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Coral reefs are important ecosystems that are increasingly negatively impacted by human activities. Understanding which anthropogenic stressors play the most significant role in their decline is vital for the accurate prediction of future trends in coral reef health and for effective mitigation of these threats. Here we present annually resolved boron and carbon isotope measurements of two cores capturing the past 90 years of growth of the tropical reef-building coral Siderastrea siderea from the Belize Mesoamerican Barrier Reef System. The pairing of these two isotope systems allows us to parse the reconstructed pH change into relative changes in net ecosystem productivity and net ecosystem calcification between the two locations. This approach reveals that the relationship between seawater pH and coral calcification, at both a colony and ecosystem level, is complex and cannot simply be modeled as linear or even positive. This study also underscores both the utility of coupled delta B-11-delta C-13 measurements in tracing past biogeochemical cycling in coral reefs and the complexity of this cycling relative to the open ocean.

Plain Language Summary Coral reefs worldwide are in decline due to multiple anthropogenic stressors (e.g., ocean warming and acidification in response to anthropogenic CO2 release). Here we explore a new way to examine the relationship between these anthropogenic stressors and critical ecosystem-scale processes using paired annual-resolution boron and carbon isotope measurements of coral cores spanning the past century. This isotopic pairing allowed us to compare changes in seawater pH, net ecosystem productivity, and net ecosystem calcification between forereef and backreef locations of the southern Belize Mesoamerican Barrier Reef System. We specifically show that the forereef colony, which exhibits declining growth over the past century, recorded unexpectedly steady pH over the same interval. The backreef colony, in contrast, exhibited accelerating growth over this interval, while recording surprisingly rapid ocean acidification. This apparent contradiction is explained with carbon isotope data from the cores showing that increasing primary production on the forereef has offset anthropogenic acidification. Likewise, the increasing rate of coral calcification in the backreef (which generates acidity) has exacerbated anthropogenic acidification of backreef waters. Thus, although ocean acidification and warming influence calcification and primary production within reef systems, the resulting changes can drive biogeochemical feedbacks capable of eliminating or amplifying the anthropogenic CO2 signal.