Phytoplankton growth and grazing mortality through the oligotrophic subtropical North Pacific

The subtropical North Pacific has been historically considered as a stable and homogenous oligotrophic marine ecosystem. The consistently low phytoplankton biomass has been attributed to a close balance between phytoplankton growth and grazing mortality. However, phytoplankton summer blooms were frequently observed in the central North Pacific near the Hawaiian Islands. To determine whether this is a result of unbalanced phytoplankton growth and microzooplankton grazing, we conducted a trans-Pacific cruise through the subtropical North Pacific. The growth and microzooplankton grazing mortality rates of the phytoplankton community and specific groups in the surface layer (10 m), were examined by dilution experiments. Positive phytoplankton net growth rates (0.34 +/- 0.29 day(-1)) were observed under the depletion of dissolved inorganic nitrogen (DIN, < 40 nM) throughout the study area. However, considering the grazing of micro- and mesozooplankton collectively, the phytoplankton growth was largely consumed (net growth rate of 0.08 +/- 0.15 day(-1)), except in the central North Pacific (net growth rate of 0.42 +/- 0.11 day(-1)). Phytoplankton biomass accumulation in this area was also demonstrated by the satellite-observed Chl a, although this was just sustained for a few days. The high phytoplankton growth rate in the central North Pacific (0.84 +/- 0.26 day(-1)) was a result of the dominant Prochlorococcus, which contributed 88% of the community Chl a. The weak response of Prochlorococcus growth to ammonium addition indicates their growth (1.14 +/- 0.55 day(-1)) was not limited by the ambient DIN and was likely a result of their advantage of utilizing various dissolved organic nitrogen.