Parameterization of vertical chlorophyll a in the Arctic Ocean: impact of the subsurface chlorophyll maximum on regional, seasonal, and annual primary production estimates

[摘要] Predicting water-column phytoplankton biomass from near-surface measurementsis a common approach in biological oceanography, particularly since theadvent of satellite remote sensing of ocean color (OC). In the Arctic Ocean,deep subsurface chlorophyll maxima (SCMs) that significantly contribute toprimary production (PP) are often observed. These are neither detected byocean color sensors nor accounted for in the primary production modelsapplied to the Arctic Ocean. Here, we assemble a large database of pan-Arcticobservations (i.e., 5206 stations) and develop an empirical model to estimatevertical chlorophyll a (Chl a) according to (1) the shelf–offshoregradient delimited by the 50 m isobath, (2) seasonal variability alongpre-bloom, post-bloom, and winter periods, and (3) regional differencesacross ten sub-Arctic and Arctic seas. Our detailed analysis of the datasetshows that, for the pre-bloom and winter periods, as well as for high surfaceChl a concentration (Chl a_surf; 0.7–30 mg m⁻³)throughout the open water period, the Chl a maximum is mainly located at ornear the surface. Deep SCMs occur chiefly during the post-bloom period whenChl a_surf is low (0–0.5 mg m⁻³). By applying our empiricalmodel to annual Chl a_surf time series, instead of theconventional method assuming vertically homogenous Chl a, we produce novelpan-Arctic PP estimates and associated uncertainties. Our results show thatvertical variations in Chl a have a limited impact on annualdepth-integrated PP. Small overestimates found when SCMs are shallow(i.e., pre-bloom, post-bloom > 0.7 mg m⁻³, and the winter period)somehow compensate for the underestimates found when SCMs are deep(i.e., post-bloom < 0.5 mg m⁻³). SCMs are, however, importantseasonal features with a substantial impact on depth-integrated PP estimates,especially when surface nitrate is exhausted in the Arctic Ocean and wherehighly stratified and oligotrophic conditions prevail.

[学科分类] 地球化学与岩石