Evaluation of autonomously measured alkalinity, pH, and pCO2 variability on a coral reef

Currently, our understanding of alkalinity (AT) variability in highly dynamic
environments such as coral reefs is limited by the dearth of AT measurements. In order to better characterize these environments, high temporal resolution AT data are needed. This work employed the newly developed Submersible Autonomous Moored Instrument for Alkalinity (SAMI-alk), a fully autonomous in situ AT analyzer, to study seawater AT variability. The main goals of this research were to evaluate the utility of combining the SAMI-alk data with currently available in situ measurements of pH and partial pressure of carbon dioxide (pCO2) to characterize the inorganic carbon cycle, and to measure AT variability and determine what drives it on a coral reef. Autonomous AT and pH sensors (SAMI-alk and SAMI-pH) were deployed along with existing pCO2 (MAPCO2) and pH (SeaFET) sensors in Kanoehe Bay, HI from June 4 – 21, 2013. The results show that the pH – AT combination can provide important information about autonomously measured in situ data quality, and that it can be used to fully characterize the inorganic CO2 system in seawater. The SAMI-alk data were also used to examine AT variability and thereby calcification rates on coral reefs in Kaneohe Bay. AT varied by more than 100 µmol kg-1 on a diel basis due to CaCO3 production and dissolution. Dissolved inorganic carbon (DIC), calculated from the pH – AT sensor pair, varied by more than 200 µmol kg-1 , due primarily to biological metabolism on the reef. Reef calcification and metabolism dramatically alter the seawater chemistry from the open ocean source water and drive the large diel changes in all measured inorganic carbon parameters (i.e. aragonite saturation state (Ωarag), pH, pCO2, AT, DIC). This data set demonstrates the value of a high-quality in situ AT analyzer in a coral reef environment; making it possible to determine combined CO2 system variability with unprecedented temporal resolution. These data show that NEC can be consistently sustained (net CaCO3 production) until a threshold level of net respiration (NEP) is reached, around -50 (mmol m-2 h-1), which corresponds to an AT : DIC ratio of about 1:1.

Peterson B., 2018. Evaluation of autonomously measured alkalinity, pH, and pCO2 variability on a coral reef. MSc thesis, University of Montana, 97p. Thesis.

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