UC San Diego

This study of an upwelling cross-shore filament in the California Current System attempts to follow a parcel of water from upwelling source to offshore in order to characterize changes in silica cycling and associated impacts on carbon cycling under the influence of iron limitation. Water column samples were collected throughout the euphotic zone in conjunction with sediment traps following the filament in Lagrangian fashion (cycles); additional samples were taken across the filament (transects). The filament progressed from iron-replete to iron-limited throughout the study, identified by low silicic acid to nitrate ratios (Si:N) and elevated nitrate to iron (N:Fe) ratios in situ. Increases in biogenic silica to POC ratios (bSi:POC) in the water column were apparent with increasing iron limitation of the diatom-dominated community. The Lagrangian nature of the study improves overall understanding of the dynamics of biogenic silica as an upwelling filament develops to iron-limited, low Si:N water conditions. This work builds upon and adds context to previous studies which have shown that iron limitation affects silicic acid to carbon (Si:C) and silicic acid to nitrate (Si:N) uptake ratios in marine diatoms. These ratios have potential implications for both carbon and silica cycling in the ocean, impacting uptake and export of these elements. The filament was shown to transport biogenic silica laterally away from the coast, and enhanced silica ballasting was shown to be associated with increased export efficiency offshore.