Diatom abundance counted on water bottle samples from the EisenEx cruise to the Southern Ocean

The ratios were consistent over time for the various species but varied widely between them. The species-specific variation can be explained by differences in both growth and mortality rates, the latter partly due to either selective grazing or avoidance by the large protozoo- and metazooplankton populations present. Selective predation by the abundant copepod populations on protistan grazers (ciliates and heterotrophic dinoflagellates) of diatoms apparently aided diatom biomass build-up.

The response patterns of populations of the phytoplankton species present fall into 6 categories comprising disparate species, indicating that phylogeny is a poor predictor of ecology. The group that did not respond to fertilization was the most diverse and included both endemic and cosmopolitan as well as background and bloom-forming species. This lack of response to the advent of favorable growth conditions indicates that proximate factors during EisenEx triggered growth only in some species but had little effect on others.

We attribute the differences in behavior to ultimate factors such as seasonal effects on life cycles and other internal constraints on growth rates. The implications for our understanding of the evolutionary ecology of phytoplankton and its impact on global biogeochemical cycles are pointed out.