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Consequences of Nearshore Low Oxygen and Low pH for Coastal Resources of Southern California

Lisa Levin (Lead PI), Ed Parnell (Co PI), Todd Martz (Assoc. Investigator)

Scripps Institution of Oceanography

University of California, San Diego

llevin@ucsd.edu / 858-534-3579

PIC

Figure 1: Map of the proposed La Jolla study sites including locations of CTD casts (blue
triangles), the telemetered Send Lab mooring with temperature, current, pH, and O2 sensors
(red star), SeapHOx moorings (green circles), rocky shelf outcrop ROV survey sites (light blue
rectangles), squid egg-bed ROV survey and sampling areas (red rectangles), and submarine canyon
wall ROV survey areas (green rectangles). Depth contours are in meters (5m intervals). Kelp forests
are indicated as light green patches and land elevation is shown (cooler colors indicate greater
elevations).

 


Contents

List of Figures

1 Introduction and Background

The southern California coastline sits at the confluence of multiple oceanographic influences - with winds,
tides, currents, upwelling, and climatic events from the north and south shaping the marine environment.
The most economically and ecologically important oceanic waters per unit area along the California shelf
are managed by the state (?3 nm from shore). These waters support valuable habitats for sea urchins, squid
spawning and deep-water reef fishes. These habitats experience dynamic conditions that include
both seasonal, and event-scale exposure to low O2 and low pH (low pHOx), which occur in
tandem (Nam, Send, Martz, Tanner, Levin, unpubl.). Climate change (both warming and ocean
acidification) have increased the likelihood and severity of such events globally (Keeling et al. 2010,
Stramma et al. 2008, 2010) and within the southern California bight (SCB) (Bograd et al. 2008,
McClatchie et al. 2010) and are encroaching into state-managed waters (Feeley et al. 2008). These
are likely to pose an escalating threat to key nearshore California fisheries and to ecosystem
health.

Effective resource management requires an understanding of how different stressors affect individual
species and in turn how these effects cascade through the ecosystem. Characterizing the nature of pHOx
stress events and understanding the ecological effects that result from such events are critical for
ecosystem-based management. Nearshore low pHOx events could result in avoidance and redistribution of
mobile species, and mortality or sublethal growth and reproduction responses in sessile/demersal species or
life stages. This project addresses how pHOx stress, a largely overlooked, but significant threat to the health
of San Diego shelf waters, affects local marine ecosystems at water depths from 20 m to 300
m.

This project focuses on most of Sea Grants programmatic themes including climate change, fisheries,
marine ecosystem function and structure, ocean stressors, physical ocean processes related to climate
change, and resilience and adaptability to climate change. In addition, our project will provide the public
with access to information and data through the California Coastal Atlas, and will engage ocean governance
agencies regarding low pHOx events. We intend to provide regular updates so that stakeholders
can make informed decisions in the arenas of management and resource use. Outreach efforts,
including programs at the Ocean Discovery Institute and the Cabrillo National Monument,
will increase ocean education and environmental literacy on the topics of acidification and
hypoxia.

 

1.1 O2 Trends

Recent studies highlight both natural and anthropogenically induced global declines of dissolved O2 in the
ocean (Whitney et al. 2007, Diaz and Rosenberg, 2008, Keeling et al. 2010, Rabalais et al. 2010). Negative
trends in O2 concentration are now well documented in tropical and subtropical Oxygen Minimum
Zone (OMZ) waters between 200 and 700 m over the past 50 years (Stramma et al. 2010).
Warming-induced increases in stratification, reduced ventilation and lower O2 solubility in
warm waters combined with changes in ocean circulation all contribute to observed trends. The
hypoxic upper boundary (1.4 ml/l O2) has shoaled by at least 90 m in the SCB in the past several
decades (Bograd et al. 2008; McClatchie et al. 2010) and is encroaching into state-regulated shelf
waters. Upwelling-enhanced primary production combined with declining O2 concentrations in
source waters contribute to seasonal dead zones on the inner shelf in the California Current
Ecosystem (CCE), as has occurred off Washington state since the 1960s (Connolly et al. 2010) and
more recently off Oregon (Grantham et al. 2004; Chan et al. 2008, Gewin 2010). Respiration of
upwelling-enhanced primary production exacerbates the low O2 concentrations in upwelled waters;
alongshore and cross shore advection of these waters may then generate localized O2 stress inshore
(Connolly et al. 2010, Nam et al., submitted). Sometimes compounded by algal blooms, these
events can generate episodic mass mortalities of coastal species. Such events have been observed
off Oregon (Chan et al. 2008), in south La Jolla CA, during 2002 (sea urchin fishermen, pers.
comm.), at Punta Erendira (J.Ayon-Hernandez et al. pers. comm.) and off Punta Eugenia, Baja
Norte, Mexico (F. Micheli, pers. comm.). Lesser events with sublethal effects on the distribution
and abundance of key resources are expected to be far more widespread. Model projections
of the US West Coast predict increased upwelling over the next century (Auad et al., 2006
doi:10.1029/2005JC003219), with likely effects on the duration, frequency, severity, and extent of low
pHOx events.

 

1.2 Southern California Bight (SCB)

The CalCOFI program provides quarterly O2 data for the SCB (to 500 m) since 1949. These records show seasonally
lower O2 each spring. Continuous O2 records from off Pt. La Jolla in 2005 and from off Del Mar at 35 m during
2006-2010 http://mooring.ucsd.edu/index.html\?/projects/geoce/geoce_intro.html
demonstrate that O2 levels can reach much lower concentrations than documented in quarterly surveys
by CalCOFI at nearby station 93.3 (30-40 m off Del Mar) (Fig. 1A). On shorter (event) time
scales there are low O2 events associated with upwelling followed by relaxation that last for a
few days-2 weeks, On the inner SCB shelf (30-100 m) O2 levels regularly fall to 2.5- 3 ml/l
(concentrations that generate avoidance responses [Vauquer-Sunyer and Duarte 2008; Cenr 2010]), and
will occasionally reach hypoxic levels (1.4 ml/l - with potentially lethal effects) at times (Fig.
1B, 2A). On the outer shelf and upper slope, hypoxia occurs routinely and expands in spring
and summer (Bograd et al. 2008, McClatchie et al. 2010). Less than three weeks prior to the
submission of this proposal, hypoxic waters were observed at depths ? 140 m just off of Pt.
Loma, CA (Fig. 1B). There are also large diurnal fluctuations in O2 and pH forced by internal
tides that are manifested at depths from 7-90 m (Send, Nam, Martz, Tanner, Levin unpubl). In
this proposal we will focus on low pHOx conditions that occur on multi-day to seasonal time
scales.