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Autumn phytoplankton blooms represent key periods of production in temperate and high-latitude seas. Biogenic silica (bSiO(2)) production, dissolution, and standing stocks were determined in the Celtic Sea (United Kingdom) during November 2014. Dissolution rates were in excess of bSiO(2) production, indicating a net loss of bSiO(2). Estimated diatom bSiO(2) contributed <= 10% to total bSiO(2), with detrital bSiO(2) 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 bSiO(2) present. Negative net bSiO(2) 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.

Plain Language Summary Small marine microalgae called diatoms are responsible for significant levels of primary production in support of marine ecosystems. Diatom cells are formed from silica dissolved in seawater; however, diatom cells may also readily dissolve in seawater. Observationsof silica uptake and dissolution during the autumn period of enhanced microalgal productivity in a shelf seafound a nonliving detrital pool of diatomaceous material, which was dissolving faster than the few diatomspresent were making new cells. These observations highlight that certain periods of the year may be associated with rapid rates of dissolution and hence are important for recycling of material prior to the winter period when nutrient budgets in the water-column are set for the following year.