The effects of biogenic silica detritus, zooplankton grazing, and diatom size structure on silicon cycling in the euphotic zone of the eastern equatorial Pacific

Two novel and independent couplings of field data sets suggest that only, similar to 10-20% of the biogenic silica (bSi) in the surface waters of the eastern equatorial Pacific upwelling zone is associated with living diatoms. Accounting for the, similar to 80-90% contribution of detrital bSi reconciles discrepancies between estimates of diatom growth rates obtained by different methods. Approximately 50% of the bSi associated with living diatoms was found to be in cells whose greatest linear dimension was > 40 mu m. The contribution of that > 40-mu m fraction to bSi production was less than its contribution to biomass, as smaller diatoms, between 10 and 40 mu m, contributed 59-100% of bSi production. A steady-state model, parameterized using field data, appears to explain the origin of the large detrital bSi pool, relative to the living fraction. Because specific rates of diatom growth are several times higher than those of detrital bSi dissolution, bSi must be predominantly detrital to balance gross rates of silica production and dissolution. Zooplankton grazing is the dominant process creating the detrital bSi pool, and the balance between diatom growth and zooplankton grazing rates maintains a living bSi pool that is a small fraction of the total bSi. Our results show near equivalence between the gross rates of detrital silica production by mesozooplankton grazing and bSi export from the euphotic zone, and also between the rates of detrital silica production by microzooplankton grazing and bSi dissolution, suggesting distinct roles for the two zooplankton size classes in regional Si biogeochemistry.