Significant Pico- and Nanoplankton Contributions to Biogenic Silica Standing Stocks and Production Rates in the Oligotrophic Eastern Indian Ocean

Abstract

The present study involves the first measurements of size-fractionated biogenic silica (bSi) standing stocks and production rates in the oligotrophic Eastern Indian Ocean. The 150-m integrated bSi standing stocks in the pico- and nanosized fractions averaged 49% and 39%, respectively, of the total; the contributions of pico- and nanoplankton to total bSi production rates were 43% and 38%, respectively, together suggesting that these smaller plankton contributed a significant proportion of both the total bSi standing stock and its rate of production. The total bSi variability appeared to be driven by smaller-plankton dynamics. Also, our results suggest that the detrital bSi stocks were potentially sustained by the pico- and nanosized siliceous organisms (for example, Minidiscus and Synechococcus) instead of broken fragments of large diatoms or other siliceous microplankton. In particular, the atomically dense structure from Synechococcus cell lysis was enriched in Si element compared with intact cells. The factors controlling size-fractionated bSi standing stocks may be biological rather than physical processes, and the clear correlation between picoeukaryotes and the < 2 μm bSi further confirmed the possibility of Si accumulation by other smaller plankton.