Contribution of phytoplankton and bacterial cells to the measured alkalinity of seawater

We report the first direct measurements of the contribution of phytoplankton and bacterial cells to the measured alkalinity of unfiltered seawater. Phytoplankton and bacterial cells suspended in seawater make a significant contribution to the measured alkalinity of unfiltered seawater; their contribution is probably next to that of borate ion in most seawater samples. This nonnegligible contribution of particulate organic matter to the measured alkalinity is due largely to the presence of negatively charged surface groups on the phytoplankton and bacterial cells that react with protons during titration with hydrochloric acid. The contribution of organic particles to the measured alkalinity of unfiltered seawater could potentially be an important factor when evaluating the accuracy of presently available carbonate thermodynamic models using at-sea carbon system parameters that include measured alkalinity. The concept of the total alkalinity of seawater has evolved in tandem with improvements in analytical techniques and in our understanding of the acid–base processes that occur in seawater (Dickson 1992). Seawater is not a simple solution containing only carbonic and boric acids; it often contains additional bases that react with protons during the titration of seawater with acid. The total alkalinity of seawater (AT) is thus defined as the number of moles of hydrogen ions equivalent to the excess of bases (proton acceptors) formed from weak acids with pK $ 4.5 at 258C and zero ionic strength over acids (proton donors) with pK , 4.5 in 1 kg of seawater sample (Dickson 1981), 22 2 2 22 A 5 [HCO ] 1 2[CO ] 1 [B(OH) ] 1 [OH ] 1 [HPO ] T 3 3 4 4 32 2 2 1 2[PO ] 1 [SiO(OH) ] 1 [NH ] 1 [HS ] 1 · · · 4 3 3 1 2 2 [H ] 2 [HSO ] 2 [HF] 2 [H PO ] 2 · · · (1) F 4 3 4 where all concentrations are micromoles per kilogram (mmol kg21) and [H]F is the concentration of free hydrogen ions. For most seawater samples, HCO , CO , and B(OH) are 2 22 2 3 3 4 the most important bases that contribute to AT. In open ocean water, the concentrations of NH3 and HS are usually so low that these species make an insignificant contribution to AT; however, their contributions might be significant in anoxic water. The ellipses represent additional minor acid or base species that are either unidentified or are present in insig1 Corresponding author ([email protected]).