Responses of Southern Ocean phytoplankton to the addition of trace metals

Trace metal enrichment experiments were performed under ultraclean conditions with natural oceanic plankton populations in the Southern Ocean along 6°W in October–November 1992. Five Fe-enrichment experiments were conducted, as well as a further eight experiments with single or combined addition of Fe, Mn, Co and Zn. Water was incubated from the Polar Frontal region at the beginning and again later in the middle of a bloom, from the Antarctic Circumpolar Current south of the Front, from the sea-ice edge area and from the ice-covered northern Weddell Sea. Growth responses of phytoplankton to the addition of Mn (2.5 nM), Co (0.4 nM) or Zn (2 nM) were not very clear (slight enhancement), which may partly have been due to low initial biomass. Growth enhancements by Fe (2 nM) were more pronounced, albeit only where initial biomass was relatively high and where distinct growth in the controls was observed simultaneously. Overall, rates of chlorophyll a increased and final yields increased, relative to the controls, by about 10% to more than 50%. Additions of 5 and 10 nM Fe yielded about 10% higher specific growth rates than additions of 2 nM. Combining Fe with one or more of the other trace metals did not result in significantly higher yields or growth rates as with Fe alone, even at high initial biomass. Moreover, the accumulation of cellular 55Fe over time was neither enhanced nor suppressed by Mn, Co, and Zn, further suggesting that co-limitation by the other metals did not occur. Fe is evidently the most important trace metal controlling phytoplankton development. Microzooplankton grazing was determined (dilution method) at the end of one 9 day experiment where a very clear response by phytoplankton was observed following addition of 2 nM Fe. Gross production and grazing rates (as chlorophyll) were 0.71 and 0.34 day−1, respectively, in the control and 0.89 and 0. 30 day−1 in the Fe enrichment. Apparently only algal production was stimulated by Fe enrichment. We conclude that in the Southern Ocean biomass build-up by phytoplankton is limited by decreasing availability of trace metals, notably Fe, under conditions of sufficient light and low grazing pressure.