Relationships between phytoplankton growth and cell size in surface oceans: Interactive effects of temperature, nutrients, and grazing

We compile two data sets from C-14 uptake and dilution experiments conducted in surface waters of the global ocean to investigate the relationship between phytoplankton mass-specific growth rate and cell size. After temperature correction, both data sets suggest that this relationship might be described by a unimodal quadratic curve with the modal size (the size corresponding to the maximal growth rate) being 2.8 and 5.4 mu m in the C-14 and dilution data sets, respectively. Nutrient enrichment does not change the qualitative nature of the relationships, and we conclude that inherently low maximal growth rates of picophytoplankton, not ambient nutrient effects, play the major role in determining the positive relationships over the size range where phytoplankton size is below the modal size. Temperature-corrected phytoplankton grazing mortality rate is positively correlated with phytoplankton average size, but the proportion of daily primary production consumed by microzooplankton is negatively correlated with cell size, suggesting a reduced grazing effect as size increases. The unimodal relationship between phytoplankton growth rate and cell size is consistent with theoretical considerations and might reflect an adaptive response of phytoplankton to varying extents of nutrient limitation and grazing effect in marine systems.