70. Monitoring Carbonate Chemistry of Algal Ridges at One Tree Island Reef using Spectrophotometric System

Vikashni Nand (1)*, Michael Ellwood (1), Steve Eggins (1), Bill Maher (2), Aero Leplastrier (1)

1 Research School of Earth Sciences, Australian National University, Canberra, ACT, 2601, Australia.
2 Institute of Applied Ecology, Faculty of Applied Science, University of Canberra, Canberra, ACT, 2601, Australia.

Carbon emissions and ocean chemistry are factors that affect the corals and reef systems. One component of coral reefs that has received little attention with respect to ocean acidification is the algal ridges that form the margins of many coral atolls and reefs. These ridges form protective features and are typically found on the windward margins of coral islands and atolls. The principal binding and cementing agent of algal ridges is metastable high-Mg calcite (Mg0.14-0.20Ca0.80-0.86CO3); the most soluble carbonate polymorph and consequently the most susceptible to ocean acidification. The aim of this study was to understand the processes that influence production and dissolution of these on algal ridges.

Here we made spectrophotometric measurements of pH and alkalinity of the algal ridges pools at One Tree Island. A number of dome experiments were conducted to quantify carbonate production and dissolution within these algal pools when isolated from the atmosphere and surrounding waters for approximately 3– 4 hours. The algal turf and red coralline algae communities in these algal ridge pools are subject to large diurnal changes in dissolved inorganic carbon (DIC) and alkalinity along with dissolved oxygen due to the accumulation/consumption of products/reactants of photosynthesis, respiration and calcification. During the day the pools show a strong DIC decrease and dissolved O2 increase and a significant decrease in alkalinity (~100 mol/kg, relative to offshore seawater) that reflects the production of high-Mg calcite by the red coralline algae. The ΔAlk-ΔDIC slope is significantly less than a nearby coral-rich back-reef site, which shows a slope near the theoretical 2:1 value for calcification. At night the reef crest pools become highly oxygen depleted (~40% saturated) despite being in contact with the atmosphere, and accumulate large amounts of metabolic CO2 and alkalinity due to the dissolution of a carbonate phase. The trajectory for increasing DIC and alkalinity is close to the equilibrium saturation state for high-Mg calcite dissolution with 15 mol% Mg. Thus the study contributes to a better understanding of. carbonate dynamics on One Tree Island algal ridges.