Multi-scale seasonal variability in Net Community Production and
Chlorophyll in the Kuroshio Extension
Abstract
The Kuroshio current separates from the Japanese coast to become the
Kuroshio Extension (KE) characterized by a strong latitudinal density
front, high levels of mesoscale (eddy) energy, and high chlorophyll
(CHL). Recent work has also shown that the KE carries subsurface
nutrients into the region horizontally. While satellite measurements of
CHL show evidence of the impact of eddies on the standing stock of
phytoplankton, there have been very limited in situ estimates of
productivity over synoptic scales in this region. Here, we present
highly spatially resolved estimates of net community production (NCP)
for the KE region derived from underway O2/Ar measurements made in
spring, summer, and early autumn. We find large seasonal differences in
the relationships between NCP, CHL, and sea level anomaly (SLA, a proxy
for local thermocline depth deviations driven by mesoscale eddies). The
KE front is a pronounced hotspot of NCP in spring when NCP is almost
completely decorrelated with CHL. Conversely, we find that NCP and CHL
are strongly correlated in summer away from the front. We explore the
mechanistic underpinnings of the relationship between NCP and CHL and
suggest that the KE nutrient stream as well as vertical motions
associated with mesoscale eddies might be a key factor in supporting an
NCP hotspot that is seasonally decoupled from CHL at the KE front. Our
observations also highlight seasonal and regional (de)coupling between
NCP and CHL which may impact the accuracy of CHL-based estimates of
productivity.