General study design:
Seawater samples and environmental measurements were collected across the Heron Island coral reef during three reef-scale surveys in the morning and evening in October of 2015. Seawater samples were analyzed for dissolved inorganic carbon chemistry parameters. The study was designed to characterize the natural spatio-temporal variability of carbonate chemistry and environmental parameters across the entire coral reef system. The spatial surveys were complemented with autonomous sensors making high frequency measurements at three locations.

Methods description:
Seawater samples for carbonate chemistry analysis were collected from 0.5 m depth using a bucket and line at 28 stations inside the Heron reef lagoon and 1 station outside of the reef. Seawater samples were collected in 250 ml Pyrex Corning glass bottle and immediately poisoned with 100 μL HgCL₂ following standard protocols (Dickson et al., 2007). Seawater temperature, salinity, and dissolved oxygen were also measured at each sampling location using a handheld YSI multiprobe. Each survey took approximately 2 hours to complete.

Analytical Methods:
All seawater samples were transported to the Scripps Coastal and Open Ocean Biogeochemistry lab and analyzed for TA via an open-cell potentiometric acid titration system developed at Scripps Institution of Oceanography (SIO) by A. Dickson (Dickson et al. 2007) and DIC via an automated infra-red inorganic carbon analyzer (AIRICA, Marianda Inc).

Quality Control:
Samples for seawater carbon chemistry analysis were collected and analyzed following standard protocol (Dickson et al., 2007). The handheld YSI multiprobe was calibrated prior to each survey with and accuracy of ±0.2°C for temperature and 1% for salinity. The accuracy and precision for DIC and TA samples were - 0.86 ± 1.78 and 1.37 ± 2.79, respectively and were evaluated using CRMs provided by the laboratory of A. Dickson at SIO. CRMs were analyzed every 5 samples for DIC and every 10 for TA.