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Abstract

Context and purpose of this Report Card Increasing atmospheric CO2 concentration is causing increased absorption of CO2 by the world’s oceans, in turn driving a decline in seawater pH and changes in ocean carbonate chemistry that are collectively referred to as ocean acidification. Evidence is accumulating to suggest ocean acidification may directly or indirectly affect many marine organisms and ecosystems, some of which may also hold significant social and economic value to the Australian community. This report card aims to provide a brief overview of the current state of scientific knowledge regarding the process of ocean acidification; current and future projected levels of ocean acidification; and, observed and projected impacts of current and future predicted levels of ocean acidification on marine organisms and ecosystems in the region. This Report Card also briefly discusses potential social and economic implications, policy challenges, and the key knowledge gaps needing to be addressed.
Marine Climate Change in Australia
Impacts and Adaptation Responses 2012 REPORT CARD
Ocean acidification
William R. Howard
1
, Merinda Nash
2
, Ken Anthony
3
, Katherine Schmutter
4
,
Helen Bostock
5
, Donald Bromhead
6
, Maria Byrne
7
, Kim Currie
5
, Guillermo Diaz-
Pulido
8
, Stephen Eggins
9
, Michael Ellwood
9
, Bradley Eyre
10
, Ralf Haese
11
, Gustaaf
Hallegraeff
12
, Katy Hill
13
, Catriona Hurd
14
, Cliff Law
5
, Andrew Lenton
15
, Richard
Matear
15
, Ben McNeil
16
, Malcolm McCulloch
17
, Marius N. Müller
12
, Philip
Munday
18
, Bradley Opdyke
9
, John M. Pandolfi
19
, Russell Richards
20
, Donna
Roberts
21
, Bayden D. Russell
22
, Abigail M. Smith
23
, Bronte Tilbrook
15
, Anya Waite
17
,
Jane Williamson
24
1
Australian National University and Office of the Chief Scientist, GPO Box 9839, Canberra
ACT 2601
2
Research School of Physics, Australian National University, ACT 0200
3
Australian Institute of Marine Science, PMB 3,Townsville, QLD 4810
4
52 Bimberi Crescent Palmerston ACT
5
National Institute of Water and Atmospheric Research, Private Bag 14-901, Kilbirnie,
Wellington 6002, New Zealand
6
Oceanic Fisheries Programme, Secretariat of the Pacific Community, Noumea, New
Caledonia
7
School of Medical and Biological Sciences, University of Sydney, Sydney NSW 2006
8
Griffith School of Environment & Australian Rivers Institute - Coast & Estuaries, Griffith
University, 170 Kessels Road, Brisbane, Nathan QLD 4111
9
Research School of Earth Sciences, Australian National University, ACT 0200
10
Southern Cross University, Lismore NSW 2480
11
Geoscience Australia, GPO Box 378 Canberra ACT 2601
12
Institute for Marine and Antarctic Studies, University of Tasmania, Hobart TAS 7001
13
Integrated Marine Observing System, University of Tasmania, Hobart TAS 7001
14
Department of Botany, University of Otago, PO Box 56, Dunedin, NZ
15
CSIRO Marine and Atmospheric Research, Hobart, TAS 7001
16
Climate Change Research Centre, University of New South Wales Sydney, NSW 2052
17
Oceans Institute, University of Western Australia, Crawley, WA 6009
18
ARC Centre of Excellence for Coral Reef Studies and School of Marine and Tropical
Biology, James Cook University Townsville QLD
19
ARC Centre of Excellence for Coral Reef Studies and School of Biological Sciences, The
University of Queensland, St. Lucia, Queensland 4072
20
Centre for Coastal Management, Griffith University, Gold Coast, Queensland 4222
21A
ntarctic Climate & Ecosystems Cooperative Research Centre, University of Tasmania,
Hobart, TAS 7001
22
Southern Seas Ecology Laboratories, School of Earth and Environmental Sciences,
University of Adelaide, SA 500
23
Department of Marine Science, University of Otago, Dunedin, NZ
24
School of Biological Sciences, Macquarie University, Sydney, NSW 2109
Howard, W.R., et al. (2012) Ocean acidification. In A Marine Climate Change Impacts and Adaptation
Report Card for Australia 2012 (Eds. E.S. Poloczanska, A.J. Hobday and A.J. Richardson).
<http://www.oceanclimatechange.org.au>. ISBN: 978-0-643-10928-5
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What is happening
Most conclusions about biological responses to ocean acidification in Australian
waters come from laboratory manipulations rather than observations. However,
reduced calcification rates observed in Southern Ocean zooplankton suggest ocean
acidification is already impacting biological systems.
What is expected
Great Barrier Reef corals and coralline algae will continue to experience reduced
calcification rates. Benthic calcifiers, such as molluscs and deep-water corals in
Antarctic and southern Australian waters, will show reduced calcification and/or
increased dissolution.
What we are doing about it
Research is underway to improve the methods and equipment used for high-precision
carbonate chemistry measurements. Monitoring of carbon chemistry in the open ocean
and some shallow coastal systems, including the Great Barrier Reef, has already
commenced. Research is underway to investigate effects of ocean acidification on
whole coral ecosystems in the Great Barrier Reef.
Introduction
Context and purpose
Increasing atmospheric CO
2
concentration is causing increased absorption of CO
2
by
the world’s oceans, in turn driving a decline in seawater pH and changes in ocean
carbonate chemistry that are collectively referred to as ocean acidification. Evidence
is accumulating to suggest ocean acidification may directly or indirectly affect many
marine organisms and ecosystems, some of which may also hold significant social and
economic value to the Australian community.
This report card aims to provide a brief overview of the current state of scientific
knowledge regarding the process of ocean acidification; current and future projected
levels of ocean acidification; and, observed and projected impacts of current and
future predicted levels of ocean acidification on marine organisms and ecosystems in
the region. This Report Card also briefly discusses potential social and economic
implications, policy challenges, and the key knowledge gaps needing to be addressed.
What is ocean acidification?
Anthropogenic CO
2
emissions arising from fossil-fuel combustion, land-use practices,
and concrete production during and since the industrial revolution first enter the
atmosphere, but a large proportion of are absorbed into the ocean by physical and
biological processes that are normal parts of the natural carbon cycle. These emissions
have resulted in a ~40% increase in atmospheric CO
2
concentrations over pre-
industrial levels [Tans and Keeling, 2011]. Over the same period, the ocean has
absorbed approximately 30%-50% of these emissions [Sabine et al., 2004; Sabine and
Tanhua, 2010]. Although the absorption of anthropogenic CO
2
by the ocean has
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115
provided a degree of buffering against global warming (which results largely from
increased CO2 in the atmosphere), the increase in dissolved CO
2
is accompanied by
chemical reactions that increase oceanic hydrogen ion concentrations (thus reducing
seawater pH) and bicarbonate ion concentrations while reducing carbonate ion
concentrations.
The result is more CO
2
dissolved in the world’s oceans. Seawater is a weakly-alkaline
solution (with a pH of ~ 8.1), but this extra CO
2
changes the carbonate chemistry of
the surface ocean, driving ocean pH and carbonate ion concentrations lower. While
the relative acidity or alkalinity of seawater (typically measured as pH) shows
significant spatial and temporal variation throughout the world’s oceans, seawater is
on average a weakly-alkaline solution (with a mean pH of ~ 8.1). Ocean acidification
is estimated to have lowered the mean pH of the ocean from its pre-industrial state by
about 0.1 pH units [Friedrich et al., 2012]. The declining trend in pH has been verified
in recent decades by measurements e.g. [Byrne et al., 2010; Doney et al., 2009].
The process of ocean acidification is already underway and discernible in the ocean
[Feely et al., 2004]. By the end of this century pH levels are likely to drop 0.2 0.3
units below pre-industrial pH. The level of atmospheric CO
2
is now higher than at any
time in at least the past 650,000 years, and probably has not been as high as present
levels for approximately 4-5 million years [Hönisch et al., 2012; Pagani et al., 2010].
The current rate of increase of CO
2
in the atmosphere is one hundred times greater
than the most rapid increases during major climate changes over the last 650,000
years, and the concomitant rate of carbonate chemistry change in the ocean is
similarly rapid [Friedrich et al., 2012; Lüthi et al., 2008; Midorikawa et al., 2012].
Nearly half the fossil-fuel CO
2
emitted to date has now dissolved into the ocean [Le
Quéré et al., 2009; Sabine et al., 2004].
CO
2
- driven acidification, in addition to lowering seawater pH, shifts the proportion
of dissolved carbon dioxide away from carbonate ion and to bicarbonate ion, and thus
towards lower saturation states for carbonate mineral. Calcium carbonate precipitation
at a decreased saturation state requires higher energetic demands from shell-making
organisms.
Why is ocean acidification a concern?
The marine environment is home to a vast and diverse range of organisms and
ecosystems, all of which will directly or indirectly experience, to a greater or lesser
degree, changes in ocean chemistry associated with ocean acidification. For many
marine organisms, marine carbonate chemistry and pH are known to play important
roles in key physiological processes (e.g. calcification in corals and shellfish,
acid/base balance, fertilisation etc) that ultimately influence their behaviour, growth,
development and/or survival. Some research suggests that ocean acidification has
already begun to have detectable impacts on some biological processes.
Most conclusions about biological responses to ocean acidification in Australian
waters come from laboratory manipulations rather than in situ observations. However
there is observational data documenting already-underway changes in calcification in
Southern Ocean zooplankton [Moy et al., 2009; Roberts et al., 2011] and in Great
Barrier Reef corals [Cooper et al., 2008; De'ath et al., 2009]. Though unambiguous
attribution of these observed trends to acidification is still uncertain, they suggest
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116
acidification may have already begun to have detectable impacts on biological
processes.
Similarly, a range of physiological processes are sensitive to pH itself. Changing pH
also influences other important aspects of seawater chemistry, such as the availability
of nitrogen and iron (both necessary for marine plant production) [Shi et al., 2010].
Given that a significant proportion of the global (including Australian) human
population is directly or indirectly reliant on the ecosystem services provided by the
ocean (e.g. for food security, employment, tourism), many governments are becoming
increasingly concerned with understanding the likely ecological, economic and social
implications of ocean acidification
There are a number of key questions that must be addressed to inform decisions
regarding the management of, and response to, this issue in the short and long term:
1. What is the current degree of acidification and what level is it predicted to
reach in the short, medium and long term, a range of anticipated global carbon
emission scenarios?
2. Which organisms and ecosystems have been or will be impacted by ocean
acidification? How soon will impacts manifest themselves and are any species
or ecosystems likely to benefit?
3. How will ocean acidification interact with other ecosystem stressors (e.g.
pollution, overfishing, ocean warming, hypoxia, etc.)
4. What capacity might organisms have to adapt to ocean acidification (i.e. via
natural selection of resistant individuals over relevant timeframes)?
5. What are the social and economic implications of ocean acidification impacts?
6. What policy response is required?
The level of research into and understanding of ocean acidification and it potential
biological impacts is growing rapidly. The following sections summarise the current
state of scientific knowledge pertaining to observed and predicted changes in ocean
chemistry and biological processes resulting from ocean acidification, with special
emphasis on studies relevant to the Australasian region. This review is intended to
highlight knowledge gaps and facilitate discussion of policy implications and
challenges (including social and economic implications).
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Figure 1. Annual mean aragonite saturation state in the surface water for: a) 1800; b) 2005; c) 2035;
d) 2095. For the future years the IS92a atmospheric CO2 concentrations is used along with the CSIRO
Mk3.5 climate projection to determine project the aragonite saturation state.
Major scientific knowledge gaps in the physical response lie in several aspects of the
physical, biological, and ecological implications of ocean acidification. One area of
gap is in projecting the spatial and temporal variability in the progression of
acidification. In particular there is a critical need for regional and local-scale data on
carbonate chemistry variability. Another major class of knowledge gaps concerns the
vulnerability of different organisms and ecosystems. The major scientific knowledge
gaps in biological and ecological responses lie in understanding inter-specific and
intra-specific differences in response to acidification (“winners” versus losers”) or
the ability to internally regulate pH [McCulloch et al., 2012a; McCulloch et al.,
2012b], the potential evolutionary adaptation of organisms to acidification [Parker et
al., 2012; Sunday et al., 2011], and in the implications for the structure of ecosystems
[Hughen et al., 2004; Hughes et al., 2010; Pandolfi et al., 2011]. Similarly, though
much research has focused on marine calcifiers, the impact of shifts in carbonate
chemistry on microbial communities and processes is still little understood e.g.
[Bowler et al., 2009; Tortell et al., 1997; Witt et al., 2011]
Observed Impacts
Chemical changes to the oceans
High Confidence. The pH of surface oceans has dropped by 0.1 units since the
industrial revolution [Feely et al., 2004; Feely et al., 2009]. The carbonate mineral
saturation state for calcite and aragonite show decade-scale downward trends
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[DeVries and Primeau, 2009; Doney et al., 2009; Feely et al., 2012; Matear and
McNeil, 2003; Matear and Lenton, 2008; McNeil and Matear, 2008]. Historical pH
drops in seawater pH have been inferred using boron isotope proxies in coral archives
[Pelejero et al., 2005; Pelejero et al., 2010; Wei et al., 2009].
Biological changes to the oceans
Medium confidence. Calcification rates in Southern Ocean calcareous zooplankton
(foraminifera) have dropped 30-35 % since the pre-industrial times [Moy et al., 2009].
Great Barrier Reef corals have reduced calcification rates [De'ath et al., 2009].
Though attribution to ocean acidification alone is unclear (increased sediment runoff
and thermal stress are the other likely causes), recent declines in GBR-wide coral
calcification rates are unprecedented in at least the past 400 years. It is nevertheless
clear that changes in the marine environment together with more frequent coral
bleaching are reducing coral growth rates. Evidence is emerging of similar declines
from other coral reef regions [Tanzil et al., 2009] although some coral reefs distal
from terrestrial impacts show greater rates of calcification [Cooper et al., 2012]
consistent with the positive enhancement of warming on calcification e.g. [McCulloch
et al., 2012a]. Lightly calcified coccolithophores have shifted ranges poleward in
recent years. Though attribution to ocean acidification is unclear, the pattern is
consistent with the expected impact [Cubillos et al., 2007].
Low confidence. Calcification in pteropods in the Southern Ocean [Roberts et al.,
2011] and tropical waters [Roger et al., 2012] has decreased on decadal time scales.
Potential impacts by the 2030s and 2100s
Chemical changes to the oceans
High Confidence. The pH of surface oceans will drop by 0.2 0.3 units by ~ 2100).
The carbonate mineral saturation states for calcite and aragonite will continue their
decade-scale downward trends [Matear and Lenton, 2008]. In areas of high seasonal
variability in carbonate chemistry, such as the high-latitude Southern Ocean, aragonite
saturation thresholds may be crossed in winter by ~ 2040 [McNeil and Matear, 2008;
McNeil et al., 2011].
Medium Confidence. The entire Southern Ocean surface (south of the current Polar
Front Zone) will be undersaturated for aragonite by ~ 2100 [Orr et al., 2005]. Tropical
aragonite saturation states will decrease to a level that will prevent optimal coral
growth [Kleypas et al., 1999; Orr et al., 2005; Veron et al., 2009]. Aragonite
saturation horizons will shoal, especially in the Antarctic and Australian southern
margins, threatening a wide range of benthic calcifiers (see below).
Low confidence. pH changes in coastal systems have mirrored changes in the open
ocean. There are few long-term records of the CO
2
-carbonate system in shallow coastal
systems. In shallow coastal systems there are a number of processes that produce and
consume alkalinity that can potentially buffer or enhance the effect of ocean
acidification. These include the dissolution of carbonate minerals, sediment
denitrification, and reduced sulphur burial. Coastal systems also receive river and
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groundwater inputs of alkalinity. Alkalinity sinks in coastal systems include
precipitation of carbonate minerals, oxidation of reduced sulphides, and coupled
nitrogen fixation and aerobic respiration.
Biological changes to the oceans
Medium confidence. Calcification rates in Southern Ocean calcareous zooplankton
(foraminifera) are likely to continue to decline [Moy et al., 2009] and impacts on
pteropods are likely to emerge [Fabry et al., 2008]. Many taxa of calcifiers especially
that are unable to internally manipulate their pH (eg coralline algae and some
foraminifera) will experience reduced calcification rates [Anthony et al., 2008;
Anthony et al., 2011b; De'ath et al., 2009]. Coral growth is likely to be affected by
multiple impacts of ocean warming, bleaching and acidification [Erez et al., 2011;
Hobday and Lough, 2011; Hoegh-Guldberg, 2005; Hoegh-Guldberg et al., 2007;
Lough and Cooper, 2011; Lough and Hobday, 2011; Silverman et al., 2007;
Silverman et al., 2009]. However, corals show a diversity of responses to
acidification, and overall responses or reef ecosystems are likely to be complex. In
particular some marine organisms may have evolutionary time scales short enough to
adapt to acidification on decadal to centennial time scales e.g. [Sunday et al., 2011].
Many mid- and high-latitude benthic calcifiers such as deep-water and cold-water
corals [Guinotte et al., 2006; Maier et al., 2009], coralline algae e.g. [Martin et al.,
2008; Martin and Gattuso, 2009; Russell et al., 2009; Russell et al., 2011b],
bryozoans [Smith, 2009; Smith and Lawton, 2010] and other benthic calcifiers
[McClintock et al., 2009] are likely to show the effects of increased dissolution on
their exposed carbonate skeletons as aragonite saturation horizons shoal [McCulloch
et al., 2012b]. The effects on calcification will variable, dependent on species specific
internal processes to modulate pH regulation and likely enhancement of calcification
from warming of cold-water environments [McCulloch et al., 2012a].
Economically important taxa such as shellfish may show reduced growth and/or
calcification [Barton et al., 2012].
Ecosystems will show signs of restructuring as changes to ecosystem services like
calcification alter benthic substrata, and to the extent non-calcifiers are advantaged.
Low confidence. Possible reduction in fertilisation in some marine invertebrates
[Havenhand et al., 2008; Havenhand and Schlegel, 2009; Parker et al., 2009; Parker et
al., 2012], but not others [Byrne, 2012]. Some reef fishes may experience impaired
olfactory-based navigation under lower pH, hindering their ability to find suitable
habitats [Munday et al., 2009b].
Overall responses of fish taxa may be highly varied and thus difficult to simply
predict, but have significant implications for economically-important ecosystems and
fisheries. Experimental evidence available to date suggests ocean acidification is not
likely to have significant direct effects on the growth, development and survival of
most adult fish taxa. However elevated CO
2
may affect sensory systems and
behaviour. Recruitment adult populations would decline if increased mortality of
larvae and juveniles results from acidification. Reduced aerobic capacity in some fish
could exacerbate climate change impacts.
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Low confidence. pH changes in coastal systems will mirror predicted changes in the
open ocean. Shallow coastal systems have a number of alkalinity sources and sinks
that may potentially buffer against, or enhance, ocean acidification (see above). In
addition, other stressors such as eutrophication and hypoxia will also interact with
ocean acidification to modify its effect in shallow coastal systems.
Adaptation Responses
The likely peak of atmospheric carbon dioxide levels well above present
concentrations, even with emissions-reduction measures, means ocean acidification
impacts will be inevitable and marine ecosystem management strategies (e.g. marine
protected areas) will have to factor in some acidification impacts.
Knowledge Gaps
There exist a number of important scientific knowledge gaps pertaining to the
response of physical and biological systems to ocean acidification. These gaps also
hinder our ability to understand the social and economic implications and hinder the
capacity of decision makers to develop appropriate policy responses. Key knowledge
gaps include:
1. Physical/chemical responses: There is a lack of understanding of future fine
scale spatial and temporal variability in the progress of acidification. There is a
critical need for regional and local-scale data on carbonate chemistry
variability, such as large-amplitude tidal-cycle changes in carbonate
chemistry, pH, and net calcification [Hofmann et al., 2011; Santos et al., 2011;
Shaw et al., 2012].
2. Biological/ecological responses: There is a lack of understanding of:
a. Inter-specific and intra-specific differences in response to acidification
(“winners” versus “losers”). Much research has focused on marine
calcifiers, however the impact of shifts in carbonate chemistry on other
key ecosystem components, such as microbial communities and
processes, is still little understood e.g. [Bowler et al., 2009; Tortell et
al., 1997]
b. The potential for organisms to adapt (via natural selection for more
resistant individuals) to changes in ocean chemistry over relatively
short time frames [Parker et al., 2012; Sunday et al., 2011].
c. The implications of ocean acidification for the structure of ecosystems;
d. The effect of multiple stressors [Byrne, 2011; Ericson et al., 2012;
Wernberg et al., 2011], especially the combined impacts of likely
environmental changes such as expansion of hypoxic zones and
increased temperature [Brewer and Peltzer, 2009; Gruber, 2011] but
also of shifting patterns of salinity e.g. [Durack et al., 2012].
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Crucially there is a need to scale the understanding of ocean acidification impacts
from “narrow” taxon-specific experiments to ecosystem-level experiments. Free
Ocean Carbon Enrichment experiments or “FOCE” may provide important insights
into ecosystem-level impacts of ocean acidification [Kline et al., 2012]. Ultimately
there is a need to detect and reliably attribute ocean acidification impacts in nature. To
date there are few in-situ studies that attempt to unravel the effects of ocean
acidification (two in Australian waters [De'ath et al., 2009; Moy et al., 2009]) from
other compounding factors and there is a need for ecosystem-level impact research
[Russell et al., 2011a]. So far insights into ecosystem-level impact come from studies
of naturally-acidified systems where CO
2
outgasses through volcanic vents [Fabricius
et al., 2011; Hall-Spencer et al., 2008; Tunnicliffe et al., 2009].
Policy implications and adaptation responses
Key policy challenges
The previous sections have identified that:
a. at its current level, ocean acidification may already be affecting some marine
organisms (e.g. foraminifera) and ecosystems (GBR) and
b. ocean acidification will continue to increase through and beyond the end of
this century, thus having the potential to have much wider and long-lasting
impacts on marine ecosystems.
c. there remains, however, significant scientific uncertainty regarding the
medium- and long-term degree and extent of possible impacts.
Governments are guided by legislation and policy frameworks that define their
environmental, economic and social objectives (amongst others). On current evidence
and scientific understanding of its observed and potential impacts, ocean acidification
may pose a threat to the long term achievement of environmental objectives (an
example, in the Australian context, is the Environmental Protection and Biodiversity
Conservation Act). Through potential impacts on marine ecosystem services, ocean
acidification may also pose risks to economic and social objectives (for example,
those associated with maximising economic returns from fishing).
Policy makers are faced with the challenge of obtaining better information pertaining
to the risks posed by ocean acidification, and of determining whether and how to
develop mitigation and/or adaptation strategies.
Mitigation and adaption considerations
Ocean acidification presents some unique policy as well as scientific challenges.
Ocean acidification differs from global warming in that its impact derives from the
chemistry of carbon dioxide (CO
2
) in seawater, rather than from its physical action as
a greenhouse gas in the atmosphere. This means that increasing atmospheric CO
2
will
inevitably increase ocean acidity, largely independent of the rate of global warming
and its impacts, and independent of climate-model projections. Ocean acidification
will need to be considered in the context of setting stabilisation targets for
atmospheric CO
2
and the timelines on which the targets need to be reached. There are
natural time lags involved in the marine carbon cycle, both in the uptake of CO
2
by
the ocean as well as in the centuries needed to reverse the acidification already under
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122
way [Archer et al., 2009; Goodwin and Ridgwell, 2010]. These lags place a penalty
on delaying limits on carbon emissions and a premium on early action. A further
policy challenge arises because the only mitigation options available are reductions in
carbon dioxide emissions or some form of carbon dioxide sequestration, or both.
Ocean acidification would not be easily ameliorated by most proposed
“geoengineering” strategies [Matthews et al., 2009], though some such strategies
would specifically act by adding alkalinity and thus buffer the ocean [Kheshgi, 1995;
Schuiling and Krijgsman, 2006] The thresholds for atmospheric CO
2
levels at which
acidification impacts begin may differ from those which trigger warming impacts, so
mitigating acidification may require different emissions-limitation targets than
mitigating global warming. Similarly, because acidification arises only from CO
2
emissions, limiting other greenhouse gases (such as nitrous oxide) will not mitigate
ocean acidification.
Detailed Assessments of Key Oceanographic and Ecosystem
Components, and Processes
Observations and Modelling
The long-term secular changes in carbonate chemistry in the open ocean are now
relatively well established [Doney et al., 2009; Doney, 2010]. A key challenge is
characterising the progress of acidification in nearshore and shallow marine
environments, such as coral lagoons and estuaries. These environments have high
natural variability in carbonate chemistry and pH e.g. [Hofmann et al., 2011]. In some
shallow-water environments, diurnal (through tidal and sun cycles) variability can
exceed the mean decadal-scale change anticipated over the current century [Nguyen et
al., 2012; Santos et al., 2011; Shaw et al., 2012], adding complexity to predictions of
future carbon chemistry changes under ocean acidification. Bio-geochemical
processes and their interaction with ocean acidification are only just starting to be
identified and understood. Importantly, there are few baseline carbon chemistry
measurements for these shallow environments. Physical changes, including warming,
circulation patterns, intensity and frequency of storms, patterns of precipitation and
sea level change, may all interact with ocean acidification.
Research is underway in Australia, New Zealand and overseas to improve the
methods and equipment used for high-precision carbonate chemistry measurements.
One initiative is the development of pH sensors that do not need constant calibration,
which would enable improved consistency of measurements. These could also be
used for remote deployment to deliver reliable, high-precision in-situ carbonate
chemistry measurements. Another project is currently assessing the possibility of
using commonly-measured oceanographic variables (e.g. depth, temperature, salinity
and oxygen) to estimate alkalinity and dissolved inorganic carbon. This would enable
translation of large-scale and long-term data sets (e.g. satellite estimates of sea-surface
temperature) currently collected by the Australian Bureau of Meteorology into pH for
large areas of ocean. Alkalinity and DIC will still need to be measured to assess
changes in anthropogenic uptake of CO
2
.
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123
Work is being undertaken to identify suitable proxies for carbonate chemistry from
sediments, massive corals and coral reef limestone (palaeoceanographic records) that
can be used to build a picture of marine carbonate chemistry prior to the Industrial
Age. Understanding past changes through geological archives (e.g. deep-sea sediment
cores and coral) will provide longer pH and ecological variability records than the
historical observational record. In particular the past record of carbonate variability is
the only source of documented changes in ocean carbon chemistry of comparable
magnitude to those anticipated over the coming decades and centuries.
Monitoring of carbon chemistry in the open marine environment and some shallow
coastal systems has already commenced in the Australasian region. The Integrated
Marine Observing System (IMOS) delivers and integrates a range of data that
contribute to research into ocean acidification
1
. Ships and moorings are used to
measure CO
2
concentrations in surface ocean waters, and estimate fluxes of CO
2
between the atmosphere and ocean. Wavegliders are also showing potential for taking
surface observations. There are currently three coastal carbonate chemistry moorings
around the Australian continental shelf, and a time series station in the Southern
Ocean. Bi-monthly measurements of marine carbonate chemistry have been made in
the surface waters associated with the Subtropical Front of east New Zealand since
1998, and coastal pH is being measured in Wellington Harbour.
The viability of using oceanographic variables other than carbonate parameters
themselves as proxies for carbon chemistry patterns and change shows promise for
open ocean areas [McNeil et al., 2007; McNeil, 2010]. Spectrophotometric measuring
systems, which do not require as much ongoing probe calibration as other systems,
have potential for remote deployment in autonomous systems such as moorings and
floats [Byrne and Yao, 2008; Martz et al., 2009; Seidel et al., 2008]. A particular
observational challenge is represented by alkalinity, an often-utilised parameter for
characterising the carbonate system and especially for inferring carbonate
precipitation from seawater [Fransson et al., 2011; Ilyina et al., 2009].The use of
commonly-measured oceanographic variables (e.g. temperature, salinity and oxygen)
to estimate alkalinity and dissolved inorganic carbon (DIC) in the open ocean may
help complement in-situ direct measurements of the carbonate system, especially DIC
and alkalinity e.g. [González-Dávila et al., 2010]. These approaches are so far limited
in their applicability to shallow-water environments, where biological processes and
sediment-water interactions strongly influence seawater carbonate chemistry [Kleypas
et al., 2011; Santos et al., 2011]
Proxies of carbonate chemistry from coral and sediment archives are proving useful to
reconstruct past carbonate chemistry parameters in order to build a pre-industrial
baseline of carbonate chemistry.
Modelling ocean acidification and its impacts faces the challenge of a wide range of
spatial scales of variability, spanning from individual organisms to entire ecosystems
and spatial scales ranging from local to global extent. Models are now focused on
chemical and physical drivers of the carbonate system. Whereas physical transport and
1
IMOS National Science and Implementation Plan
http://imos.org.au/plans.html#c1210; IMOS
Observations for Acidification Research, http://imos.org.au/imosobsresearch.html
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124
atmosphere-ocean fluxes of carbon are mainly well represented in models, pelagic and
benthic biological processes and their role in modifying the ocean carbon chemistry are
often highly simplified [Hood et al., 2006; Jin et al., 2006; Vichi et al., 2007]. At
present, projections of the impacts of ocean acidification cannot be captured by a single
model. Rather, hierarchies of models in which spatial, temporal and biological
responses at the full range of spatial, ecological, and temporal scales can be separated
and investigated are needed. Modelling socio-economic impacts is still in its infancy,
however work is underway to extend the known organism-scale impacts of ocean
acidification on molluscs to global economics and food security. This work has shown
that the potential impacts of declining mollusc growth could be detrimental for some
developing nations [Cooley and Doney, 2009; Cooley et al., 2009]. Such socio-
ecological systems modelling is critical to help understand the impacts of ocean
acidification on human societies now and in the future.
Calcification processes
One implication of ocean acidification for Australian marine ecosystems is the impact
acidification has on the process of calcification the making of shells, plates and
skeletons out of calcium carbonate (CaCO
3
) for the variety of calcifiers important in
Australian marine, and global ocean, communities such as corals, shelled plankton and
others. Though calcification is only one of many biological processes likely to be
affected by acidification, it is an important process in the formation of reef habitats and
benthic substrates in a wide range of ecosystems, as well as a key process in the global
carbon cycle. In a purely non-biological system, carbonate mineral formation would
depend mainly on carbonate ion concentration, e.g. in household “hard water” calcium
deposits. Biological calcification, however, is more complex than simple mineral
precipitation, given the many biocalcifiers utilizing bicarbonate and metabolic CO
2
e.g
[Roleda et al., 2012] . Acidification affects the rate and energetic cost of calcification,
as well as the dissolution of existing skeleton. Many experiments on calcifiers, whether
they utilize bicarbonate or carbonate ion, show an apparent dependence of calcification
on carbonate ion concentration. However many corals have the ability to up-regulate
their internal pH during calcification e.g. [McCulloch et al., 2012a; Venn et al., 2011]
and therefore exhibit a lower sensitivity than that predicted from decreases in seawater
saturation state alone. The calcification response to increased bicarbonate ion is also
complex and variable, and some organisms may be able to take advantage of increased
bicarbonate availability to maintain or increase calcification e.g.[Jury et al., 2010;
Marubini and Thake, 1999].
The majority of research to date suggests that ocean acidification will reduce overall
calcification in calcifying animals in both larval and adult life history stages [Byrne,
2010; Gattuso et al., 2011]. However many experiments suggest mixed impacts of
ocean acidification on calcification processes for some organisms [Kroeker et al., 2010;
McCulloch et al., 2012a; Miller et al., 2009; Pandolfi et al., 2003; Ries et al., 2009].
The calcifying algae coccolithophores in particular show mixed species and strain-
specific responses to acidification [Iglesias-Rodriguez et al., 2008; Riebesell et al.,
2000; Riebesell, 2004]. Some of the variable and species-species responses however,
are likely to be influenced by variable experimental approaches [Byrne, 2012; Schlegel
et al., in press].
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Calcification occurs with a variety of physiological mechanisms and in different tissues
so generalisations are difficult[Turley et al., 2010]. Some taxa have unprotected
external skeletons (e.g. abalone) directly exposed to changing ocean chemistry whereas
others (eg. sea urchins) have internal skeletons protected by overlying tissue, a
difference that influences vulnerability.[Byrne et al., 2011]. Many scleractinian corals
appear to have the ability to up-regulate internal pH which effectively acts to raise their
carbonate saturation state at the site of calcification [McCulloch et al., 2012a; Venn et
al., 2011]. Species sensitivities and the potential mitigating effects on ocean
acidification and energetic costs remain to be investigated. The potential of evolution
and adaptation to a changing ocean (mainly in regard to temperature and CO
2
/pH) is
also not well understood as few experiments have been carried out through multiple
generations of organisms. Micro-organisms with short generation times (e.g. bacteria
and phytoplankton such as coccolithophores) may be able to genetically adapt to a new
environment [Collins and Bell, 2004; Lohbeck et al., 2012; Müller et al., 2010],
however evolutionary timescales may vary among larger taxa as well.
Carbonate mineralogy affects the vulnerability of calcification and carbonate net
accumulation to ocean acidification. Three calcium carbonate (CaCO
3
) polymorphs
occur commonly in nature: aragonite, low-magnesium (Mg) calcite (LMC) and high-
Mg calcite (HMC; >4mol% MgCO
3
) [Andersson et al., 2008]. Aragonite is more
soluble than LMC [Mucci, 1983], and HMC with is in turn more soluble than calcite
and aragonite [Morse et al., 2007]. Whereas aragonitic organisms are considered the
most vulnerable to ocean acidification, high-Mg calcite organisms may be equally, if
not more, susceptible. Physiologically, however, aragonite calcifiers may have pH-
regulation mechanisms that may confer some resilience to acidification [McCulloch et
al., 2012a]
A wide range of marine organisms produce skeletons/shells containing significant
amounts of Mg, including echinoderms (2-12mol% Mg), benthic foraminifera (2-
16%), coralline algae (7-20%), crustaceans (5-12%) [Chave, 1954]. Some Mg-calcite
coralline algae can also form (Ca
0.5
Mg
0.5
CO
3
) and magnesite (MgCO
3
) [Nash et al.,
2011] and it is not yet understood how rising CO
2
will affect these organisms.
Ocean acidification (OA) leads to reduced concentration of carbonate ions and in turn
lowered carbonate mineral saturation states required to maintain shells. Thus OA
poses a two-fold problem for species with calcium carbonate structures: (1) Exposed
calcium carbonate structures such as shells may start to dissolve if saturation states
fall low enough [Rodolfo-Metalpa et al., 2010] and (2) individuals would have to
work harder to maintain their shells due to reduced carbonate concentrations in sea
water [Cummings et al., 2011], reducing energy available for other processes such as
growth and reproduction.
In addition to impacts on calcification, increased levels of pCO
2
, may affect essential
physiological processes, such as metabolism and acid-base balance [Langenbuch and
Pörtner, 2003; Munday et al., 2009b; Pörtner, 2008; Pörtner and Peck, 2010]. If
experimentally-detected physiological responses to acidification occur in nature, they
may result in a reduction of fitness for many species with repercussions on ecosystem
function. CO
2
–related physiochemistry and ocean warming may work synergistically
[Pörtner, 2008].
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Corals
Changes in coral reef ecosystems driven by acidification and other impacts, could affect
societies that depend upon these ecosystems [Raven et al., 2005]. Calcification in corals
throughout the Great Barrier Reef has declined by 14.2% since 1990 [De'ath et al.,
2009] and projections of the saturation levels of the form of CaCO
3
precipitated by
corals aragonite suggest that calcification rates in many warm-water corals may
decrease over the next century [Gattuso et al., 1998; Langdon and Atkinson, 2005].
However the joint effects of changing temperature [Cooper et al., 2012], and of internal
pH-maintenance mechanisms [McCulloch et al., 2012a] complicate the projection of
future acidification to coral growth. Similarly both coral and algal growth can alter
seawater chemistry so as to mask or exacerbate the impacts of acidification [Anthony et
al., 2011a]. Experiments suggest that ocean acidification will affect coral reefs by mid
century, with risks arising both from reduced coral calcification rates in many taxa
[Kleypas et al., 1999] and reductions in net community calcification [Silverman et al.,
2009]. However, there is a great deal of variability in coral response to acidification and
other impacts e.g. [Pandolfi et al., 2011], as well as physiological scope for resilience in
calcification for many taxa [McCulloch et al., 2012a]. The impacts of warming-induced
bleaching are also of concern [Anthony et al., 2011b]. These major stressors, warming
and acidification, do not operate in isolation, with synergistic impacts observed in
experiments [Anthony et al., 2008].
Cold-water corals may also experience difficult water chemistry conditions in the
coming decades resulting in projected losses as high as 70% by 2100 [Orr et al., 2005]
and some as early as 2020 [Guinotte et al., 2006]. Manipulative experiments show that
reductions in pH significantly reduce cold-water coral calcification rates (30% and 56%
respectively when pH drops by 0.15 and 0.3 units [Maier et al., 2009], however there is
potential for acclimation to long-term shifts in carbonate chemistry [Form and
Riebesell, 2012]
Holopelagic Calcifiers: coccolithophores, foraminifera, shelled pteropods and other
plankton
Impacts on calcification in the planktonic CaCO
3
producers coccolithophores,
foraminifera and shelled pteropods are less well reported but of equal concern given
these calcifiers account for nearly all of the export flux of CaCO
3
from the upper ocean
to the deep sea [Fabry, 2008; Schiebel, 2002].
Coccolithophores planktonic unicellular shelled algae are considered to be the
most productive calcifying organisms on Earth [Iglesias-Rodriguez et al., 2008;
Riebesell et al., 2000] and the cosmopolitan species Emiliania huxleyi is one of the
best-studied species in regard to ocean acidification. The process of photosynthetic
carbon assimilation may be enhanced in coccolithophores under future ocean
acidification [Iglesias-Rodriguez et al., 2008; Riebesell, 2004]. CaCO
3
production on
the other hand, shows a diversity responses to increased CO
2
in experiments [Lohbeck
et al., 2012]. However, the majority of studies suggest a reduction in calcification in
response to ocean acidification [Riebesell and Tortell, 2011], but this appears species-
and even strain-specific. However, as with other marine organisms, the combined
effects of multiple variables need to be considered [Lefebvre et al., 2011].
Coccolithophores are major contributors to marine primary production, and dominate
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127
the vertical supply of CaCO
3
to the deep ocean, and are important components of open
ocean and coastal marine ecosystems as well as the global ocean carbon cycle [Balch et
al., 2011; Honjo et al., 2008; Iglesias-Rodriguez et al., 2002].
Foraminifera unicellular shelled protists are important calcifiers in planktonic and
benthic ecosystems [Gooday and Jorissen, 2011; Schiebel, 2002]. Laboratory
experiments suggest ocean acidification would reduce calcification in foraminifera,
leading to lighter shells [Bijma et al., 2002; Lombard et al., 2010] and recently natural
populations of planktonic foraminifera in the Southern Ocean have been found to have
30-35% lighter shells than their counterparts from pre-industrial times [Moy et al.,
2009]. Similarly, recent-deposited planktonic foraminifera in Arabian Sea cores also
show reduced calcification, also likely due to acidification [de Moel et al., 2009].
Pteropods – planktonic shelled gastropods – can reach densities of more than 10,000
individuals per cubic metre in high-latitude areas [Bathmann et al., 1991; Pane et al.,
2004] and are important components of polar food webs, contributing to the diet of
carnivorous zooplankton, North Pacific salmon, mackerel, herring, cod and baleen
whales [LeBrasseur, 1966; Takeuchi, 1972]. Pteropods also contribute to carbonate
fluxes in a range of marine environments [Accornero et al., 2003; Almogi-Labin et al.,
1988; Bednarsek et al., 2012; Fabry and Deuser, 1992; Hong and Chen, 2002; Hunt et
al., 2008; Jasper and Deuser, 1993; Meinecke and Wefer, 1990; Mohan et al., 2006;
Pilskaln et al., 2004; Singh and Conan, 2008; Tsurumi et al., 2005]. Observations of
pteropod populations in subantarctic waters since 1997 [Howard et al., 2011] suggest
that their numbers and calcification may be declining in these waters [Roberts et al.,
2011]. Similarly, observations of shell thickness and porosity in shelled pteropods in
tropical Australian waters suggest a decadal reduction in calcification [Roger et al.,
2012]. Impacts on calcification processes in these calcifiers is of particular cause for
concern as pteropods make shells of aragonite, the more soluble from of CaCO
3
than
the calcite produced by coccolithophores and foraminifera, and polar waters in both
hemispheres are likely to be especially at risk of aragonite undersaturation by the end
of the century [McNeil, 2010; Orr et al., 2005; Steinacher et al., 2009]. Indeed, current
laboratory experiments show reductions in pteropod calcification under higher CO
2
[Comeau et al., 2009].
Similarly, experimental data on krill, a key component of the pelagic food web in the
Southern Ocean, show impairment of embryonic development at CO
2
levels likely to be
seen by the end of this century [Kawaguchi et al., 2010].
Benthic calcifiers: non-coral invertebrates (eg. benthic foraminifera, mollusks and
echinoderms)
In addition to corals a large suite of benthic species calcify and some of these such as
molluscs and echinoderms are also major habitat providers and play key ecological
functions. These groups also include species that calcify across both their planktonic
and benthic life history stages. Larval shells are among the smallest and most fragile
shells in the ocean, so the vulnerability of calcification in these life stage is of
particular concern and is still poorly understood [Byrne, 2011]. Vulnerable early life
history stages may be bottlenecks for species persistence [Byrne, 2010; 2011; Dupont
et al., 2010].
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Benthic foraminifera
Benthic foraminifera are important carbonate producers in many reef environments
[Hallock, 2005]. Like their planktonic counterparts a number of benthic foraminiferal
taxa show decreased calcification and faunal diversity under elevated CO
2
conditions
[Dias et al., 2010; Dissard et al., 2010; Kuroyanagi et al., 2009; Uthicke and
Fabricius, 2012]
Molluscs and Echinoderms
Molluscs and echinoderms play key roles in marine habitats by filtering and controlling
habitat heterogeneity [Coen et al., 1999; Rodney and Paynter, 2006]. Echinoderms are
keystone predators (e.g. sea stars) or grazers (e.g. urchins).
Molluscs and echinoderms are also a key source of food and deleterious effects on
commercial species are of great concern. A recent review presents a model of marked
production of shell fish in the future due to ocean acidification, at a time when
increased human populations and food security will be a considerable challenge
[Cooley et al., 2009].
Studies of bivalves and echinoids indicate that larvae reared under ocean acidification
and hypercapnia are smaller and have less skeletal material, as well as evident
abnormal development [Byrne, 2010; 2011; Dupont et al., 2010; Gazeau et al., 2010;
Kurihara, 2008]. The stunting effect of ocean acidification may be caused by impaired
calcification under lower mineral saturation conditions, hypercapnic developmental
delay/depressed metabolism, teratogenic effects and energetic constraints in acid-base
regulation, or a combination of these [Chan et al., 2011; Martin et al., 2011; Sheppard
Brennand et al., 2010; Stumpp et al., 2011]. Adult bivalves also show reduced
calcification under elevated pCO
2
[Gazeau et al., 2007]. Reduced larval size in a high
pCO
2
ocean would have a negative impact on feeding and swimming ability and make
larvae more vulnerable to predation [Allen, 2008; Przeslawski et al., 2008].
Depending on the species, and perhaps the developmental stage at which experimental
incubations are initiated (eg. juveniles, adults [Byrne, 2012]) may also stunt growth of
benthic life stages through reduced larval production and midstage growth e.g.
[Barton et al., 2012].. Warming (up to a point) may ameliorate the negative effects of
acidification on calcifiers by stimulating growth [Byrne, 2011; Sheppard Brennand et
al., 2010; Walther et al., 2010]. However some calcifiers may not reach the calcifying
larval stage in a warmer ocean [Brierley and Kingsford, 2009]. Non-calcifying
echinoderm larvae appear to be more sensitive to warming than acidification [Nguyen
et al., 2012].
Macroalgae
Other important calcifiers include calcareous benthic algae (especially crustose
coralline algae or “CCA”s) that precipitate either high-magnesium calcite or aragonite
and perform the important function of ‘gluing’ the skeletons of corals together to create
reefs. These organisms’ vulnerability to ocean acidification is still being studied but
because they can secrete high-Mg calcite they are likely to be affected earlier than other
groups of calcifiers [Anthony et al., 2008; Kuffner et al., 2007; McClintock et al., 2009;
Nash et al., 2011], and so represent a key area of vulnerability in shelf ecosystems from
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129
the tropics to the Antarctic.
Experimental studies in the tropics and temperate localities [Russell et al., 2009]
suggest high sensitivity of physiological and population-level processes of CCAs to
ocean acidification. In particular, calcification, primary production and abundance are
reduced, whereas skeletal dissolution and mortality increase with elevated pCO
2
.
Interactions with other anthropogenic processes, such as warming and eutrophication,
may exacerbate the these responses of calcifying algae [Anthony et al., 2011b; Diaz-
Pulido et al., 2012]. Natural experiments with elevated pCO
2
confirm experimental
findings [Fabricius et al., 2011; Hall-Spencer et al., 2008]. Upright calcified
macroalgae (e.g. Halimeda) are also important producers of sediment to reef
environments; these calcifiers’ response to acidification also suggest reduced
calcification under acidification [Sinutok et al., 2011]. Brown algae, which do not
necessarily calcify, but are important carbonate producers in shallow marine
ecosystems, also show reduced calcification under elevated pCO
2
near volcanic vents
[Johnson et al., 2012]. Elevated pCO
2
may enhance the competitive ability of some
seaweeds to overgrowth corals, potentially tipping the balance in favour of non-
calcified organisms [Diaz-Pulido et al., 2011].
Key knowledge gaps include: variability in responses across taxa and habitats, potential
for adaptation to high CO
2
, identification of molecular, cellular and physiological
mechanisms involved in the responses observed, and mineralogical responses.
Impacts on fish
Despite the ecological importance of fishes in marine ecosystems, and their
substantial socio-economic significance, relatively little research has been conducted
into the effects of ocean acidification on fishes. One risk to fish is the acidosis (high
pCO
2
in the bloodstream) induced under exposure to elevated environmental CO
2
,
potentially causing acidosis which, at high levels may affect many cellular processes.
Fish are generally considered to be more resistant to direct impacts of ocean
acidification because they do not have extensive calcium carbonate skeletons.
However they have a range of physiological vulnerabilities to elevated pCO
2
and
associated changes in ocean chemistry. Impacts may include: reduced respiratory
capacity and energetic costs of acid–base maintenance [Munday et al., 2009a;
Munday et al., 2009c; Munday et al., 2011a]; impaired sensory performance and
altered behaviour r[Dixson et al., 2010; Munday et al., 2009b], especially in larval
fish; effects on otolith (earbone) calcification (though in some cases this formation
appears to be unaffected or even enhanced [Checkley et al., 2009; Munday et al.,
2011b]).
Most of the research in Australia on the impacts of ocean acidification on fish has
focused on small coral-reef fishes that are amenable to experimental research, and
focused on species that lay their eggs on the substratum (demersal spawners),
including many commercially important species. Few studies have examined
acidification impacts on species that release their eggs directly into the ocean
(broadcast spawners). The latter may represent a critical knowledge gap - eggs and
larvae of broadcast spawners may be more sensitive to elevated CO
2
if they develop
in the open ocean where CO
2
is more stable than in shallow-water environments.
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Similarly, pelagic fishes may be more sensitive to elevated CO
2
if they are similarly
adapted to stable CO
2
chemistry in the open ocean [Munday et al., 2008].
Previous studies show that the mortality of adult fishes is not directly affected by
small increases in ambient CO
2
. Similarly, fertilization and egg survival appears to be
tolerant to high CO
2
, at least in the species studied to date. Recent experimental
results indicate that the growth and development of larval and juvenile reef fishes is
also relatively unaffected by CO
2
levels that might be experienced in the ocean by the
end of this century [Munday et al., 2009c; Munday et al., 2011a]. Whether growth and
development of species from other habitats, especially pelagic species, are similarly
tolerant is unknown. Calcification of otoliths does not appear to be retarded by ocean
acidification. Instead, otoliths are larger in larval fish exposed to high CO
2
, possibly
due to changed bicarbonate concentrations from acid-base compensation.
Fish exposed to high CO
2
exhibit behavioural changes and sensory impairment that
affects their capacity to detect appropriate habitats and avoid predators [Domenici et
al., 2012; Ferrari et al., 2011; Munday et al., 2009b; Munday et al., 2010; Simpson et
al., 2011]. Field experiments show that these behavioural changes could increase
mortality rates of newly recruited fish and could lower population replenishment and
affect patterns of population connectivity.
Elevated CO
2
has been shown to reduce respiratory capacity in some reef fishes, but
the ecological consequences are currently unknown, but one consequence could be
reduced metabolic scope needed in a warmer ocean [Pörtner, 2008; 2010; Pörtner and
Farrell, 2008]. In general, the metabolic performance of species and life stages with
high oxygen demand, such as pelagic species and pelagic larvae, are predicted to be
most sensitive to elevated oceanic CO
2
levels.
Microbial Processes
The impact of shifts in carbonate chemistry on microbial communities and processes is
still little understood e.g. [Bowler et al., 2009; Joint et al., 2011; Tortell et al., 1997;
Witt et al., 2011]. However, a number of studies, including those carried out in NZ
waters, have identified an increase in the activity of bacterial extracellular enzymes
which indicates a potential increase in the breakdown of organic matter e.g. [Piontek et
al., 2010]. Increasing seawater pCO
2
also stimulates fixation of nitrogen (N
2
) by
cyanobacteria in some experiments e.g. [Hutchins et al., 2009] though not in all cases
[Law et al., 2012]. Possible reductions in nitrification under acidification may limit the
the supply of nitrate by this microbial pathway [Hutchins et al., 2009].
Paleoceanographic perspective and buffering by deep-sea and shelf carbonate
sediments
The injection of carbon to the ocean during the Paleocene-Eocene Thermal Maximum,
with its associated acidification and carbonate dissolution, is often cited as an “analog”
to the current acidification of the ocean [Hönisch et al., 2012; Leon-Rodriguez and
Dickens, 2010; Ridgwell and Schmidt, 2010; Zachos et al., 2005]. The glacial-
interglacial cycles of the Late Pleistocene also provide constraints on the response of
marine ecosystems to repeated changes in carbonate chemistry of similar magnitude to
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131
anthropogenic acidification of the ocean to date [Hönisch and Hemming, 2005;
Hönisch et al., 2009].
The response of calcite-dominated deep-sea sediments will depend on their eventual
exposure to undersaturated water as fossil-fuel CO
2
penetrates the deep ocean. The
depth to which the fossil-fuel carbon dioxide must be absorbed is a function of the
calcite saturation horizon and its manifestation in carbonate dissolution. The CSH
varies from 3100-2800 m in the basins of the SW Pacific [Bostock et al., 2011].
Evidence from sedimentary cores suggests calcite saturation has varied over
glacial/interglacial cycles. In the Pacific Ocean CaCO
3
concentrations are greatest
during the deglaciations and lowest during the transition from interglacial to glacial
[Farrell and Prell, 1989; Hodell et al., 2001; Marchitto et al., 2005], whereas in the
Atlantic and parts of the Southern Ocean carbonate concentrations are highest in
interglacials [Crowley, 1983; Howard and Prell, 1994]. The processes responsible for
this variability have been debated over the last 50 years and are focused around
production versus preservation driven by both ocean circulation-driven partitioning and
shelf-basin partitioning in carbonate deposition e.g. [Berger, 1970; Opdyke and Walker,
1992]. The underlying cause represents a fundamental link between ocean
biogeochemistry and climate change. Models for the glacial ocean have suggested that
increasing the global net dissolution rate of sedimentary CaCO
3
by 40% could reduce
atmospheric CO
2
to glacial levels [Archer and Maier-Reimer, 1994]. The CaCO
3
content of deep sea sediments, however, is a complex interplay of changes in overlying
carbonate production, dilution by terrestrial sediment/biogenic silica, transport by ocean
currents, as well as dissolution from low [CO
3
2-
] deep waters, or corrosive pore waters,
from organic matter degradation [Archer, 1996]. Several proxies have been used to
assess changes in the carbonate ion concentration of the water column over time
including shell weight, foraminiferal shell fragmentation and more recently the
development of boron isotopes as proxies for pH and B/Ca ratios as proxies for
carbonate ion concentration [Foster, 2008; Rae et al., 2011; Sanyal et al., 1995; Yu et
al., 2007]. Reconstructions applying these proxies suggest the glacial ocean was 0.15
pH units higher than in interglacials [Hönisch and Hemming, 2005]. The current
average surface ocean pH is ~0.1 units lower than at any time over the past 1 million
years. Studies of the rates and distribution of the marine sedimentary response to past
carbon cycle change will help inform our understanding of the future buffering of
acidification.
The Australian continental shelf is composed of 80 to 100% carbonate in many areas,
including unique ecosystems such as the Great Barrier Reef and extensive bryozoan
and seagrass communities in the southern margin and in Torres Strait, respectively.
The very high carbonate content reflects the contribution of calcifying organisms to
ecosystem diversity and functioning. Calcification provides a key ecosystem service,
producing hard substrate for sessile organism attachment [Wood, 1995] and by
generating reef-stabilizing cements [Manzello et al., 2008]. Calcification produces a
range of carbonate mineralogies, with implications for the timing of dissolution-
mediated buffering of ocean acidification, with high-Mg calcite being the earliest to
dissolve from marine sediments [Andersson et al., 2008]. The diversity of
calcification is still being studied [Smith et al., 2006; Smith and Girvan, 2010; Smith
and Lawton, 2010], with many taxa showing a range of mineralogies. Recently,
dolomite was observed in living coralline algae [Nash et al., 2011] and aragonite in
hydrocorals in Antarctica [Riddle et al., 2008]. Though dolomite is stable in the
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132
marine environment it is, in principle, thermodynamically unlikely to form in these
environments. A recent pilot study found high Mg-calcite to be the most abundant
carbonate mineral in four northern Australian shelf regions with abundances between
35 and 50% (unpubl. data, R. Haese). As calcification of this mineral fraction is
predicted to cease within this century due to ocean acidification, the implications for
shelf habitats are profound [Andersson et al., 2007]. Sediment porewater chemistry
also represents a still poorly-understood source of feedback processes to acidification
especially the interaction with advection in permeable carbonate sediments [Santos et
al., 2011] and high diurnal variability in reef environments [Shaw et al., 2012].
Extending the mapping of carbonate mineral distribution and predictions of future
mineral stability to other continental shelf regions will assist in establishing a spatial
context to changes in calcification and identifying the most threatened ecosystems.
Research Priorities
While pH changes in the open ocean are relatively predictable and now well-
documented, less is known about natural variations in the carbonate chemistry of
shallow coastal systems and how these systems might respond to ocean acidification.
The coast is dynamic so projections for the oceans can only partially guide what will
happen to coastal regions, although an overall depression of pH levels from current
baseline conditions seems likely e.g.[Christensen et al., 2011; McElroy et al., 2012].
We urgently need baseline observations of the carbonate chemistry of a range of
shallow coastal systems with different sources and sinks of alkalinity. This fundamental
information on the CO
2
-carbonate system of coastal systems is essential to inform
ocean acidification experiments with marine organisms.
Ocean acidification has the potential to significantly affect calcification and a range of
other processes in economically significant habitats (e.g., coral reefs, oyster beds), food
webs, regionally important ecosystems (e.g. Southern Ocean pteropods) and with
implications for planetary geochemical cycles (e.g. through corals, foraminifera,
coccolithophores). However, our present understanding of the impact of ocean
acidification on physiological processes is informed largely from short-term laboratory
experiments whilst we currently know very little about the response of individual
organisms, populations, and communities in natural settings and under gradual change
scenarios[Doney et al., 2009]. Along with observations of carbonate chemistry. There is
a need for baseline observations of important marine populations and wider community
responses to acidification in key Australian marine ecosystems (e.g. Southern Ocean,
Great Barrier Reef). In concert with this fundamental research, we need to understand
how impacts on calcification and other processes will affect the overall structure and
function of entire ecosystems and what the consequences of significant changes are
likely to be in terms of those ecosystems especially important to the millions of
Australians that depend on them for food, livelihoods, and tourism.
We also need to understand the potential for acclimatization (phenotypic plasticity)
and evolutionary (genetic) adaptation of organisms to ocean change stressors
especially for ecologically and economically important taxa. There are several major
gaps in knowledge. A better understanding of the molecular and cellular mechanisms
underlying the responses to will allow us to discern levels of perturbation.
Determination of potential for evolutionary change will require targeted genetic e.g.
[Sunday et al., 2011] and multigenerational studies e.g. [Lohbeck et al., 2012].
Ocean acidification
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Recommendations
Efforts need to focus on establishing a better understanding of the various calcification
processes at different parts of the life cycle for key marine calcifiers and how these are
expressed on community and ecosystem levels. We need more information on how
non-calcifiers may respond to ocean acidification. The impacts of ocean warming and
other impacts in multi-stressor studies are also important to consider.
Ecosystem approaches required include establishing baselines of current calcifiers
population ‘health’, and monitoring in key areas (e.g. Southern Ocean, Great Barrier
Reef, temperate systems such as the Great Australian Bight) as well as targeted
laboratory process studies, manipulative experiments, community and ecosystem scale
mesocosm and free ocean carbon dioxide enrichments (FOCE) experiments and ocean
acidification model development on a range of temporal and spatial scales.
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... For example, if poleward currents weaken, the ability of marine species to undergo the range shifts necessary to track their climatic envelopes could be severely compromised. In the Southern Hemisphere, a number of ocean currents [e.g. the Indonesian Throughflow (ITF) and Leeuwin Current] are predicted to weaken under future climate change scenarios (Poloczanska et al., 2012;Sun et al., 2012). By contrast, western boundary currents (WBCs) (e.g. the East Australian Current, EAC) are predicted to intensify ( Fig. 1) Sun et al., 2012). ...
... Using currently available climate change models, in combination with relevant biogeographical and biological data, we predict that climate-driven oceanographic changes will either enhance or reduce species dispersal by strengthening, weakening and altering the structure of oceanic dispersal pathways. Changing oceanic and coastal circulation patterns are already altering the dispersal pathways of many marine taxa, driving major changes in marine ecosystems (Pitt et al., 2010;Banks et al., 2011;Poloczanska et al., 2012;Tanaka et al., 2012;Cetina-Heredia et al., 2015). Given projections for nearfuture climate-driven oceanographic changes (Hobday & Lough, 2011;Kirtman et al., 2013;Hobday & Pecl, 2014), our review indicates that changes in marine population connectivity and range shifts will continue into the future, probably at accelerated rates. ...
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Aim: The dispersal and distribution patterns of many marine organisms are driven by oceanographic conditions, which are influenced by global climate. Climate-driven oceanographic changes are thus likely to result in biogeographical changes. We assess how recent and predicted oceanographic changes affect the dispersal capacities and distributions of ecologically important (especially habitat-forming) marine organisms. Location: We include studies from tropical, temperate and sub-polar regions to draw globally relevant conclusions. Methods: We review biogeographical, biological and oceanographic studies to critically evaluate emerging trends in biogeographical responses to climate-driven oceanographic changes, and predict how future changes will affect marine ecosystems. Results: Many oceanic dispersal pathways are being altered by climate change. These changes will affect marine ecosystems by differentially affecting the replenishment potential and connectivity of key habitat-forming species. In particular, the length of propagule pre-competency periods, propagule behaviour and the geographical distance between areas of suitable habitat will be critical in determining how oceanographic changes affect the pattern and success of dispersal events, including the likelihood of species experiencing poleward range shifts in response to a warming climate. Main conclusions: Future climate-driven oceanographic changes are likely to strengthen or weaken different oceanic dispersal pathways, which will either increase or decrease the potential for dispersal and connectivity in various marine taxa according to the interaction between the local oceanographic, geographical and taxon-specific biological factors. A key focus for future work should be the development of fine-scale near-shore ocean circulation models that can be used to assess the dispersal response of key marine taxa under various marine climate change scenarios.
... In the present study, the annual average surface water temperature was 28°C and it often reached 33°C during summer and monsoon months. Moreover, admixture of freshwater having high nutrient, low electrical conductivity from waters having low nutrient, high electrical conductivity from the marine end often reduces the primary productivity rate of the present phytoplankton community (Jeffries et al. 2016;Richardson and Poloczanska 2009), which was evident from the consistently higher CR in this region compared with GPP. Since the predominance of freshwater and adjacency to urban setting both coincided in the riverine stretch of Hooghly, it was difficult to differentiate between the contribution of urban effluent and freshwater towards the increase in pCO 2 (water). ...
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Urbanized rivers flowing through polluted megacities receive substantial amount of carbon from domestic sewage and industrial effluents which can significantly alter the air-water CO2 flux rates. In this regard, we quantified the partial pressure of CO2 in the surface water (pCO2(water)), air-water CO2 fluxes, and associated biogeochemical parameters in the Hooghly River, India, flowing through two of the most polluted cities of the country, Kolkata and Howrah, over a complete annual cycle during spring tidal phase (SP) and neap tidal phase (NP). This urbanized part of Hooghly River was always supersaturated with CO2 having an annual mean pCO2(water) and air-water CO2 flux of ~ 3800 μatm and ~ 49 mol C m−2 year−1, respectively. Significant seasonal variability was observed for both pCO2(water) and air-water CO2 flux (pre-monsoon, 3038 ± 539 μatm and 5049 ± 964 μmol m−2 h−1; monsoon, 4609 ± 711 μatm and 7918 ± 1400 μmol m−2 h−1; post-monsoon, 2558 ± 258 μatm and 4048 ± 759 μmol m−2 h−1, respectively). Monthly mean pH and total alkalinity varied from 7.482 to 8.099 and from 2437 to 4136 μmol kg−1, respectively, over the annual cycle. pCO2(water) showed significant positive correlation with turbidity and negative correlation with electrical conductivity and gross primary productivity (GPP). High water discharge could have facilitated high turbidity, especially during the monsoon season, which led to depletion in GPP and enhancement in pCO2(water) which in turn led to very high CO2 effluxes. The CO2 efflux rate in this urbanized riverine stretch was substantially higher than that observed in previous studies carried out in the less urbanized estuarine stretch of Hooghly. This indicates that the presence of highly urbanized and polluted metropolis potentially enhanced the pCO2(water) and CO2 effluxes of this river. Similar observations were made recently in some Asian and Australian urban rivers.
... Although pCO 2 in the mid sections of the estuary are negatively correlated to Chl-a and OSat, pCO 2 is also positively correlated to temperature and salinity. The influence of hydrodynamic mixing between the low salinity, high nutrient waters of the upper estuary (Jeffries et al., 2016) with the high salinity, nutrient poor water mass of the lower estuary (Richardson and Poloczanska, 2009) causes large fluctuations in phytoplankton production rates. Additionally during the generally dry conditions experienced in autumn the estuary was generally well mixed with incursions of saline marine water all the way to the upper estuary. ...
Article
Full-text available
Annual organic and inorganic carbon budgets were constructed for the Sydney Harbour Estuary. Net ecosystem metabolism was the main control on carbon fluxes in the system. Sydney Harbour Estuary was slightly net heterotrophic, which is consistent with a small CO2 emission of 0.8 × 10⁸ mol C yr⁻¹. Terrestrial carbon loads were 70% dissolved inorganic carbon, 21% dissolved organic carbon, and 9% particulate organic carbon. Dissolved inorganic carbon was exported to the ocean (4.19 × 10⁸ mol C yr⁻¹), and an import of organic carbon (1.92 × 10⁸ mol C yr⁻¹) from the ocean was required to balance the budget. Sydney Harbour had high sediment organic carbon burial rates similar to other river valley estuaries including the Hudson River and Chesapeake Bay. Productivity was the main sink of inorganic carbon followed by sediment burial, emissions of CO2 to the atmosphere, and oyster sequestration. This study highlights the importance of determining the sources, sinks, and transformations of all carbon forms in constructing estuarine budgets.
... Based on elevated rates of ocean warming, southwest and southeast Australia are recognized as global warming hotspots (Wernberg et al., 2011). It is virtually certain that the increased storage of carbon by the ocean will increase acidification in the future, continuing the observed trends of the past decades in Australia as elsewhere (Howard et al., 2012; see also WGI AR5 Sections 3.8, 6.44). ...
... Although pCO 2 in the mid sections of the estuary are negatively correlated to Chl-a and OSat, pCO 2 is also positively correlated to temperature and salinity. The influence of hydrodynamic mixing between the low salinity, high nutrient waters of the upper estuary (Jeffries et al., 2016) with the high salinity, nutrient poor water mass of the lower estuary (Richardson and Poloczanska, 2009) causes large fluctuations in phytoplankton production rates. Additionally during the generally dry conditions experienced in autumn the estuary was generally well mixed with incursions of saline marine water all the way to the upper estuary. ...
Article
The Sydney Harbour Estuary is a large drowned river valley adjacent to Sydney, a large urban metropolis on track to become a megacity; estimated to reach a population of 10 million by 2100. Monthly underway surveys of surface water pCO 2 were undertaken along the main channel and tributaries, from January to December 2013. pCO 2 showed substantial spatio-temporal variability in the narrow high residence time upper and mid sections of the estuary, with values reaching a maximum of 5650 matm in the upper reaches and as low as 173 matm in the mid estuary section, dominated by respiration and photosynthesis respectively. The large lower estuary displayed less variability in pCO 2 with values ranging from 343 to 544 matm controlled mainly by tidal pumping and temperature. Air-water CO 2 emissions reached a maximum of 181 mmol C m À2 d À1 during spring in the eutrophic upper estuary. After a summer high rainfall event nutrient-stimulated biological pumping promoted a large uptake of CO 2 transitioning the Sydney Harbour Estuary into a CO 2 sink with a maximum uptake of rate of À10.6 mmol C m À2 d À1 in the mid-section of the estuary. Annually the Sydney Harbour Estuary was heterotrophic and a weak source of CO 2 with an air-water emission rate of 1.2e5 mmol C m À2 d À1 (0.4e1.8 mol C m À2 y À1) resulting in a total carbon emission of around 930 tonnes per annum. CO 2 emissions (weighted m 3 s À1 of discharge per km 2 of estuary surface area) from Sydney Harbour were an order of magnitude lower than other temperate large tectonic deltas, lagoons and engineered systems of China, India, Taiwan and Europe but were similar to other natural drowned river valley systems in the USA. Discharge per unit area appears to be a good predictor of CO 2 emissions from estuaries of a similar climate and geomorphic class.
... Australian scientists contributing to the government's Marine Climate Change 2009 Report Card explained the urgency of long-term marine plankton monitoring (Richardson et al., 2009). By the next Report Card in 2012, a national coastal plankton monitoring programme utilizing a network of survey stations was underway and planned for perpetuity (Richardson, 2012). Important as is this development, there is still a need for small scale inshore and estuary monitoring. ...
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... Based on elevated rates of ocean warming, southwest and southeast Australia are recognized as global warming hotspots (Wernberg et al., 2011). It is virtually certain that the increased storage of carbon by the ocean will increase acidification in the future, continuing the observed trends of the past decades in Australia as elsewhere (Howard et al., 2012; see also WGI AR5 Sections 3.8, 6.44). ...
... 0.3 m for Wellington and Christchurch, up to 0.45 m rise for Auckland where tide range is higher ( • Average global pH of surface waters has decreased by 0.1 since the mid-19th century to a current value of approximately 8.11. This corresponds to a 26% increase in acidity (Howard et al. 2012) • Global prediction of further decrease in pH ranging from 0.06-0.32 units (15%-109% increase in acidity) by 2100 for different RCP scenarios Wind and storm events ...
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The east coast of Africa is renowned as an adventure tourism getaway destination, and scuba dive tourists from around the world frequent the region to dive on its pristine coral reefs. Environmental change, however, is becoming a key driver for dive tourists’ decisions to travel, and these tourists are avoiding areas where coral reefs are no longer pristine. Indirect environmental risks, occurring outside the borders of countries and outside the control of dive operators, impact coral reef tourism within those countries and on the greater Blue Economy. Indirect environmental risks include climate change, industrialisation of coastal regions, increased shipping and boating activities, as well as marine and land-based pollution. This paper reports on the perceptions of dive operators on the indirect environmental risks which threaten the future of the dive tourism industry in the East African Marine Ecoregion. The research followed an exploratory and descriptive research design within a post-positivism paradigm. Via purposive sampling, surveys were conducted with 34 dive operators from Kenya, Tanzania, Mozambique and South Africa. Research findings indicate two overall areas of concern impacting coral reefs—activities relating to climate change, followed by coastal development and industrialisation of coastal areas. In terms of specific indirect risks, marine pollution is significant, followed by coral bleaching and extreme weather events. There is an increasing level of concern that continued environmental disturbances will impact business in future. This is not just a problem for the coastal tourism industry but also for coastal communities who share this precious resource. It is vital to cast a spotlight on the turbulent waters of environmental risks, which ultimately affect the sustainability of marine tourism and the wider Blue Economy. By doing so, regional plans can be shaped to respond to these threats in a more discerning way.KeywordsDive operatorsEast African Marine EcoregionEnvironmental changeIndirect environmental risksMarine tourismScuba diving industryJEL ClassificationQ53Q54Z31Z32
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