Seasonal Net Ecosystem Metabolism of the Near-Shore Reef System in La Parguera, Puerto Rico

Abstract. Changes in ocean chemistry as a direct response to rising atmospheric carbon dioxide (CO₂) concentrations is causing a reduction of pH in the surface ocean. While the dynamics and trends in carbonate chemistry are reasonably constrained for open ocean waters, the ways in which ocean acidification (OA) manifests within the shallow near-shore waters, where coral reefs reside, is less understood. Constraining near-reef variability in carbonate chemistry and net ecosystem metabolic processes across diel, seasonal, and annual scales is important in evaluating potential biogeochemical thresholds of OA that could result in ecological community changes. The OA Test-Bed at La Parguera Marine Reserve in Puerto Rico provides long-term carbonate chemistry observations at high-temporal resolution within a Caribbean near-shore coral reef ecosystem. A 1-D model was developed using the carbon mass balance approach to yield information about net ecosystem production and calcification processes occurring in the water column adjacent to the reef. We present results of nine years of sustained monitoring at the Enrique mid-shelf forereef, which provides for the characterization of temporal dynamics in carbonate chemistry and net ecosystem metabolic processes encompassing near-shore and upstream locations. Results indicate that net heterotrophy and net dissolution dominate over most of the year, while net autotrophic conditions coupled with calcification dominated from only January to mid-April. The average carbonate dissolution rate observed during summer is estimated at −2.19 g CaCO₃ m⁻² day⁻¹ and net community dissolution persists 76 % of the seasonal year despite the water column remaining super-saturated with respect to aragonite. This corresponds to −0.62 kg CaCO₃ m⁻² year⁻¹, classifying the Enrique fore-reef and off-reef areas in a net dissolutional state. The combination of thermodynamically-driven depressed aragonite saturation state and high rates of respiration during the summer cause conditions that jeopardize the most soluble carbonate minerals and the free energy in the system for calcification. These data suggest that the reef area and associated ecosystems upstream of the sampling location are experiencing a net loss of CaCO₃, possibly compromising coral ecosystem health and reef accretion processes necessary for maintenance as sea level increases. Resiliency from other climate-scale stressors including rising sea surface temperatures and coral bleaching is likely to be compromised in a system exhibiting net carbonate loss.