Abstract
The Transition Zone Chlorophyll Front (TZCF) is a dynamic region of
elevated chlorophyll concentrations in the Northeast Pacific that
migrates from a southern winter (February) extent of approximately 30°N
to a northern summer (August) extent of approximately 40°N. The
transition zone has been highlighted as important habitat for marine
animals and fisheries. We re-examine the physical and biological drivers
of seasonal TZCF variability using a variety of remote sensing,
reanalysis, and in-situ datasets. Satellite-based remote sensing
estimates of chlorophyll and carbon concentrations show that seasonal
TZCF migration primarily reflects a seasonal increase in the chlorophyll
to carbon ratio, rather than changes in phytoplankton carbon. We use our
data compilation to demonstrate how the seasonality of light and
nutrient fluxes decouple chlorophyll and carbon seasonality at the
transition zone latitudes. Seasonal mixed-layer-averaged light
availability is positively correlated with carbon and negatively
correlated with chlorophyll through the transition zone, while
climatological nitrate profiles show that chlorophyll to carbon ratios
are facilitated by wintertime nitrate entrainment. These empirical
results are consistent with physiological data and models describing
elevated chlorophyll to carbon ratios in low light, nutrient-replete
environments, demonstrating the importance of latitudinal structure in
interpreting seasonal chlorophyll dynamics at the basin scale.