Enrichment of alkanes within a phytoplankton bloom during an in situ iron enrichment experiment in the western subarctic Pacific

Abstract

During the Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study ΙΙ (SEEDS-II), we monitored variations in the concentrations of non-methane hydrocarbons (NMHCs), CH 3Cl, N 2O, and CH 4 within a phytoplankton bloom. Stable isotopic compositions were also determined to evaluate the sources of the variations. Although there was little variation in either the concentrations or the stable isotopic compositions of alkenes, CH 3Cl, N 2O, and CH 4 during the 23-day observation period, alkane concentrations increased substantially as the phytoplankton bloomed. The column-integrated quantities of alkanes increased to 3 times pre-bloom levels for C 2H 6, 5 times for C 3H 8, and 20 times for n-C 4H 10. The δ 13C values of both C 2H 6 and C 3H 8 remained almost constant while concentrations increased, whereas that of n-C 4H 10 increased by about 12‰. To evaluate the sources of the alkanes produced during the bloom, we compared their δ 13C values with those of alkanes produced in axenic phytoplankton cultures in our laboratory. We concluded that during the SEEDS-ΙΙ experiment the major portions of C 2H 6 and C 3H 8 were produced during the autolysis of diatoms cells, whereas n-C 4H 10 was produced during autolysis of other phytoplankton cells such as cryptophytes and dinoflagellates.