Degradation process of shell density of marine calcified plankton in different pH conditions in the North Pacific

Decreasing of oceanic pH by ocean acidification gives severe impact to marine ecosystems. Especially decreasing of carbonate ions induces imperfect developing of marine calcifiers, but its impact on the pelagic ocean is still less known. In 2016, we deployed a bio-geochemical mooring with the Hybrid pH Sensor (HPHS, glass electrode pH sensor with colorimetric pH sensor) and sediment traps through a year at St K2 (47°N 160°E), western North Pacific in order to observe biological impacts and in-situ pH. HPHS and sediment trap were deployed at 200 m and 500 m water depths, respectively. We focused on the shells of single species of planktic foraminifer (Globigerina bulloides) and thecosomatous pteropod (sea butterfly, Limacina helicina) who have calcium carbonate skeleton. Planktic foraminifers and pteropods have shells which are made by calcite and aragonite, respectively, therefore they are suitable to monitor the behavior of different biominerals in the water column. Their dead shells quickly sink to deeper water as sinking particles and are captured into the cup of sediment trap and fixed in buffered formalin.

Sinking particles were completely recovered through the year from June 2016 to July 2017. Shell morphometrics of planktic foraminifers and pteropods was analyzed by the Microfocus X-ray CT (MXCT) and the Scanning electron microscope (SEM). In summer which was observed higher pH conditions, shell bulk densities of both species were higher. In contrast, in winter which was observed lower pH conditions, shell bulk densities were decreased 37 ~ 46 %, and sometimes pteropod shells had been severely damaged and/or completely lost. It indicates that shell degradation has close relationship with pH condition in the water column.