Resilience of Pore-Water Chemistry and Calcification in Photosynthetic Zones of Calcifying Sediments

Photosynthetically driven calcification was investigated in diatom-dominated carbonate sediments from Bait Reef, Australia. Laboratory measurements conducted over complete diel cycles, using O₂, pH, $\text{CO}_{3}^{2-}$ , and Ca²⁺ microsensors, confirmed that photosynthesis and respiration drive calcification and calcium release via their respective effects on the local pH. However, the dark situation does not simply mirror the light situation. Profiles showed that calcification and calcium release are not necessarily tightly coupled to the light cycle and that mass transfer phenomena need to be considered in diel chemical dynamics. The magnitude and timing of pH and $\text{CO}_{3}^{2-}$ concentration changes did not simply follow the light cycle. The pH in the upper 3 mm of the sediment changed more rapidly upon illumination than upon darkening. Consequently, photosynthetically induced calcification began shortly (within 1 h) after illumination, but the pH remained elevated and calcification continued for ∼7 h after darkening. Thus, calcification in marine phototrophic sediments is not limited to light periods, but may continue for extended periods after darkening. This decoupling of light, photosynthesis, and calcification has profound consequences for estimates of daily calcification rates, which have previously been made from measurements assuming close to steady states and 12: 12 h light: dark calcification and decalcification. In Bait Reef sediments, such an assumption underestimates daily calcification rates by two- to threefold.