Analysis of Autonomous Seawater pH Data and Instrument Calibration Verification Method

Abstract

Recently ocean acidification (OA) has received increased attention from scientists due to its potential adverse effects on marine ecosystems, so obtaining an accurate and precise pH measurement in the coastal ocean is critical. This project aims to obtain the most precise pH measurements possible using a pH spectrophotometer, and will use these measurements to correct time series data collected by three PMEL MAPCO2 buoys around Oahu and determine the data’s reliability. Two of the buoys are located on fringing reefs of the south shore and one on the barrier reef in Kaneohe Bay. Other parameters with time series data are then used to analyze influences on the coastal pH around Oahu. For each buoy location, bottle samples were collected in situ at times corresponding to when the SeaFET, a pH instrument attached to the buoy, is programmed to measure pH. Bottle data were used to calibrate the time series data collected by the SeaFET. Spectrophotometrically measured pH differed from the SeaFET measurements by a small, yet significant value. Analysis of the data shows positive correlations between pH and dissolved oxygen at both south shore buoys, as well as a less well defined relationship with chlorophyll and turbidity. There are also clear diel cycles in pH due to photosynthesis, and seasonal changes in the times at which pH peaks each day. The pH data collected at each of the three locations can be considered reliable based on the spectrophotometric measurements, and each of the three locations isa influenced by slightly varying factors.