Development and Application of the Carbonate Measurement System to Monitor Ocean pH and Alkalinity

• Main Author: Nand, Vikashni
• Format: Thesis
• Language: English
• Published: The Australian National University 2019
• Subjects: Ocean acidification
• Online Access: https://dx.doi.org/10.25911/5d4d5557d8e30; https://openresearch-repository.anu.edu.au/handle/1885/158103

Anthropogenic carbon dioxide released into the atmosphere is taken up by the oceans. The most profound change has been the decline in ocean pH as it is likely to impact calcifying marine organisms.. In chapter 2, a method utilising purified Bromophenol Blue (BPB) as an indicator to make simple one-point alkalinity measurements via spectrophotometric detection was developed. Our purified BPB dye was characterised over a range of temperatures and salinities and using the absorbance ratio (R (t) = 25 A590/A436) via equation: pH=pK_a +log[((R_([25])-e_1))/((e_2-R_([25]).e_3))] where e1, = 0.00533, e2 = 2.232, e3 = 0.0319, the pH was calculated. The pKa of purified BPB was also determined as 3.513. The method used for the alkalinity of ANU in-house standard and CRM had an uncertainty of +- 1.5 umol kg-1. Comparing the results to unpurified BPB dye, the uncertainty for alkalinity measured was slightly higher approximately plus minus 3-4 umol kg-1. Thus, for experiments that does not require high precision alkalinity measurements (>=4 umol kg-1), then utilisation of the impure BPB could be suitable. The one-point titration method using purified BPB was tested on seawater samples collected from algal flats at One Tree Island, Australia. The method proved to be suitable for producing accurate and precise alkalinity data i.e. approximately 10-15 samples can be determined per hour. In chapter 3 we present the results of open algal pools and the dome experiments conducted on algal ridges to quantify fluxes of calcium carbonate production and dissolution which are isolated from surrounding ocean waters for approximately 3 - 4 hours. The pH and total alkalinity measurements were made for samples collected from algal pools and the dome experiments. During the day, pH increased from 8.1 to 8.7 pH units while alkalinity decreased from around 2200 to 2000 umol kg-1 this is consistent to photosynthesis and high magnesium- calcite production. During the night the opposite was observed, pH decreased from 7.8 to 7.5 and alkalinity increased from around 2300 to 2500 umol kg-1, via respiration and the dissolution of organic matter. The net production and dissolution rates were half of those observed for a nearby back reef when compared to day and night time periods. An automated spectrophotometric system that monitors changes in ocean pH and alkalinity in culture and field experiments simultaneously was developed in chapter 4. The pH-alkalinity spectrophotometric system was calibrated against multiple certified reference materials (CRM) and the in-house ANU standards. High precision range of pH and total alkalinity that is +- 0.0024 - 0.0059 pH units (n = 10) and +-0.84 - 1.07 umol kg-1 (n=10) were obtained respectively. The system was connected to ANU coral reef culturing tank to measure pH and total alkalinity for three months (mid-July, 2015 to mid-October, 2015). The system was also tested at One Tree Island, Australia. The field deployment of the system reveals significant diurnal variability in both pH and alkalinity at One Tree Island. Carbonate chemistry and metabolic processes on coral reefs over time enables the changes in coral calcification rates to be quantified and often used to predict future changes to calcium carbonate production. The pH and total alkalinity measurements will give an understanding of how ocean acidification and other environmental factors will impact coral reefs. Our new automated spectrophotometric pH-alkalinity system is portable and could be easily adapted for in situ measurements on ships and remote locations.