Effects of iron enrichment on phytoplankton in the Southern Ocean during late summer: active fluorescence and flow cytometric analyses

Eight shipboard iron-enrichment experiments were carried out during the late summers of 1997 and 1998 in the Ross Sea and the Polar Front, respectively, as part of the US JGOFS Southern Ocean program. Using active fluorescence techniques (pump-during-probe flow cytometry/microfluorometry and fast repetition rate fluorometry) and flow cytometry, we examined responses of phytoplankton to iron enrichment over time scales of days. Results of both individual cell and bulk water measurements suggest that physiological iron limitation was widespread in the Ross Sea gyre in the late summer, but that in the region just south of the Polar Front other factors were limiting phytoplankton growth. In the five experiments in which responses to enrichment occurred, all the phytoplankton groups we examined, with the exception of cryptophytes, responded to iron enrichment by increasing normalized variable fluorescence (Fv/Fm) over several days. Normalized variable fluorescence of cryptophyte cells was typically higher than that of other cells and often near the maximum observed. Significant correlations were observed between ambient iron concentrations and normalized variable fluorescence at the beginning of each experiment, and also between ambient iron and the response of normalized variable fluorescence to enrichment. These relationships, which have not been previously documented, support the use of ambient active fluorescence measurements to predict iron-limiting conditions without conducting incubations.