How will marine organisms respond?

Brain coral with Christmas snails. Christine Kellogg, USGS.

Most of what is known about organismal responses to ocean acidification has been obtained from relatively short-term laboratory experiments on single species. Such experiments use simplified versions of the natural environment, but give an indication of potential responses in the ocean²⁸.

In a growing number of laboratory and field experiments multiple organisms are studied together, for example, in gradients of pH in naturally acidified ecosystems and in mesocosms with natural communities including numerous species.

Results from a broad range of marine organisms show various responses, including decreased survival, calcification, growth, development and abundance. There is considerable variation in the sensitivity and tolerance of marine organisms to ocean acidification, sometimes even within a single species. Other organisms show a positive response to the increased availability of CO₂. More active organisms, such as mobile crustaceans and fish, seem to be less sensitive to ocean acidification. Fleshy algae, some phytoplankton and some seagrasses may benefit from an increase in carbon availability. The impacts on individual species – whether they decline or benefit from ocean acidification – may cause cascading disturbances in other parts of the food web.

Confidence levels
Confidence levels are expressed in this document with the qualifiers “low”,
“medium”, “high” and “very high”. These qualifiers synthesise the authors’
judgments about the validity of findings as determined through evaluation
of evidence and agreement. The analysis builds on statements of confidence
derived from peer-reviewed synthesis such as the European Project on
Ocean Acidification synthesis book²⁸ and the Intergovernmental Panel on
Climate Change (IPCC) Fifth Assessment Report. The most recent metaanalyses,
of 228 ocean acidification studies on marine organisms²⁹ and 167
studies on marine animals¹⁴, provided further evidence to aid the authors
in analysing and summarising the outcomes of the experimental evidence.
Increasing levels of evidence and degrees of agreement are correlated with
increasing confidence, as outlined in the IPCC’s guidance note on
the treatment of uncertainties³⁰ in the Fifth Assessment Report.

• VERY HIGH CONFIDENCE
• HIGH CONFIDENCE
• MEDIUM CONFIDENCE
• LOW CONFIDENCE

Anthropogenic ocean acidification will adversely
affect many calcifying organisms [MEDIUM CONFIDENCE]
Most studies demonstrate that calcification – the ability for some organisms
to produce shells or skeletons – decreases with ocean acidification²⁹. These
include planktonic calcifiers (such as foraminifera, coccolithophorids and
pteropods), corals and molluscs, as well as echinoderms (e.g., urchins) and less
so crustaceans (e.g., crabs).
An analysis of ocean acidification studies shows that many calcifying
organisms also show a decrease in survival, growth, development and
abundance²⁹. In many calcifying groups, early life stages are most sensitive to
CO₂-induced changes in seawater chemistry. Crustaceans are less affected than
corals, molluscs or echinoderms¹⁴.

Molluscs (such as mussels, oysters and pteropods) are
one of the groups most sensitive to ocean acidification
[HIGH CONFIDENCE]
Early life stages of many molluscs (larvae and juveniles) as well as adults have
shown reduced calcification, growth and survival. This makes molluscs one of
the groups most sensitive to ocean acidification¹⁴.

Pteropod (marine snail) shells are already dissolving
[MEDIUM CONFIDENCE]
The high-latitude oceans are already becoming corrosive to some species.
The shells of pteropods, small marine snails that are key species in the food
web, are already dissolving in parts of the Southern Ocean, which surrounds
Antarctica³⁵. They have special importance in the food web in polar regions,
for example forming a key food source for pink salmon36.

If CO₂ emissions continue on the current trajectory,
coral reef erosion is likely to outpace reef building
sometime this century [HIGH CONFIDENCE]
Ocean acidification alone is likely to cause reef building to cease by the end of
the 21st century on the current CO₂ emissions trajectory³⁷. If coral bleaching
due to ocean warming is also taken into account, then the rates of erosion
on most reefs could outpace the overall reef building by corals and other
organisms once CO₂ levels reach 560 ppm (by mid-century under the current
emissions trajectory)³⁸. If this happens, the degradation and loss of coral reefs
will affect whole ecosystems dependent on reefs as habitat, with consequences
for biodiversity, fisheries and coastal protection. Very aggressive reductions
in CO₂ emissions are required to maintain a majority of tropical coral reefs in
waters favourable for growth¹¹.

Cold-water coral communities are at risk [HIGH CONFIDENCE],
and may become unsustainable
By 2100, it is estimated that 70% of cold-water corals will be exposed to
corrosive waters, although some will experience undersaturated waters as
early as 2020³⁹. Undersaturated conditions will increase the dissolution rate of
the dead skeletons (the base of these deep-water coral communities), which
will lead to a disintegration of the cold-water coral ecosystems^40,41. Their loss
would have consequences for food webs⁴², as they provide habitat, feeding
grounds and nursery areas for many deep-water organisms.

[This text is from the Ocean Acidification Summary for Policy Makers, 2013, and is available online as a PDF with full references.]