Responses of pico- and nanophytoplankton to artificial iron infusions observed during the second iron enrichment experiment in the western subarctic Pacific (SEEDS II)

How pico- and nanophytoplankton responded to artificial iron infusions was investigated using flow cytometry during SEEDS II, the second mesoscale in situ iron enrichment experiment in the western subarctic North Pacific. Two iron infusions on days 0 and 7 caused a remarkable increase in cellular chlorophyll fluorescence and cell size of all the four phytoplankton groups investigated: Synechococcus, cryptophytes, picoeucaryotes and nanoeucaryotes other than cryptophytes. After the second infusion, the abundance of three phytoplankton groups, excluding Synechococcus, also started to increase. After surface dissolved iron concentration decreased to <0.2 nM on day 11, chlorophyll fluorescence of all the four groups returned to a level observed before the iron infusions, suggesting that pico- and nanophytoplankton were physiologically stressed by iron deficiency. Cell concentrations of pico- and nanoeucaryotes decreased to the pre-infusion level by day 23, while that of cryptophytes remained high until day 25. Flow cytometric diagnosis showed that cryptophytes were physiologically limited during this period and effective iron uptake from suspended particles, as reported for freshwater cryptophytes, was not observed. Thus their prosperity may have been due to alleviation from grazing. Cell concentration of Synechococcus started to increase at a net specific growth rate of 0.13 d−1 after day 12, reaching more than 6 times that of the pre-infusion level on day 24. This may have been due to the elevation of surface water temperature observed during the survey period, together with trophic cascading effects of increased copepod grazing.