National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2020

Seasonal variations of carbonate chemistry at two western Atlantic coral reefs

Meléndez, M., J. Salisbury, D. Gledhill, C. Langdon, J.M. Morell, D. Manzello, S. Rodriguez-Abudo, S. Musielewicz, and A. Sutton

J. Geophys. Res., 125, e2020JC016108, doi: 10.1029/2020JC016108, View online (2020)

Time series from open ocean fixed stations have robustly documented secular changes in carbonate chemistry and long‐term ocean acidification (OA) trends as a direct response to increases in atmospheric carbon dioxide (CO2). However, few high‐frequency coastal carbon time series are available in reef systems, where most affected tropical marine organisms reside. Seasonal variations in carbonate chemistry at Cheeca Rocks (CR), Florida, and La Parguera (LP), Puerto Rico, are presented based on 8 and 10 years of continuous, high‐quality measurements, respectively. We synthesized and modeled carbonate chemistry to understand how physical and biological processes affect seasonal carbonate chemistry at both locations. The results showed that differences in biology and thermodynamic cycles between the two systems caused higher amplitudes at CR despite the shorter residence times relative to LP. Analyses based on oxygen and temperature‐normalized pCO2sw showed that temperature effects on pCO2sw at CR were largely counteracted by primary productivity, while thermodynamics alone explained a majority of the pCO2sw dynamics at LP. Heterotrophy dominated from late spring to fall, and autotrophy dominated from winter to early spring. Observations suggested that organic respiration decreased the carbonate mineral saturation state (Ω) during late summer/fall. The interactive effects between the inorganic and organic carbon cycles and the assumed effects of benthic metabolism on the water chemistry at both sites appeared to cause seasonal hysteresis with the carbonate chemistry. Improved integration of observational data to modeling approaches will help better forecast how physical and biogeochemical processes will affect Ω and carbonate chemistry in coastal areas.

Plain Language Summary. Anthropogenic activities, such as the burning of fossil fuels, interact with the chemistry of the ocean surface and cause ocean acidification (OA). The current trend and projections for OA are well defined for open ocean waters, although less is known about its effects on nearshore ecosystems, where most of the affected organisms reside. In this work, we analyzed almost a decade of carbonate chemistry data for two reef time series at Cheeca Rocks (CR), Florida, and La Parguera (LP), Puerto Rico, to assess the physical and biological processes that dictate carbonate seasonal variability. The results showed that temperature is the dominant driver of seawater CO2 changes at LP, while at CR, biological processes have major effects on CO2 chemistry. Continued observations will advance future projections of OA in nearshore areas and provide coastal managers with appropriate tools to monitor OA.

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