Plankton 13C/12C variations in Monterey Bay, California: evidence of non-diffusive inorganic carbon uptake by phytoplankton in an upwelling environment

A test of the hypothesis that photosynthetic 13C fractionation, εp, is affected by variations in diffusive transport of CO2aq into phytoplankton cells was conducted in Monterey Bay over a 4 yr period. Counter to predictions based on diffusive CO2aq transport were observations of very weak to non-existent correlations between εp and: (i) CO2aq concentration, (ii) mean phytoplankton community growth rate, (iii) cell size, (iv) carbon demand, or (v) [CO2aq]-normalized functions of ii–iv. The preceding decoupling between εp and CO2 supply/demand and the low 13C fractionation associated with carboxylation (εf) implied in this study are consistent with several recent reports of active bicarbonate uptake and utilization in similar coastal environments. If the frequent low εp seen here is indicative of such carbon acquisition, it suggests that active bicarbonate transport in situ (1) may not be restricted to diatoms, (2) can be present even in high-[CO2aq] low-growth-rate waters, (3) may be a common feature of low-latitude upwelling environments, and (4) may help explain the common 13C/12C dichotomy between marine planktonic and terrestrial C3 autotrophs.