Application of dilution experiments for measuring growth and mortality rates among Prochlorococcus and Synechococcus populations in oligotrophic environments

Dilution experiments were used to examine growth and grazing mortality rates among Prochlorococcus and Synechococcus populations in the Sargasso Sea and California Current. In these experiments, deviation from linearity in the relationship between dilution and net growth rate was significant in a large number of cases. An alternative, more conservative approach for estimating growth and grazing mortality rates (independent of the shape of this relationship) was therefore employed. Growth rates estimated by this approach ranged from 0.32 to 0.76 and 0.37 to 0.67 d-1 for Prochlorococcus and Synechococcus, respectively. Grazing mortality rates ranged from 0.25 to 0.85 and 0.13 to 0.51 d-1, respectively. Cell-cycle-based growth rate estimates were consistent with these dilution-based rates. Nutrient amendment had little affect on picocyanobacterial growth rates, but did stimulate grazing mortality (and in some cases changed the apparent functional response of the grazer community) in a number of experiments. We hypothesize that improved food quality in nutrient-replete picoplankton cells may be responsible for these changes. Diel patterns of picocyanobacterial abundance in the Sargasso Sea experiments suggest that grazing activity varied strongly over the diel cycle, with low grazing activity during the first half of the light period. Growth rate and abundance were not positively correlated among or within picocyanobacterial groups, as might be expected if physiologically mediated controls were the dominant forces regulating these populations.