Technical note: Continuous fluorescence-based monitoring of seawater pH in situ

John W Runcie,Maria Byrne,Sergio A Torres Gabarda,Christian Krause

doi:10.5194/bg-15-4291-2018

Abstract

Abstract. Electrical conductivity (salinity), temperature and fluorescence-based measurements of pH were employed to examine diel fluctuations in seawater carbonate chemistry of surface waters in Sydney Harbour over two multiple-day periods. A proof-of-concept device employing the fluorescence-based technique provided a useful time series for pH. Alkalinity with pH and temperature were used to calculate the degree of calcite and aragonite saturation (ΩCa and ΩAr, respectively). Alkalinity was determined from a published alkalinity–salinity relationship. The fluctuations observed in pH over intervals of minutes to hours could be distinguished from background noise. While the stated phase angle resolution of the lifetime fluorometer translated into pH units was ±0.0028 pH units, the repeatability standard deviation of calculated pH was 0.007 to 0.009. Diel variability in pH, ΩAr and ΩCa showed a clear pattern that appeared to correlate with both salinity and temperature. Drift due to photodegradation of the fluorophore was minimized by reducing exposure to ambient light. The ΩCa and ΩAr fluctuated on a daily cycle. The net result of changes in pH, salinity and temperature combined to influence seawater carbonate chemistry. The fluorescence-based pH monitoring technique is simple, provides good resolution and is unaffected by moving parts or leaching of solutions over time. The use of optics is pressure insensitive, making this approach to ocean acidification monitoring well suited to deepwater applications.

Highlights

Ocean carbon chemistry is predicted to vary in response to an elevated concentration of atmospheric carbon dioxide (CO2), with a decline in pH and an increase in the partial pressure of dissolved CO2 over the coming decades
Seawater carbonate chemistry in shallow nearshore environments is more likely driven by a combination of biological and local hydrodynamic processes (Santos et al, 2012) with typical variation at timescales ranging from minutes to days
The phase angle resolution of the lifetime fluorometer was stated by the manufacturer as 0.05◦, and this translated to an instrument resolution of 0.0028 pH units when calibrated against the phosphate buffer mixtures

Summary

Introduction

Ocean carbon chemistry is predicted to vary in response to an elevated concentration of atmospheric carbon dioxide (CO2), with a decline in pH and an increase in the partial pressure of dissolved CO2 (pCO2) over the coming decades. Seawater carbonate chemistry in shallow nearshore environments is more likely driven by a combination of biological (photosynthesis, respiration) and local hydrodynamic (tidal, low salinity surface- and/or ground-water input) processes (Santos et al, 2012) with typical variation at timescales ranging from minutes to days. Capturing these short-term fluctuations requires virtually continuous monitoring of water chemistry. Recent advances in electrode design including the incorporation of double- (or even quadruple-) junction salt bridges have enabled the continuous use of glass pH electrodes in shallow marine waters for over 12 months (Ionode Pty Ltd, Australia, Matthew Newman, personal communication, 2015)

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