Biological productivity and community structure of phytoplankton in the oligotrophic Philippine Sea

In the Philippine Sea, mesoscale eddies have been frequently observed, but
little is known about their contributions to the biological productivity
and community structure of phytoplankton in the region. In situ carbon and
nitrogen uptake rates and phytoplankton community structure were
investigated in the Philippine Sea based on the 13C-15N dual-tracer
technique and photosynthetic pigment analysis from late August to
September 2018. Based on the comparison of the phytoplankton community
structure among the cold eddy, warm eddy, and outer area, we found that
the phytoplankton composition was different in the warm eddy with
relatively lower contributions of large phytoplankton. However, a distinct
enhancement in phytoplankton biomass over the euphotic zone was not
detected even in eddy occurrence. In terms of community, the picoplankton
Prochlorococcus was persistently dominant, which was attributed to a low
supply and chemical composition of nutrients. The average integrated
primary productions (IPP) were 80.4, 75.9, and 76.3 mg C m−2 d−1 for the
cold eddy, warm eddy, and outer area, respectively. The IPP under the eddy
condition was not largely different from that in reference sites, even
though the vertical distribution of productivity maximum was found to be
different depending on eddy condition. These weak biological responses to
mesoscale eddies might be attributed to the sustaining
picoplankton-dominant community structure and no substantial upward inputs
of nutrients to the euphotic zone. Our findings represent an important
contribution to understanding the biological response of mesoscale eddies
in the oligotrophic Philippine Sea.