UC San Diego

Anthropogenic environmental change threatens the ability for many coral reefs to maintain the calcium carbonate structures that provide shoreline protection, fisheries provision, and tourism revenue to human populations worldwide. Rigorous quantification of the rates and drivers of coral and coral reef net ecosystem calcification (NEC) represents a significant challenge but is tantamount for understanding how these services to humanity may be affected by environmental change. This challenge is addressed here by leveraging a combination of field observations, numerical models, and statistical analyses. The major findings showed that extremely large NEC errors of –91% to +1000% can be driven by interacting ±83% uncertainties in the difficult to measure seawater depth and residence time parameters used to calculate NEC. Confidence in NEC rates can nonetheless be improved by leveraging multiple NEC methods as evidenced by the agreement between chemistry and census-based NEC calculated for Hog Reef, Bermuda. Analysis of the environmental drivers of coral and reef-scale calcification at Hog Reef and Crescent Reef, Bermuda showed that temperature was the strongest driver of coral and reef-scale calcification rates with little influence by the other environmental parameters studied. This suggests that reduced warming rates driven by lower global carbon dioxide emissions pathways could maintain Bermudan coral calcification through the twenty-first century. However, more rapid warming can cause coral bleaching that reduces NEC as evidenced by the observation of zero NEC during the fall 2015 coral bleaching event in Kāne'ohe Bay, Hawai'i. The subsequent recovery to pre-bleaching NEC rates by the following summer highlights the capacity of coral reef NEC to rapidly recover in the absence of continued stressors. Conversely, the cumulative effects of 20 years of disturbances on coral calcification capacity (CCC) across the main Hawaiian Islands, Mo'orea, Florida Keys reef tract, and St. John revealed disturbance-driven reductions in CCC and community-level shifts in contributions to CCC from competitive to weedy corals that may increase CCC resilience to future disturbances. This dissertation collectively improves projections for how the Anthropocene may change reef structures and the services they provide humanity by advancing our understanding of the rates and drivers of coral and coral reef calcification.