Global Ocean Spectrophotometric pH Assessment: Consistent Inconsistencies

Seawater physics and chemistry along the Med-SHIP transects in the Mediterranean Sea in 2016

The Mediterranean Sea has been sampled irregularly by research vessels in the past, mostly by national expeditions in regional waters. To monitor the hydrographic, biogeochemical and circulation changes in the Mediterranean Sea, a systematic repeat oceanographic survey programme called Med-SHIP was recommended by the Mediterranean Science Commission (CIESM) in 2011, as part of the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP). Med-SHIP consists of zonal and meridional surveys with different frequencies, where comprehensive physical and biogeochemical properties are measured with the highest international standards. The first zonal survey was done in 2011 and repeated in 2018. In addition, a network of meridional (and other key) hydrographic sections were designed: the first cycle of these sections was completed in 2016, with three cruises funded by the EU project EUROFLEETS2. This paper presents the physical and chemical data of the meridional and key transects in the Western and Eastern Mediterranean Sea collected during those cruises.

Toward Citizen Science-Based Ocean Acidification Observations Using Smartphone Devices

pH is a key parameter in many chemical, biological, and biogeochemical processes, making it a fundamental aspect of environmental monitoring. Rapid and accurate seawater pH measurements are essential for effective ocean observation and acidification investigations, resulting in the need for novel solutions that allow robust, precise, and affordable pH monitoring. In this study, a versatile smartphone-based environmental analyzer (vSEA) was used for the rapid measurement of seawater pH in a field study. The feasibility of the use of the vSEA algorithm for pH quantification was explored and verified. When used in conjunction with a three-dimensional (3D)-printed light-proof shell, the quality of captured images is guaranteed. The quantitative accuracy of vSEA pH measurements reached 0.018 units with an uncertainty of <0.01, meeting the requirements of the Global Ocean Acidification Observing Network (GOA-ON) for "weather" goals (permitting a maximum pH uncertainty of 0.02). The vSEA-pH system was successfully applied for on-site pH measurements in coastal seawater and coral systems. The performance of the vSEA-pH system was validated using different real-world samples, and t-test results showed that the vSEA-pH system was consistent with pH measurements obtained using a state-of-the-art benchtop spectrophotometer (t = 1.986, p = 0.7949). The vSEA-pH system is applicable to different types of smartphone devices, making it possible for vSEA-pH to be widely promoted for public citizen use. The vSEA-pH system offers a simple, accurate, and applicable method for the on-site measurement of seawater pH, assisting the large-scale monitoring of ocean acidification by allowing the contribution of citizen science-based data collection.

Pérez F, Easley RA, Álvarez M. Spectrophotometric Measurement of Carbonate Ion in Seawater over a Decade: Dealing with Inconsistencies

The spectrophotometric methodology for carbonate ion determination in seawater was first published in 2008 and has been continuously evolving in terms of reagents and formulations. Although being fast, relatively simple, affordable, and potentially easy to implement in different platforms and facilities for discrete and autonomous observations, its use is not widespread in the ocean acidification community. This study uses a merged overdetermined CO₂ system data set (carbonate ion, pH, and alkalinity) obtained from 2009 to 2020 to assess the differences among the five current approaches of the methodology through an internal consistency analysis and discussing the sources of uncertainty. Overall, the results show that none of the approaches meet the climate goal (± 1 % standard uncertainty) for ocean acidification studies for the whole carbonate ion content range in this study but usually fulfill the weather goal (± 10 % standard uncertainty). The inconsistencies observed among approaches compromise the consistency of data sets among regions and through time, highlighting the need for a validated standard operating procedure for spectrophotometric carbonate ion measurements as already available for the other measurable CO₂ variables.