THE ROLE OF A SILICATE PUMP IN DRIVING NEW PRODUCTION

In the past, the importance of silicate as a limiting nutrient for new production in the ocean, and in determining global productivity and carbon budgets, has been relegated to the lower ranks compared to the role of nitrogen and, more recently, iron. This paper describes a ''silicate pump'' that acts in diatom-dominated communities to enhance the loss of silicate from the euphotic zone to deep water compared to nitrogen, which is more readily recycled in the grazing loop, thus leading the system to silicate limitation. The impact of this silicate pump is described for the HNLC (High Nutrient-Low Chlorophyll) waters offshore from 15 degrees S, Peru and reproduced in a simulation model of a diatom-dominated ecosystem. Silicate pumping to deep water results in low silicate, high nitrate conditions in the mixed layer, shown here to be a characteristic of many HNLC areas. These areas should more accurately be termed HNLSLC (High Nitrate-Low Silicate-Low Chlorophyll) areas. Silicate dynamics may control and dominate new production processes in these areas and consequently control the rate at which newly upwelled CO2 in the surface regions is reduced by the phytoplankton. In such silicate-controlled systems, export production (i.e. production that is lost to deep water) of silicon and nitrogen are not equivalent, since export production of silicon is controlled by input of silicate, whereas export production of nitrogen is controlled by grazing rate and regeneration.