Development and deployment of an in situ sensor for seawater pH based on the spectrophotometric method

Abstract

The uptake of anthropogenic CO2 by the oceans since the onset of the industrial revolution is considered to be a serious challenge to marine ecosystems due to ensuing carbonate chemistry changes (ocean acidification). Furthermore, the CO2 uptake is reducing the ocean’s capacity to absorb future CO2 emissions. In order to follow the changes in the ocean’s carbonate system, high-quality analytical measurements with good spatial and temporal resolution are necessary. The spectrophotometric pH technique is now widely used and capable of the required high-quality measurements. Smaller and more rugged instruments are nevertheless required for a more widespread in situ application to allow routine high resolution measurements even in the most remote regions. In this thesis, a simple micro-fluidic design integrated in a shipboard instrument featuring high accuracy and precision is presented as a key step toward a targeted pH micro-sensor system. The system is particularly adapted to shipboard deployment: high quality data was obtained over a period of more than a month during a shipboard deployment in northwest European shelf waters, and less than 30 mL of indicator was consumed. The system featured a short term precision of 0.001 pH (n=20) and an accuracy within the range of a certified Tris buffer (0.004 pH). The quality of the pH system measurements have been checked using various approaches: measurements of certified Tris buffer, measurement of certified seawater for DIC and TA, comparison of measured pH against calculated pH from pCO2, DIC and TA during the cruise in northwest European shelf waters. All showed that our measurements were of high quality. The optical set up was robust and relatively small due to the use of an USB mini-spectrometer, a custom made polymeric flow cell and an LED light source. Finally, the pH data measured in the North West European Shelf Seas in summer 2011 is used to study the carbonate chemistry dynamics of the Shelf Seas surface water. A statistical approach is used to investigate which processes affect pH and their relative importance in explaining the observed pH variance along the ship’s transect. The study highlighted the impact of temperature, biological activity and riverine inputs on the carbonate chemistry dynamics of the shelf sea surface water.

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