pH in the sea surface microlayer

We describe high-resolution in situ observations of pH in the uppermost layer of the ocean, the sea surface microlayer (SML) and compare it to pH measurements in the underlying bulk water at 1 m depth (UWL). Observations were made in the Indo-West Pacific during cruise FK161010 (11 October to 10 November 2016; R/V Falkor). We observed patchy and banded slicks of cyanobacterial surface blooms and rain events in different stations. Normally, the SML is less acidic than UWL by a mean of 0.09 ± 0.02. However, during slicks or shortly after a rainfall event, the SML is more acidic (up to 0.25 units differences). It is important to note, that there is a delay in descending pH values and rainfall, so we can’t associate the drop with the more acidic raindrop water. Furthermore, these drops in pH in the SML correlate with an increase in fluorescence dissolved organic matter. We hypothesis that the acidic functional groups (for example, carboxyl and phenolic groups) of the organic matter might affect the carbonate chemistry equilibrium at the sea-air interface and therefore, lower the pH. As the system always tends to equilibrium, the pH goes back to “normal” relatively quickly. However, this strong drop in pH could have important implications in the assessment of carbonate chemistry over the surface ocean, the air-sea exchange, ocean acidification stress on the organism living there or the speciation of trace element.