Article

Emergence of Anoxia in the California Current Large Marine Ecosystem

Department of Zoology, Oregon State University, Corvallis, OR 97331, USA.
Science (Impact Factor: 33.61). 03/2008; 319(5865):920. DOI: 10.1126/science.1149016
Source: PubMed

ABSTRACT

Eastern boundary current systems are among the world's most productive large marine ecosystems. Because upwelling currents
transport nutrient-rich but oxygen-depleted water onto shallow seas, large expanses of productive continental shelves can
be vulnerable to the risk of extreme low-oxygen events. Here, we report the novel rise of water-column shelf anoxia in the
northern California Current system, a large marine ecosystem with no previous record of such extreme oxygen deficits. The
expansion of anoxia highlights the potential for rapid and discontinuous ecosystem change in productive coastal systems that
sustain a major portion of the world's fisheries.

Full-text

Available from: William T. Peterson
Emergence of Anoxia in the California
Current Large Marine Ecosystem
F. Chan,
1
* J. A. Barth,
2
J. Lubchenco,
1
A. Kirincich,
2
H. Weeks,
3
W. T. Peterson,
4
B. A. Menge
1
E
astern boundary current systems are
among the most productive large marine
ecosystems in the world. Their produc-
tivity arises from wind-driven upwelling of
nutrient-rich water into the photic zones of
coastal oceans and supports 20% of global fish-
ery yield (1). Upwelling also transports oxygen-
poor waters onto productive conti ne nt al shel ve s,
where respiration can further reduce water-
column dissolved oxygen (DO) content and
thus subject coastal ecosystems to the risk of
hypoxia or anoxia.
Of the worlds four major eastern boundary
current systems, water-column shelf anoxia is
previously known for only the Humboldt and
Benguela Current systems (2). Because oxygen
deficit can be a critical determinant of fishery,
ecological, and biogeochemical pro cesses (3, 4),
the rise or expansion of hypoxia and anoxia rep-
resent major perturbations to the structure and
functioning of coastal marine ecosystems. Here,
we report the intensification of severe inner-
shelf hypoxia (defined here as 0.5 ml l
1
)and
the novel rise of water-column anoxia in the
California Current large marine ecosystem
(CCLME) along the U.S. West Coast.
Following changes in upwelling-favorable
winds in 2006, we measured the emergence of
anoxia along the central Oregon coast. Anoxia
was evident in inner-shelf (<50 m) stations
situated within 2 km of the surf zone (Fig. 1C).
The onset of anoxia was accompanied by the
expansion of severe hypoxia across broad sec-
tions of the central Oregon shelf. At its largest,
hypoxia prevailed across all cross-shelf transect
lines between 44.25°N and 45.00°N, extending
from the shelf break to the inner shelf and en-
compassing at least 3000 km
2
. Hypoxia was
also exceptional in its vertical and temporal
extents, occupying up to 80% of the water
column in shallow (60 m) shelf waters and
persisting over mid- and inne r -s he l f water s from
June to October .
Although severe hypoxia is a permanent fea-
ture of the oxygen minimum zone (OMZ) that
intersects the continental slope (>600 m in this
system), there are no prior records of anoxia
over the continental shelf or within the OMZ
(Fig. 1A). In contrast to OMZ marine life that
possess adaptations to severe oxygen stress (3),
demersal fish and benthic invertebrate commu-
nities in these shallow shelf waters were acutely
affected by seasonally persistent anoxia and
severe hypoxia. In August 2006, submersible-
based surveys along the same previously mon-
itored (2000 to 2004) transect lines revealed
the complete absence of all fish from rocky
reefs that normally serve as habitats for diverse
rockfish (Sebastes species) communities that are
of current fishery management concern. Our sur-
veys also revealed near-comp lete mortality of
macroscopic benthic invertebrates and an accom-
panying rise of putative sulfide-oxidizing bacte-
rial mats in shallow (50 m) waters (movie S1).
The rise of anoxia has occurred against a
backdrop of recent increases in the frequency
and severity of shelf hypoxic events in this sys-
tem (Fig. 1B). Five decades of available records
show little evidence of shelf hypoxia and no
evidence of severe inner-shelf hypoxia before
2000 (Fig. 1A). Recent studies indicate that the
onset of shelf hypoxia can reflect basin-scale
fluctuations in atmosphere-ocean processes that
alter the oxygen content of upwelled water,
the intensity of upwelling wind stress, and
productivity-driven increases in coastal respi-
ration (5, 6). Strongly coupled atmospheric and
oceanic circulation underpins ecosystem dynam-
ics in wind-driven upwelling shelves and eco-
system susceptibility to modulations of upwelling
wind stress from climate warming (7, 8). The
pre se nt- d ay glob al di st rib ut io n of shelf anoxia
reflects broad cross-system differences in verti-
cal proximity to OMZs, shelf productivity, and
circulation. The novel rise of shelf anoxia in the
CCLME highlights the potential for rapid re-
organization in the distribution of anoxia and the
sensitivity of productive upwelling shelves to dis-
continuous ecosystem change.
References and Notes
1. D. Pauly, V. Christensen, Nature 374, 255 (1995).
2. J. Helly, L. A. Levin, Deep Sea Res. 51, 1159 (2004).
3. J. J. Childress, B. A. Seibel, J. Exp. Biol. 201, 1223
(1998).
4. L. A. Levin, Oceanogr. Mar. Biol. Annu. Rev. 41,
1 (2003).
5. B. A. Grantham et al., Nature 429, 749 (2004).
6. A. Bakun, S. J. Weeks, Ecol. Lett. 7, 1015 (2004).
7. H. V. McGregor, M. Dima, H. W. Fischer, S. Mulitza,
Science 315, 637 (2007).
8. J. A. Barth et al., Proc. Natl. Acad. Sci. U.S.A. 104, 3719
(2007).
9. We thank M. Robart, W. Miller, S. Pierce, and J. Peterson for
their assistance. This paper is contribution 273 of the
Partnership for Interdisciplinary Studies of Coastal Oceans
(PISCO; funded by the David and Lucile Packard Foundation
and the Gordon and Betty Moore Foundation) and
contribution 582 of the U.S. Global Ocean Ecosystem
Dynamics program (funded by NSF and National Oceanic
and Atmospheric Administration). Additional support was
providedbyNSF,theA.W.MellonFoundation,theWayne
and Gladys Valley Foundation, and the Robert and Betty
Lundeen Marine Biology Fund.
Supporting Online Material
www.sciencemag.org/cgi/content/full/319/5865/920/DC1
Movie S1
9 August 2007; accepted 28 November 2007
10.1126/science.1149016
BREVIA
1
Department of Zoology, Oregon State University, Corvallis,
OR 97331, USA.
2
College of Oceanic and Atmospheric
Sciences, Oregon State University, Corvallis, OR 97331, USA.
3
Oregon Department of Fish and Wildlife, Newport, OR
97365, USA.
4
Northwest Fisheries Science Center, National
Oceanic and Atmospheric Administration, Newport, OR
97365, USA.
*To whom correspondence should be addressed. E-mail:
chanft@science.oregonstate.edu
Fig. 1. Dissolved oxygen profiles during the upwelling season (mid-April
to mid-October) in the upper 800 m of the continental shelf and slope
of Oregon (42.00°N to 46.00°N). (A) 1950 to 1999 from the World
Ocean Database and Oregon State University archives (n = 3101 hydro-
casts, blue). (B) (A) with additional data for 2000 to 2005 (n = 834
hydrocasts, green). (C) (A) and (B) plus data for 2006 (n =220hydrocasts,
red). The black vertical line denotes the 0.5 ml l
1
threshold. (Insets)
Overlapping locations of hydrographic (blue, green, and red) and
remotely operated vehicle (black) stations through time and the 100-m
and 1000-m isobaths.
15 FEBRUARY 2008 VOL 319 SCIENCE www.sciencemag.org
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