Silica cycling within marine snow

Abstract

The silica cycle within particles of marine snow from coastal waters off central California was examined. The ratios of biogenic silica to particulate organic nitrogen and to particulate org.anic carbon in aggregates were significantly greater than Redfield ratios, suggesting a large detrital silica component within the particles. Silicic acid concentrations within aggregates were ∼10‐fold greater on average than those in the surrounding seawater, with interstitial concentrations up to 305 µM Si(OH)4. Silica production rates within aggregates were sufficient to deplete the interstitial silicic acid in 0.3–4.0 h, suggesting rapid cycling of Si within the aggregates. Efflux of Si(OH)4 from the aggregates predicted from molecular diffusion theory was 20‐fold greater than the known maximum possible rates of silica dissolution could support. Congruence between efflux rates predicted by diffusion theory and dissolution kinetics requires the diffusion coefficient for Si(OH)4 in marine snow to be 2 orders of magnitude less than in pure seawater. Experimental determination of the concentration dependence of silicic acid use by diatoms in aggregates and by those in the surrounding seawater indicated that assemblages from both environments would experience Si‐replete conditions within marine snow. Aggregation of diatoms following silicic acid depletion at the end of blooms may provide Si‐replete microhabitats suitable for the formation of the heavily silicified resting spores that are central to the life‐history of many species.