Relative mineralisation of C and Si from biogenic particulate matter in the upper water column during the North East Atlantic diatom bloom in spring 2001

The standing stocks and production rates of particulate organic carbon (POC) and biogenic silica (bSiO2) were measured in the upper water column at 10 stations in the North East Atlantic during the spring 2001 diatom bloom. The elemental composition of the particulate pool was rather homogeneous with depth, suggesting that any material being exported from the photic zone was generally similar in composition to the ambient pool. Pronounced vertical structure was observed in uptake ratios resulting from the strong light dependence of the carbon fixation and the weak light dependence of biogenic silica production. The integrated C/Si molar ratios of particulate material were found to be generally larger than the corresponding assimilation ratios. We interpret this discrepancy as implying a preferential mineralization of Si relative to C from particulate matter during the earliest stages of processing in the upper water column. The preferential mineralisation of Si relative to C in the early stages of particle processing contrasts with processes occurring deeper in the water column, where C is typically mineralised preferentially to Si, and particulate matter becomes enriched in bSiO2. In the northern North Atlantic, the balance of mineralisation of Si relative to C from sinking organic matter with depth is likely to strongly influence the role of diatoms in export production.