 2006;Reeburgh, 2007). But as shown in Table 2, the experimental work in this study indicated that part of the CH 4 contents seems to come from the surface waters and that such produced gas could be emitted to the atmosphere.…”

“…Among costal ecosystems, eastern boundary coastal upwelling areas could be major sources or hotspots of marine CH 4 (Monteiro et al, 2006;Kock et al, 2008;Naqvi et al, 2010). There, a combination of physical and biological processes must co-occur to produce high air-sea CH 4 efflux.…”

“…Physical forcing related to favorable upwelling winds promotes intense vertical advection of subsurface waters, driving fertilization but also strong outgassing of N 2 O and CH 4 (Hatton et al, 1999;Bange, 2006;Kock et al, 2008;Cornejo and Farias, 2012). On the other hand, high biological productivity linked to natural hypoxia occurs during coastal upwelling events, favoring the existence of anaerobic/microaerophilic processes which mediate CH 4 cycling; most of them in the sediments but also in free-or attached living pelagic or benthic microorganisms (Monteiro et al, 2006). Considerable focus is placed on excess quantities of biogenic CH 4 in surface oceans, apparently generated by a local methanogenesis in oxygenated surface waters (Karl et al, 2008;Rudd and Taylor, 1980).…”

Coastal hypoxia and sediment biogeochemistry

“…Naturally occurring hypoxia is found in bottom waters of silled basins and fjords with restricted circulation (e.g. Black Sea, Cariaco Basin, Kau Bay, Anderson and Devol, 1987;Middelburg et al, 1991), as a result of natural intrusions or upwelling of subsurface oxygen-depleted waters on shelf systems (northern Chile, Morales et al, 1999;northeast Pacific, Grantham et al, 2004;Costa Rica, Thamdrup et al, 1996; Namibian shelf, Monteiro et al, 2006Monteiro et al, , 2008Lavik et al, 2009;Indian shelf, Naqvi et al, 2000), or in coastal embayments such as estuaries from the heterotrophic status sustained by the delivery of terrestrial and riverine organic matter (Heip et al, 1995). Human influences on coastal hypoxia are multiple and can operate on local and regional scales (e.g.…”