The annual silica cycle in the Sargasso Sea near Bermuda

The annual cycles of silicic acid and biogenic particulate silica (BSiO2) concentrations were examined from October 1988 through December 1992 at the JGOFS time-series site near Bermuda. Lithogenic particulate silica (LSiO2) concentrations were measured from August 1991 through December 1992 at the same site. Distinctly different seasonal patterns were observed in the concentration of biogenic and lithogenic phases. Integrated BSiO2 concentrations in the upper 160 m were greatest (7.6–56.3 mmol BSiO2 m−2 during an annual diatom bloom that occurred each year between January and April. In contrast, integrated LSiO2 concentrations in the upper 160 m showed an annual maximum during July and August (2.05–2.12 mmol LSiO2 m−) probably due to greater aeolian dust inputs during summer. The export of both BSiO2 and LSiO2 was examined with sediment traps deployed for 4 days each month from August 1991 through August 1992. The annual export of biogenic silica at 150 m (47.6 ± 8.6 mmol BSiO2 m−2 y−1, s. d.) was dominated by a winter diatom bloom, which was responsible for 62% of the annual flux. In contrast, the export of LSiO2 through 150 m was highest during July and August (0.039–0.064 mmol LSiO2 m−2 day−1), coincident with the summer maxima in suspended LSiO2 concentrations. The close temporal coupling between maxima in suspended concentrations and vertical flux indicates that any delay between the appearance of BSiO2 and LSiO2 in the surface water and their subsequent export was less than our 30 day sampling interval. The standing stock of BSiO2 in the upper 160 m during non-bloom periods was very small with little temporal variability (mean = 2.7 ± 0.9 mmol m−2, s. d.). High dissolution rates of BSiO2 within sediment traps at that time (mean = 0.072 ± 0.036 day−1, s. d.) suggested that significant silica production was occurring to maintain the observed standing stocks of BSiO2 against losses due to dissolution. Comparison of estimated BSiO2 production and export during this period indicates that at least 64–82% of the BSiO2 produced during the non-bloom period was recycled within the euphotic zone. Estimates of the annual new production by diatoms, with a mole ratio of 1:1 for Si:N within sinking siliceous particles assumed, suggest that diatoms account for up to 26–48% of the new production occurring at this site.