LAUSR

Autumn phytoplankton blooms represent key periods of production in temperate and high‐latitude seas. Biogenic silica (bSiO2) production, dissolution, and standing stocks were determined in the Celtic Sea (United Kingdom) during November 2014. Dissolution rates were in excess of bSiO2 production, indicating a net loss of bSiO2. Estimated diatom bSiO2 contributed ≤10% to total bSiO2, with detrital bSiO2 supporting rapid Si cycling. Based on the average biomass‐specific dissolution rate (0.2 day−1), 3 weeks would be needed to dissolve 99% of the bSiO2 present. Negative net bSiO2 production was associated with low‐light conditions (<4 E·m−2·day−1). Our observations imply that dissolution dominates Si cycling during autumn, with low‐light conditions also likely to influence Si cycling during winter and early spring.