Changes in ocean chemistry can have extensive direct and indirect effects on marine organisms and the ecosystems in which they live. Studies of marine calcifiers (corals, crustaceans, and mollusks) indicate that most, but not all, exhibit reduced calcification with increased ocean acidification. ref
Impacts on Marine Organisms
A growing number of studies have demonstrated adverse impacts on marine organisms as a result of ocean acidification, including the following: ref
- Decreased rate of skeletal growth in reef-building corals
- Reduced ability to maintain a protective shell among free-swimming zooplankton (zooplankton include “animal plankton”, mainly small crustaceans and fish larvae, and form the base of most marine food webs)
- Reduced rate of calcium carbonate production in marine algae (crustose coralline and green algae)
- Reduced survival of larval marine species, including commercial fish and shellfish
- Impaired developmental stages of invertebrates (fertilization, egg cleavage, larva, settlement and reproduction)
- Excessive CO2 levels in the blood (CO2 toxicity) of fish and cephalopods and significantly reduced growth and fecundity in some invertebrate species
Declining pH (increasing acidity) may affect organisms in ways that extend beyond declining calcification or metabolic performance, including:
- Interactions between species during different life stages
- Shifting competitive pressures (e.g., algae outcompeting corals)
- Alterations in predation, which will come into play as communities respond to acidification
- Alteration of fish larvae behavior (due to impaired sensory function in larval fish) and reduced recruitment success ref
Interactions with other stressors (e.g., nutrient input, increased sea-surface temperature, and sea-level rise) will also affect how marine communities will change in response to high CO2 conditions.
Impacts on Calcification
One of the most critical effects of increasing ocean acidity relates to the production of shells, skeletons, and plates from calcium carbonate, a process known as calcification. Acidification shifts the equilibrium of carbonate chemistry in seawater, reducing pH and the concentration of carbonate ions available for corals and other marine calcifiers to use to build their skeletons. This decreases the rate and amount of calcification among many marine organisms that build external skeletons and shells, ranging from plankton to shellfish to reef-building corals.
The reduction of dissolved carbonate ions in seawater has many implications for coral reef ecosystems. Since reef-building corals need carbonate to build their skeletons, decreasing carbonate ion concentrations will likely lead to weaker, more brittle coral skeletons and slower coral growth rates. In the future this may cause coral reefs to erode faster than they can calcify, thus decreasing the ability of coral species to compete for space.
For corals and other calcifiers like sea urchins and shellfish, reductions in calcification may:
- Increase corals’ susceptibility to bleaching and disease
- Decrease the ability of organisms to fend off predators and compete for food and habitat
- Alter behavior patterns
- Reduce capacity to tolerate ultraviolet radiation and increased rates of bioerosion and greater damage from cyclones
Laboratory studies have examined the effects of ocean acidification on many types of corals and coralline algae, revealing a range of responses from a 3% to 60% decline in calcification rate for a doubling of atmospheric CO2. ref A recent study of brain corals in Bermuda found that calcification rates have declined by 25% over the past 50 years and ocean acidification is a likely contributing factor. ref