Nutrient and temperature control of the contribution of picoplankton to phytoplankton biomass and production

The observation that the relative importance of picophytoplankton is greate st in warm and nutrient-poor waters was tested hen based on a comprehensive review of the data available in the literature from oceanic and coastal es tuarine areas. Results show that picophytoplankton dominate (greater than o r equal to 50%) the biomass and production in oligotrophic (chlorophyll a [ Chl a] < 0.3 mg m(-3)), nutrient poor (NO3 + NO2 < 1 mu M), and warm (>26 d egrees C) waters, but represent <10% of autotrophic biomass and production in rich (Chl a > 5 mg m(-3)) and cold (<3 degrees C) waters. There is, howe ver, a strong covariation between temperature and nutrient concentration (r = -0.95, P < 0.001), but the number of observations where both temperature and nutrient concentrations are available is too small to allow attempts t o statistically separate their effects. The results of mesocosm nutrient ad dition experiments during summer in the Mediterranean Sea allowed the disso ciation of the effects of temperature from those of nutrients on picophytop lankton production and biomass and validated the magnitude at which picopla nkton dominates (greater than or equal to 50%) autotrophic biomass and prod uction obtained in the comparative analysis. The fraction contributed by pi coplankton significantly declined (r(2) = 0.76 and 0.90, respectively, P < 0.001) as total autotrophic production and biomass increased. These results support the increasing importance of picophytoplankton in warm, oligotroph ic waters. The reduced contribution of picophytoplankton in warm productive waters is hypothesized here to be due to increased loss rates, whereas the dominance of picophytoplankton in warm, oligotrophic waters is attributabl e to the differential capacity to use nutrients as a function of difference s in size and capacity of intrinsic growth of picophytoplankton and larger phytoplankton cells.