Long-term effects of nutrient and CO_2 enrichment on the temperate coral Astrangia poculata (Ellis and Solander, 1786)

摘要

Zooxanthellate colonies of the scleractinian coral Astrangia poculata were grown under combinations of ambient and elevated nutrients (5 μM NO_3~-, 0.3 μM PO_4~(-3), and 2 nM Fe~(+2)) and CO_2 (～780 ppmv) treatments for a period of 6 months. Coral calcification rates, estimated from buoyant weights, were not significantly affected by moderately elevated nutrients at ambient CO_2 and were negatively affected by elevated CO_2 at ambient nutrient levels. However, calcification by corals reared under elevated nutrients combined with elevated CO_2 was not significantly different from that of corals reared under ambient conditions, suggesting that CO_2 enrichment can lead to nutrient limitation in zooxanthellate corals. A conceptual model is proposed to explain how nutrients and CO_2 interact to control zooxanthellate coral calcification. Nutrient limited corals are unable to utilize an increase in dissolved inorganic carbon (DIC) as nutrients are already limiting growth, thus the effect of elevated CO_2 on saturation state drives the calcification response. Under nutrient replete conditions, corals may have the ability to utilize more DIC, thus the calcification response to CO_2 becomes the product of a negative effect on saturation state and a positive effect on gross carbon fixation, depending upon which dominates, the calcification response can be either positive or negative. This may help explain how the range of coral responses found in different studies of ocean acidification can be obtained.