Picoplankton contribution to biogenic silica stocks and production rates in the Sargasso Sea

The picoplankton size class (< 3 µm) was observed to contribute a measurable, and at times significant, proportion of the total biogenic silica (>0.4 µm) standing stock and to its rate of production in the Sargasso Sea. These trends were robust after correcting biogenic silica, and the calculated rates which use these data, for interference by lithogenic silica. The 100-m total integrated biogenic silica concentration was low and ranged from 0.7 – 5.0 mmol Si m^-2, with the highest value within a mesoscale eddy. Material within the picoplankton size fraction was present at every low-biomass station and provided a relatively consistent contribution to total biogenic silica (10 – 24%, average 14%). The integrated rates of total biogenic silica production were reflective of the low biomass: 0.2 – 1.8 mmol Si m^-2 d^-1 in non-eddy stations and 6.0 mmol Si m^-2 d^-1 within the eddy. The average proportion of biogenic silica production in the picoplankton was 16% (range 3 – 38%), with a lower value within the eddy. Outside the eddy, the biomass-normalized rates of silica production were similar for each size class, ~0.6 d^-1. Using Synechococcus abundance data in the upper 30 m and assuming this group was the only contributor to the picoplankton biogenic silica, we calculate an average Si quota of 229 amol Si cell^-1; this is ~two-fold higher than reported previously by direct measurement on cells using x-ray fluorescence. In the eddy, the estimated Synechococcus Si-quotas were up to 50-fold higher than estimated at other stations, suggesting that most of the pico-sized biogenic silica in this feature may have been diatom fragments. This interpretation is also supported by the eddy having the lowest biomass-normalized production rates for the picoplankton. Our results suggest picoplankton may have a small, but relatively stable, contribution to biogenic silica in this region, which underlies a more dynamic microplankton biogenic silica pool driven by diatoms.