Biogenic silica at the Bermuda Atlantic Time‐series Study site in the Sargasso Sea: Temporal changes and their inferred controls based on a 15‐year record

Abstract

Upper water column biogenic silica (bSiO2) at the Bermuda Atlantic Time‐series Study (BATS) site in the western Sargasso Sea was measured biweekly to monthly from 1989 through 2003, and changes on four distinct timescales were observed. Mesoscale physical features, such as eddies, occasionally increase vertically integrated bSiO2 in the upper 120 m (∫bSiO2) by an order of magnitude or more (this degree of enhancement was observed in only 5 of 217 total profiles). There is a strong seasonal cycle in ∫bSiO2, with a maximum in spring followed by a ∼50% decline to a fall minimum. The ∫bSiO2 also varies on timescales of 3–6 years, parallel with phase shifts in the North Atlantic Oscillation; those changes appear to reflect diatom responses to the combined effects of changes in 18°C subtropical mode water volume and in the meteorological forcing that entrains nutrients into the lower euphotic zone. Removal of the seasonal cycle reveals a secular, ∼40% decline in ∫bSiO2 over the 15 years of the BATS record, concurrent in time with increases in vertically integrated abundances of dinoflagellates and prasinophytes in the upper 120 m. At the same time there has been a linear increase in density stratification in the upper 200 m, a linear decline in winter silicic acid supply in the upper 120 m, and a linear decline in the Si:N ratio on the 26.4 kg m−3 potential density isopycnal. The apparent 15‐year shift in autotrophic community composition, with decreasing ∫bSiO2 and increasing flagellates, may have impacted future biogeochemical processes in the Sargasso Sea by reducing the role of diatoms in this system.