The size composition of phytoplankton is an important determinant of the whole community structure, fisheries production and export production. Some field observations suggest that development of ocean stratification evoked by global warming would result in shrinkage of average phytoplankton cell size through decrease in nutrient supply to the surface layer. However, most of those studies were conducted over a wide range of temperatures, ranging from the subtropical to subarctic waters, whereas the increase in the surface water temperature at the end of this century is much smaller. Moreover, scant attention has been paid to subtropical and tropical regions, which are overwhelmingly dominated by pico- and nanophytoplankton and account for contribute to ~40% of global ocean primary production. Therefore, we surveyed the size composition of pico- and nanophytoplankton by flow cytometry in the tropical and subtropical Pacific Ocean, which occupies a vast area of the world ocean, and examined which factors affect it.
The cell size of pico- and nanophytoplankton (Synechococcus, nano-sized cyanobacteria and eukaryotic phytoplankton) showed a clear diel variation, which peaks in the late afternoon. After removing the effect of the diel variation, only the average size of eukaryotic phytoplankton showed a significant relationship with environmental factors, temperature and phosphate concentration. Surprisingly, the correlation was positive with temperature and negative with phosphate concentration, suggesting that more oligotrophic conditions favor larger eukaryotic phytoplankton.
We hypothesized that the mixotrophy could be a key strategy for larger eukaryotic phytoplankton to survive under the oligotrophic condition. Thus we elucidated the latitudinal distribution of autotrophic, mixotrophic and heterotrophic nano-sized flagellated in the central North Pacific Ocean from the equator to the Bering Sea. The proportion of mixotrophic to total plastidic flagellates was higher within the oligotrophic subtropical gyre and negatively correlated with phosphate concentration. This strongly suggests that higher proportion of nanophytoplankton facilitate mixotrophy for survival in the oligotrophic condition, while obligate autotrophic pico- and nanophytoplankton are relatively disadvantageous in such a condition.
The present results show the possibility that global warming would not result in straightforward shrinkage of cell size of primary producers in the subtropical water. The expected complex response of phytoplankton to climate change might be partly due to a variety of nutritional strategies of phytoplankton, including phagotrophy, which should be included in future biogeochemical models.
