The seasonal cycle in sinking particle fluxes off Vancouver Island, British Columbia

Abstract

We have made year-long observations of sinking particle fluxes with sequential sediment traps moored at a depth of 200 m at three sites proceeding offshore from Vancouver Island, Canada in water depths of 500–2500 m. Total particle flux and biogenic silica decreased offshore as expected in an eastern boundary current regime. No obvious gradient was observed in POC, and CaCO 3 was proportionally greater offshore. Even there POC dominated over particulate inorganic carbon (as CaCO 3) indicating a net removal of CO 2 from surface water by biological activity. Common to all three sites, we observed an annual cycle in total particle flux, with highest fluxes during spring/summer, reflecting the higher primary production during these seasons. In spite of changes in total fluxes, the ratios between organic C, CaCO 3 and biogenic silica of the sedimented material did not change over the annual cycle. Atomic C organic/N total ratios were highest in the summer and lowest in the winter, possibly reflecting an increase in the relative contribution of lithogenic particles during winter. The isotopic composition of sinking particulate nitrogen ( δ 15 N total) tended to be lighter during periods of higher fluxes, most likely reflecting variation in relative nutrient utilization and length of the food chain. The δ 13 C organic composition of sinking particles varied among sites, probably reflecting differences in planktonic species composition; contributions of terrestrial organic matter appear unlikely. Currents at 50 m depth were higher and more variable at the inshore site, and temperature and salinity were also more variable, consistent with the tendency for dominance of diatoms (silica shells) in more energetic environments.