Current Observations

To date, there are limited observations of current changes in ocean biology as a result of ocean acidification. This is in part a result of a lack of chemical data with which observations can be correlated and a lack of research in this emerging area, but may also be an artefact of organisms' ability to cope with short-term variability in pH. Seawater pH in coastal and shelf sea water columns can fluctuate by up to 0.9 depending on time, season, position in water column and fresh water influence [10,22]. That is, there may be only short periods of low pH with the periods of high pH allowing the organisms to recover. Impacts may not become apparent until they are subjected to longer periods of lower pH or the whole pH range that they experience is reduced. In contrast, seasonal pH variation of open ocean surface waters is around 0.07 (Fig. 3a) which may make these regions more sensitive to current and future acidification. Indeed, the detected change in pH (—0.1) since the pre-indus-trial already exceeds the open ocean seasonal variation. Observed changes, for example, in species distribution which have been attributed to changes in climate, pollution, ecosystem detioration and so on, may have masked the role of ocean acidification. Further work at the international level needs to be carried out to explore current and future impacts of ocean acidification.

Observed differences in the cold-water coral ecosystems between the North Atlantic and the North Pacific may be indicative of biological responses to changes in ocean chemistry. Cold-water corals in the North Pacific are found living close to the ASH, as it is much shallower than in the North Atlantic; however, they do not flourish or form large structures, such as are found in the North Atlantic [23]. Only 10% of all known stylasterid corals produce calcite instead of aragonite, yet in the North Pacific six out of seven stylasterid corals used calcite to form their spicules and skeletons [24]. Near the ASH, it may be less costly for these corals to produce calcite thereby reducing the affects of dissolution.

Coral on the Great Barrier Reef (GBR), Australia, have shown a 21% decrease in net calcification and 30% decrease in growth over the period 1988 2003 [25]. Sea surface temperature does not appear correlated to this decline, as might be expected if increasing temperature was causing bleaching events or decreasing health of the corals. The change in carbonate chemistry observed in our oceans (Fig. 1a and b) could be impacting the growth and net calcification of corals, but as yet there is no chemical data directly from the GBR to confirm this. However, reefs in the Red Sea have shown correlated responses in net calcification rate to natural fluctuations in O and temperature [26], providing observational evidence of a response of corals to changes to today's carbonate conditions.

Spatial variation in sea bed organisms has been observed across a large pH range at natural marine volcanic CO2 vent sites. A number of key ecosystem changes are apparent, for example, calcareous algae were replaced by non-calcareous algae and sea-grasses with the latter increasing their primary production. There was a large reduction in biodiversity, particularly a loss of calcifying organisms at low pH levels. A number of taxa appear to be more susceptible to acidification impacts than others, for example, echinoderms (particularly sea urchins) did not appear below pH 7.6, whereas molluscs (limpets) and crustaceans (barnacles) were present until pH 6.5 [19].

Coccolithophores, microscopic plants that secrete CaCO3 platelets called liths, occur over a variety of environmental conditions throughout the worlds oceans yet they are excluded from certain locations, for example, the Baltic Sea. Areas known to have an extremely large seasonal cycle of calcite saturation states, with wintertime values declining to <1, appear to be areas where coccolithophores are absent [27] implying that the saturation state may have a large influence on their distribution, although low salinity or differences in the magnitude of the spring bloom will also contribute [27,28].

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