An application of the AquapHOx-Logger was demonstrated by continuous monitoring of pH and dissolved oxygen at the coast of Rovinj, Croatia. The sensors were deployed on a pier in approximately 2 m water depth and measured continuously with a sampling interval of 2 minutes for 8 weeks during June, July and August 2019 (40 000 data points).

Set-Up and Study Site

The AquapHOx Logger is a stand-alone optode for monitoring of pH or dissolved oxygen. It contains a battery and data storage for deployments of up to one year. In this case study we demonstrate the application of this device to monitor pH and dissolved oxygen at the coast of Rovinj, Croatia. Six devices were deployed in two packages for 8 weeks in the summer of 2019 and recorded approximately 40 000 data points each.

Every package consisted of one oxygen and two pH sensors. All sensors were 2-point calibrated at the start, after 3 weeks and after 8 weeks (end) of the deployment. The two packages were mounted on two columns of a jetty in approximately 5 m distance.

The water was warm (25 to 29°C) and rich in nutrients. This led to heavy biofouling during the deployment. A copper-guard was used to protect the sensors but was not sufficient to fully prevent biofouling (Fig. 2). Multiple samples for spectrophotometric reference measurements were taken at the start, after 3 weeks and at the end of the deployment.

Results and Discussion

In Fig 3 the results of the deployment are presented. The optodes recorded the day-night fluctuations of pH and dissolved oxygen over 8 weeks, with good correlation between both analytes. The results of the pH-Optodes were drift corrected by interpolating the calibrations (drift ca. 0.005 /day). The oxygen results are displayed without any drift correction.

In Fig 4 the deviation of the pH-Optodes to the collected samples is displayed. The deviation is low at the beginning of the experiment and increases at the end. This is probably caused by biofouling of the sensor materials.

Fig 5 shows an interesting result of the deployment. At a distance of only 5 m a strong variation of pH and dissolved oxygen could be observed.

Conclusion and Outlook

In conclusion the application of the AquapHOx-Logger for monitoring of pH and dissolved oxygen was demonstrated successfully over 8 weeks. Every sensor recorded with a sampling interval of 2 minutes (approximately 40 000 data points total). The pH-sensors and oxygen sensors were in good agreement and showed good correlation.

Interestingly, strong spatial differences in pH were observed already at a distance of 5 m. This was only possible because the weather was very calm and the measurement was conducted close to the shore. Nevertheless, this demonstrates the advantages of a close mesh of sensors and highlights the disadvantages of sampling.

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