Alkalinity to calcium flux ratios for corals and coral reef communities: variances between isolated and community conditions

Abstract

A series of outdoor flume experiments was conducted using a range of experimental "reef communities" in near natural conditions to test whether the alkalinity anomaly technique accurately predicts calcification. Such calcification measurements have been used to describe coral reef metabolism and to monitor reef health. For the past 40 years it has been assumed that half the total alkalinity flux (ΔTA/2) equals calcification and that there is no significant effect of organic metabolism on the overall alkalinity. The hypothesis that ΔTA/2 equals calcification was tested by determining if the relationship between alkalinity and calcium uptake is constant for various communities and community components. Assumptions of the alkalinity anomaly technique were tested by measuring the alkalinity and calcium fluxes of isolated components (corals, phytoplankton, algae and the sediment) in reference to that of the combined community. Natural sunlight, realistic hydrodynamic regime and natural levels of nutrients, plankton and organic matter were available to the organisms. Groups of corals were run separately and in conjunction with other reef components (live rock, filamentous algae and sandy sediment). The alkalinity to calcium flux ratios were consistently higher during coral-only runs (2.01 ± 0.19) than in the mixed community (1.61 ± 0.14, p-value =0 .011) where additional sources of alkalinity from the sediment and algae caused a depressed ratio. Additionally, pH was higher and more stable when sediment was included with the corals (7.52 ± 0.07 vs. 7.94 ± 0.03, p-value = 3 x 10-5). Aragonite saturation state (Ωarag) showed the same pattern (1.12 ± 0.14 vs. 2.51 ± 0.2, p-value = 2 x 10-6), indicating corals may receive benefits from living in the same environment with carbonate sediments and photosynthetic organisms. Additional experiments where macro-algae and a sediment community were tested separately revealed that these components of alkalinity can be a significant source of error for calcification measurements when they are dominant on a reef. Alterations of nutrients and organic matter due to photosynthesis, oxidation, reduction and remineralization in the non-coral components can cause deviations from ΔTA/ΔCa2+ = 2.