Re-evaluating total alkalinity (TA) and dissolved inorganic carbon (DIC) relationships in dynamic coral reef systems

Tyler Cyronak1, Yuichiro Takeshita2, Kai Schulz3, Rebecca Albright4, Ken Caldeira5, Bradley Eyre3, Alyssa Jean Griffin6, David Koweek7, Manoela Romanó de Orte7 and Kennedy Wolfe8, (1)Georgia Southern University, Institute for Coastal Plain Science, Statesboro, GA, United States, (2)Monterey Bay Aquarium Research Institute, Moss Landing, CA, United States, (3)Southern Cross University, Centre for Coastal Biogeochemistry, Lismore, NSW, Australia, (4)California Academy of Sciences, San Francisco, CA, United States, (5)Carnegie Institution for Science, Department of Global Ecology, Stanford, United States, (6)Scripps Institution of Oceanography, La Jolla, United States, (7)Carnegie Institution for Science, Department of Global Ecology, Stanford, CA, United States, (8)University of Queensland, Brisbane, QLD, Australia
Abstract:
Vector analysis of seawater total alkalinity (TA) and dissolved inorganic carbon (DIC) measurements can provide insight into the biogeochemical processes occurring within an ecosystem. The slope of a linear regression through TA and DIC data is often used to infer the ratio between net ecosystem calcification (NEC) and production (NEP) in coral reefs. However, we find that this slope does not generally indicate NEC:NEP ratios, and that NEC and NEP do not necessarily exhibit a constant ratio through time and/or space.

This study uses a combination of high-resolution TA and DIC measurements from a coral reef flat near Lizard Island, Australia and numerical modelling to demonstrate how linear regressions of TA and DIC data do not necessarily reflect ratios of NEC:NEP. Measured ratios of NEC:NEP varied from -0.53 to 0.35 over an average diel cycle at Lizard Island. However, NEC:NEP ratios were most unstable during dusk and dawn, and stabilized at relatively constant, but different values during the day (0.25) and night (-0.01). This most likely reflects different processes dominating the carbon cycle during the day (e.g., photosynthesis and calcification) and night (e.g., respiration and CaCO3 dissolution).

Modelling results suggest that linear regressions of TA and DIC measurements taken from within an ecosystem cannot be used to infer absolute NEC:NEP ratios unless the starting conditions, such as offshore TA and DIC, are known. Instead, TA and DIC regressions using within-system measurements are informative of the temporal and spatial dynamics of biogeochemical processes. In ecosystems where residence times are low (<3-4 hours), measurements of TA and DIC made during different times of the day could be informative of coefficients from photosynthesis- and calcification- irradiance curves. Monitoring changes to these metabolic irradiance curves through time could be useful for detecting long-term changes in the carbon cycle of photic ecosystems. Our study stresses the need for a more comprehensive set of measurements to characterize and understand the dynamics of NEC and NEP in coral reef systems.

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