Changes in environmental conditions are presently occurring at a rate not experience before as a consequence of increased green house gasses in the atmosphere due to human activities. However, the impacts of these changes on the marine ecosystems are not so clear. Several species and different biogeochemical processes are at risk to be seriously affected. Examples of the processes are: Calcification, Primary production, Hypoxia, Nitrogen fixation.
If sensitivity of these processes to Ocean Acidification varies between different species, then changes in community composition, and in turn in trophic levels and food webs might also occur. The next table, taken from Turley and Gattuso (2012) describes the level of evidence, agreement among experts and confidence on the different statements regarding the impacts of Ocean Acidification.
- Calcification: We have seen that Ocean Acidification is expected to decrease in 50% the CO3– in the ocean by the end of this century. CO3– is used in the process of calcification by different organisms (phytoplankton such as coccolithophores, zooplankton such foraminifera, and also pteropods, echinoderms, mollusks, corals, etc.). During the process CO3– reacts with calcium to form CaCO3. However, the reaction depends on the concentration of both ions on the water and other factors such as temperature, pressure, salinity and the type of mineral to be form (calcite or aragonite for example), the relation of these factors describing the so called calcium carbonate saturation state, which need to be greater than 1 for calcification to occur. However, with less CO3– the saturation state also decreases. Many of the calcifying species show reduced calcification in laboratory experiments under high CO2 conditions; however, studies are still limited for many important species, or combination of parameters, or testing more than one species, etc.
- Primary production: Ocean Acidification is also likely to cause changes in Primary production. During photosynthesis, phytoplankton consumes CO2 and produce particulate organic carbon. Photosynthesis might be below saturation under present day CO2 levels due to the low affinity for CO2 of the rubisCO enzyme that catalyses the reaction between the CO2 and a simple carbon molecule, the sugar ribulose, during carbon fixation. However, many species are known to have developed Mechanisms to concentrate CO2 near this enzyme. In some species, nevertheless these mechanisms are absent or not so effective. Therefore some species could increase primary production and other could just not care.
- Hypoxia: An increased primary production, can of course, help drawing down atmospheric CO2, but increasing the amount of organic carbon that sinks in the ocean will also enhance its oxidation in deeper layers or for benthic communities. If this happens, oxygen levels at these sites could decrease to anoxic states, harming the organisms that need oxygen for its metabolism.
- Nitrogen fixation: This is a process driven by small cianobacterias that transforms nitrogen molecules non-suitable for phytoplankton into suitable nitrogen. It has been observed to increase but also to decrease under experimental conditions. The decrease has been related to iron limitation. If this process is affected by ocean acidification in any direction, it could be important because in most oceanic regions primary production is limited by nitrogen.
Other concerns about Ocean Acidification are:
- Significant shift in key nutrients
- Shift in phytoplankton diversity
- Reduced growth, production and life span of adults, juveniles & larvae of Echinoderms, Fish, Mollusks, others.
- Reduced tolerance to other environmental fluctuations
- Changes to fitness and survival
- Changes to species biogeography
- Changes to key biogeochemical cycles
- Changes to food webs
- Changes to ecosystem & their services