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
deg. N to a northern summer (August) extent of approximately 40 deg. 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 suggest the seasonal
TZCF migration primarily reflects a seasonal increase in the chlorophyll
to carbon ratio, rather than changes in carbon biomass. Chlorophyll to
carbon ratios increase due to photoacclimation to low light availability
and elevated nutrient supply in the transition zone winter. Seasonal
mixed-layer-averaged light availability is positively correlated with
carbon and negatively correlated with chlorophyll. Analysis of
climatological nitrate profiles show that chlorophyll to carbon ratios
are further enhanced 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 need to incorporate phytoplankton
ecophysiology into biogeochemical interpretations of remote-sensing
observations.