The contributions of phytoplankton and bacteria cells to alkalinity (A_T) were measured in seawater samples obtained from 205 locations including the East Sea, the North Pacific Ocean, the Bering Sea, the Chukchi Sea, and the Arctic Ocean. We attributed the differences in A_T values measured for unfiltered versus filtered samples to A_T components contributed by phytoplankton (retained on a 0.7 µm filter) and by phytoplankton and bacteria combined (A_T-BIO; retained on a 0.45 µm filter). The A_T-BIO values reached 10-19 μmol kg^-1 in the East Sea and the North Pacific Ocean, and progressively decreased to a level of 1 μmol kg^-1 with distance towards the Arctic Ocean. The study shows that the A_T-BIO values are non-negligible in coastal and open ocean environments and need to be considered when assessing the accuracy of carbon parameters calculated using the thermodynamic models that use measured A_T as an input parameter.

Sampling locations

Sampling was carried out on board the R/V ARAON during the study from 17 July 2020 to 15 September 2020. The sampling locations were approximately evenly spaced along a transect from the East Sea to the North Pacific Ocean, the Bering Sea, the Chukchi Sea, and the Arctic Ocean. A total of 205 surface seawater samples were collected in 4-L polycarbonate bottles from the underway clean seawater line. To obtain depth profiles, a total 24 discrete samples were collected in 1-L high density polyethylene bottles using a rosette sampler (SeaBird Electronics, SBE 911 plus) at two sites (76.003ºN, 170.588ºW; 74.499ºN, 162.262ºW) in the Arctic Ocean.

Determination of A_T, A_T-BIO, A_T-PLANKTON, and A_T-BACTERIA

For each seawater sample the A_T values of an unfiltered and a filtered (0.7 μm and 0.45 μm) sample were measured within 12 h of collection. The total contribution of all biogenic particles to the A_T of the unfiltered seawater (A_T-BIO; Equation 2) was determined as the difference between the measured A_T values of the unfiltered sample and the 0.45 μm-filtered sample, followed by subtraction of the CaCO₃ contribution (A_T-PIC), which was calculated from the particulate inorganic carbon (PIC) concentration determined as described in Text S1. In this calculation we assumed that the particulate matter contribution to A_T mostly comprised phytoplankton and bacteria cells, and CaCO₃ shells. The contribution of phytoplankton cells to A_T (A_T-PLANKTON; Equation 3) was determined as the difference in measured A_T between the unfiltered sample and the 0.7 μm-filtered sample, followed by subtraction of A_T-PIC. The contribution of bacteria (A_T-BACTERIA; Equation 4) was determined by calculating the difference in measured A_T values between the 0.7 μm-filtered sample and the 0.45 μm-filtered sample.

          A_T-BIO = A_T-PLANKTON + A_T-BACTERIA = A_T-unfiltered – A_{T-0.45 μm-filtered} – A_T-PIC      (2)

          A_T-PLANKTON = A_T-unfiltered – A_{T-0.7 μm-filtered} – A_T-PIC                                      (3)

          A_T-BACTERIA = A_{T- 0.7 μm-filtered} – A_{T-0.45 μm-filtered}                                           (4)

The precisions of measurements for three biological A_T components were estimated from the root sum square of the standard deviations of all parameters involved ( ; see those relevant parameters in Equations 2, 3, 4, and Text S2) and were found to be 1.1 μmol kg^-1 for A_T-BIO, A_T-PLANKTON, and A_T-BACTERIA. Note that the A_T-PIC component was not considered in the estimation of the precisions for A_T-BIO and A_T-PLANKTON because measurable PIC values were found only for 6 of the 205 samples.

For more details, see the methods section in the manuscript "Contribution of marine phytoplankton and bacteria to ocean alkalinity: an uncharacterized component".