Potential causes for the unequal contribution of picophytoplankton to total biomass and productivity in oligotrophic waters

Size-fractionated chlorophyll a and primary production measurements during 7 latitudinal cruises (56 stations) in the Atlantic Ocean indicate that in subtropical and tropical regions the relative contribution of large (L) (> 2 μm) phytoplankton to total (T) chlorophyll a biomass (B) is smaller (BL:BT ratio = 0.25 ± 0.01 SE) than their contribution to total primary production (P) (PL:PT ratio = 0.45 ± 0.02). This implies a lower assimilation number for picoplankton than for larger cells: 1.5 ± 0.2 and 3.7 ± 0.3 mg C mg chlorophyll a h^-1, respectively (n = 90). During short-term time-course experiments in subtropical and tropical waters of the North Atlantic, a > 50% reduction in the abundance of picoplankton, mainly Prochlorococcus spp., took place as early as the first 2 h of incubation. This suggests that long incubation times in bottle experiments can lead to underestimations of primary production rates, at least in oligotrophic waters. The phytoplankton size-structure remained fairly constant throughout the experiments, suggesting that the high PL:PT ratios in these environments cannot be due to enhanced loss rates of small-sized phytoplankton as a result of bottle enclosure. Our results indicate that the observed disagreement between BL:BT and PL:PT ratios is not necessarily an experimental artifact, but could be related to a higher light utilization efficiency of larger phytoplankton.