Net biogenic silica production and the contribution of diatoms to new production and organic matter export in the Costa Rica Dome ecosystem

We determined the net rate of biogenic silica (bSiO₂) production and estimated the diatom contribution to new production and organic matter export in the Costa Rica Dome during summer 2010. The shallow thermocline significantly reduces bSiO₂ dissolution rates below the mixed layer, leading to significant enhancement of bSiO₂ relative to organic matter (silicate-pump condition). This may explain why deep export of bSiO₂ in this region is elevated by an order of magnitude relative to comparable systems. Diatom carbon, relative to autotrophic carbon, was low (<3%); however, the contribution of diatoms to new production averaged 3 and 13% using independent approaches. The 4-old discrepancy between methods may be explained by a low average C:Si ratio (∼1.4) for the net produced diatom C relative to the net produced bSiO₂. We speculate that this low production ratio is not the result of reduced C, but may arise from a significant contribution of non-diatom silicifying organisms to bSiO₂ production. The contribution of diatoms to organic matter export was minor (5.7%). These results, and those of the broader project, suggest substantial food-web transformation of diatom organic matter in the euphotic zone, which creates enriched bSiO₂ relative to organic matter within the exported material.