State the relationship between atmospheric carbon dioxide and calcification rates.

Suggest one advantage of using a mesocosm in this experiment.

Outline one way in which reef-building corals are affected by increasing atmospheric carbon dioxide.

Negative correlation/inverse relationship (Do not accept “negative” alone)

Exponential decrease in calcification as pCO₂ /concentration/atmospheric CO₂ rises

As CO₂ increases calcification decreases

Allows atmospheric CO₂ concentration to be controlled/varied

Control other variables

Closed system so no external processes affect experiment

Alternative 1
Corals have parts/shells/«exo»skeleton made of calcium carbonate

Calcium carbonate «shells» dissolve in acid conditions

Acid/high CO₂ conditions reduces availability of calcium carbonate «for forming shells»
OR
negatively affects enzymes involved in calcification

Alternative 2
CO₂ is «significant» greenhouse gas causing rise in ocean temperatures

Higher ocean temperatures/acidification leads to rejection of zooxanthellae
OR
higher ocean temperatures/acidification leads to coral bleaching

The vast majority were able to read the graph correctly, noting that as CO₂ increased, calcification decreased. Only stronger candidates used terms such as ‘negative correlation’ or ‘inverse relationship’. 

The majority were also able to get the mark here, usually for noting that the mesocosm allowed for control of all variables although many seemed doubtful as to what a mesocosm was.

Candidates found this difficult and few could correctly outline why increasing CO₂ levels affected coral. Many simply repeated the stem of the question. Few seemed to know that coral had an exoskeleton or shell made of calcium carbonate, instead referring in general to the ‘reef’. Few also knew the relationship between increasing CO₂ levels and coral bleaching.