Geochemical Evidence of Hourly Ecosystem Photosynthetic Plasticity within a Pristine Coral Atoll - Supplementary Material
Description
Here, we use co-located measurements of total dissolved inorganic carbon, total alkalinity, and the stable carbon isotope composition of DIC (δ13C-DIC) over a 27+ hour period from Tetiaroa Atoll, French Polynesia to isolate steady-state (i.e. near dusk and near dawn) values for the photosynthetic fractionation factor between stable carbon isotopes in DIC and organic matter, εorg. We then interpret these isolated values in the context of ambient carbonate chemistry to investigate εorg as a proxy for monitoring the composition and behavior of primary producers within coral reef ecosystems. Quantitatively decomposed values of steady-state εorg over the sampling period (-13.9 ± 2.8‰ ≤ εorg ≤ -4.5 ± 1.8‰) suggest that inorganic carbon physiology of the reef algal community is plastic on daily timescales or longer, potentially as functions of ambient CO2 and acidity within the ecosystem. While more work is needed to understand the applicability of our approach to reef ecosystems of a variety of morphologies and benthic community compositions and on a variety of timescales, we argue these results support the “proof-of-concept” isolation of εorg as a geochemical fingerprint of primary producer trophic dynamics and community composition within and between coral reef ecosystems – particularly as environmental stressors compound. Data is organized with explanatory column headers. Total alkalinity (µmol*kg-1) was determined through open-cell automated titration (876 Dosimat plus, Metrohm AG) with a solution of 0.1M hydrochloric acid (HCl) + 0.6M sodium chloride (NaCl). Total dissolved inorganic carbon (µmol*kg-1) was obtained through coulometric determination (VINDTA 3D, Marianda with UIC coulometer). Remaining carbonate system parameters were calculated via CO2SYS v2.1 for Microsoft Excel using the total pH scale with carbonate equilibrium constants refit from Mehrbach et al. (1973) by Dickson and Millero (1987); borate alkalinity was calculated using the boron/chlorinity (salinity) relationship provided by Uppström (1974) and equilibrium constants from Dickson (1990). Long-term precision on DIC and TA based on repeated measurements of CRM materials was ± 3.7 µmol*kg-1 (n = 46; 2σ std. dev.) and ± 4.3 µmol*kg-1 (n = 41; 2σ std. dev.), respectively. δ13C of DIC (in the form of CO2) extracted from the seawater was measured at the University of Washington School of Oceanography via mass spectrometry according to the procedure described in Quay et al. (Quay et al., 1992). The reproducibility of all δ13CDIC values based on replicate internal seawater standard measurements is ±0.1‰ (1σ std. dev.). Temperature (± 0.005ºC) and salinity (± 0.005) at the sampling site were obtained using an SBE 16plus V2 SeaCAT CTD Recorder.