Temporal stability of marine phytoplankton in a subtropical coastal environment

We investigated the temporal stability of phytoplankton at a subtropical coastal site for 9 months by conducting chlorophyll and flow cytometric measurements at relatively high frequency (roughly at 2–5 day interval). Phytoplankton cells were grouped based on their sizes obtained from flow cytometric signals. We also conducted dilution experiments to estimate the growth and grazing mortality rates of different phytoplankton groups to assess whether the temporal stability of phytoplankton abundances was related with phytoplankton growth/grazing rates. Based on size-fractionated chlorophyll measurements, there was some indication that smaller phytoplankton cells were more stable than larger ones. However, by cytometric counting, there was no evidence for greater stability in small cells. Synechococcus, which had the lowest stability and dominated the <1 μm size class, showed a strong seasonal cycle that was highly dependent on temperature whereas eukaryotes did not have evident seasonal cycles. In general, biomass of a phytoplankton group consisting of several size classes was more stable than that of its sub component, consistent with the hypothesis that higher diversity leads to higher stability, probably related with the effect of statistical averaging (portfolio effect). Stability of heterotrophic bacteria was much higher than that of phytoplankton, leading to the speculation that bacteria were more diverse than phytoplankton. Phytoplankton stability was not related with their growth or grazing mortality rates. Our study suggests that species diversity should be taken into account when considering the temporal stability of phytoplankton.