Continuous monitoring of seawater pH and pCO2 in a temperate estuary

Chair: Libby Jewett

John Runcie (1)

1 Aquation Pty Ltd, Umina Beach, NSW, 2257, AUSTRALIA

Background
Understanding the carbon chemistry of seawater is essential to ocean acidification research. The drivers of small-scale and short-term changes in nearshore carbon chemistry and their interactions are complex. Continuous measurements that identify fluctuating chemistry can help in the identification and characterisation of these drivers. Recent research efforts to continuously measure seawater carbon chemistry have focussed on pH determination using ISFET and spectrometric techniques. Here we present results of an alternate approach to pH measurement using fluorescence. In addition we also report pCO2 determinations using the same fluorescence-based technology.

Methods
Specially made artificial substrates respond to changes in pH and pCO2 by changes in the lifetimes of fluorescence decay. These changes are compared against the lifetimes of an invariant reference. Both pH and pCO2 measurements were made from a single device located in an estuary in NSW, with virtually simultaneous measurements made regularly for several weeks.

Findings
The fluorescence-based technique provided a useful time-series for both pH and pCO2 of surface waters in the estuary. Variations over the course of the day and night were subtle but could reasonably be distinguished from background variability. Drift due to photodegradation of the fluorophore was kept to a minimum by minimising exposure to ambient light. Seawater carbon chemistry composition was calculated using the measured parameters.

Conclusions
The fluorescence lifetimes-decay approach used here to measure both pH and pCO2 in situ provides sufficient information to calculate the carbon chemistry of ambient seawater. The simplicity of the technique is attractive and means it is less susceptible to mechanical failure. An additional potential advantage of this approach is an independence to external pressure, however this will be examined in detail in a separate future study.