Phytoplankton carbon isotope fractionation during a diatom spring bloom in a Norwegian fjord

Abstract

The stable carbon isotope composition (δ13C) of particulate organic carbon (POC) was measured in 3 size fractions (POCtotal, POC>20 µm, POC 20 µm size fraction, which was dominated by diatoms, was ca 2o/oo heavier than that of the 6 µmol l-1). A positive correlation between e p and [CO2(aq)] in the diatom size fraction was obtained for the period of exponential growth. Deviation from this correlation occurred after the peak in cell density and chlorphyll a (chl a) concentration, when POC still continued to increase, and may be related to changing phytoplankton growth rates or to possible effects of nutrient (nitrate) limitation on e p. Comparison of these results with those of previous field studies shows that, while an inverse relationship is consistently observed between e p and the ratio of instantaneous growth rate and CO2 concentration {µi/[CO2(aq)]}, considerable scatter exists in this relationship. While this scatter may have partly resulted from inconsistencies between the different studies in estimating phytoplankton growth rate, it could also reflect that factors other than growth rate and CO2 concentration significantly contribute to determining isotope fractionation by marine phytoplankton in the natural environment