Effects of nearshore processes on carbonate chemistry dynamics and ocean acidification

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. This dissertation synthesizes autonomous and bottle observations to model carbonate chemistry and to understand how physical and biological processes affect seasonal carbonate chemistry at both locations. The autonomous carbonate chemistry and oxygen observations are used to examine a mass balance approach using a 1-D model to determine net rates of ecosystem calcification and production (NEC and NEP) from communities close (<5km) to the buoys. The results provide evidence to suggest that seasonal response between benthic metabolism and seawater chemistry at LP is attenuated relative to that at CR because their differences in benthic cover and how benthic metabolism modifies the water chemistry. Simple linear trends cannot explain the feedback between metabolism and reef water chemistry using long-term observations over natural variations. The effects of community production on partial pressure of CO2 (pCO2sw) make these interactions complex at short- and long-term scales. Careful consideration should be taken when inferring local biogeochemical processes, given that pCO2sw (and presumably pH) respond on much shorter time and local scales than dissolved inorganic carbon (DIC) and total alkalinity (TA). The observations highlight the need for more comprehensive observing systems that can reliably measure both the fast-response (pCO2sw, pH) and slow-response (DIC) carbon pools.

The metabolism rates are shown to be robustly modeled using a mass balance approach in two coastal reef systems at two fixed assets that could be employed elsewhere to monitor OA and its impacts within coral reef ecosystems. The data can be applicable to other sites with the similar auxiliary data and can be used in combination with other approaches, such as the turbulent flux, to estimate long-term metabolic rates in the field. Both sites were net heterotrophic and net dissolutional from late summer to fall, with occasional periods of net calcification and net autotrophy from winter to early summer. High respiration rates at CR and LP observed in the fall generated a local source of DIC to the system, causing a decrease in carbonate saturation states. During this time of the year, these processes may affect the reef’s susceptibility to other climate pressures and decrease the ability of upstream communities (e.g., seagrasses at CR) to serve as OA refugia. Surface waters at LP are likely to be affected by OA sooner and more strongly than surface oceanic waters due to the significant annual changes respiration and calcification have in coastal carbonate saturation states. Our results suggest that tropical Caribbean reef ecosystems are exhibiting long periods of net dissolution of highly soluble carbonate minerals based on similarities in environmental characteristics. Future research efforts should be directed to improve our understanding of the drivers of both calcification and organic production, at long-term and ecosystem scales.

Melendez M., 2020. Effects of nearshore processes on carbonate chemistry dynamics and ocean acidification. PhD thesis, University of New Hampshire, 146 p. Dissertation.

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