Development of a low-cost marine pCO2 sensor to characterise the natural variability of coastal carbonate chemistry in the context of global change

• Main Author: Hill, Kirsty Shona
• Format: Thesis
• Language: English
• Published: 2018
• Subjects: G Geography (General); QD Chemistry; Carbonic acid; Ocean acidification
• Online Access: http://theses.gla.ac.uk/30899/; http://theses.gla.ac.uk/30899/1/2018HillPhD.pdf; https://eleanor.lib.gla.ac.uk/record=b3331726

Increasing concentrations of CO2 are being released into the atmosphere from anthropogenic sources which consequently dissolves into the ocean creating carbonic acid. The effect of this is to decrease seawater pH and change the composition of marine carbonate chemistry as a whole. In certain areas of the world’s ocean, there is already a substantial natural variability in carbonate chemistry which, in some cases, can exceed the projected figures for long-term anthropogenic acidification. This is especially true of coastal areas which can be subject to increased human activity but also a larger variation of naturally forced biological activity and hydrographically induced fluctuations of water column properties. Anthropogenic acidification will therefore be layered on top of this natural variability and this could have potentially adverse effects on the marine ecosystem. Studies of coastal areas can aid in ocean acidification research by highlighting how organisms cope under the decreasing levels of alkalinity. Because of this, it is vital that we characterise and quantify the drivers of the natural patterns in the marine carbonate system as well as the anthropogenically forced changes that are now evident. Partial pressure of CO2 (pCO2) is one of the most important parameters to be measured in conjunction with ocean acidification and carbonate chemistry research. High frequency temporal and spatial measurements of pCO2 will provide some understanding of the fluxes and their variability and forcing parameters. To aid the investigation into natural variability of coastal carbonate chemistry, pCO2 sensors are an invaluable tool for ease of in-situ data collection. However, these sensors can require not only specific expertise of utilisation but are also inaccessible to many due to high cost. In lieu of an expensive sensor, the most common way to measure pCO2 in seawater is with discrete sampling of water and subsequent analysis for two of the three parameters of the carbonate system (dissolved inorganic carbon (DIC), Total alkalinity (AT) or pH) which is then used to calculate a final pCO2 value. This method requires a substantial amount of cost, time and labour to not only retrieve seawater from depth, but also employ precise expertise in analyses with each step being potentially fraught with human error. This research addressed these issues by developing a low-cost, easy-to-use sensor which efficiently and accurately measured coastal marine pCO2. This required a research and development stage where the sensor and housing design was tested at The University of Glasgow (Chapter 2 and 3) and also deployed in a temperate (Chapter 4) and tropical (Chapter 5) field environment. Seawater samples were also taken and their carbonate chemistry analysed in conjunction with sensor readings to calibrate and confirm the accuracy of the sensor. Along with the developed sensor and the collection of in-situ pCO2 data, other marine variables were also measured (pH, dissolved oxygen, chlorophyll, salinity, temperature, depth, photosynthetically active radiation, dissolved inorganic carbon and total alkalinity) to obtain a characterisation of the areas and an analysis of the drivers behind these variables. The observed variability in the temperate area of Caol Scotnish, Loch Sween, Scotland was shown to be highly dependent on biological activity and the tidal action which exchanged different water masses into and out of the site. The observed variability in the tropical area of El Quseir, Egypt was shown to be highly dependent on biological activity, temperature and weather events. The sensor coped well in characterising the concentrations of pCO2 in both sites. There is a larger fluctuation of pCO2 in the tropical site than compared with the temperate site which is dictated by the relative hydrography in each area and the particular weather conditions experienced. This research provides industry, scientists and interested parties with a means of monitoring pCO2 levels in the marine environment in an efficient, easy and low-cost manner and contributes to the demand for the development of these sensors to monitor anthropogenically-forced global change which is layered over already in-flux natural carbonate chemistry.