Silica fluxes in the inner Elbe Estuary, Germany

Estuaries are major pathways for silica fluxes towards the ocean and considered to be a sink for dissolved silica (DSi). However, the data basis for DSi, but also biogenic silica (BSi) fluxes through estuaries is relatively thin. To test the hypothesis, a silica budget for the inner Elbe Estuary was constructed, based on 18 cruises between 2009-2011 and historical monitoring data. A strong positive correlation between particulate organic carbon (POC) and BSi concentration was identified, allowing the reconstruction of historical BSi time series from POC monitoring data. The budget revealed that the Elbe Estuary is a source for DSi and a strong sink for BSi, contradicting the general assumption of estuaries being a sink for DSi. The Elbe River delivered 2.8 x 10(9) mol DSi a(-1) and 0.9 x 10(9) mol BSi a(-1) to the estuary. There, 17 % of DSi were added along the inner estuary and 88 % of BSi were lost. Estuarine tributaries represented the largest source of additional DSi to the estuarine waters, while sedimentation was the single largest sink for BSi. It was calculated that large amounts of BSi were removed permanently from the estuarine cycle by sediment dredging. As data on Si transformation in estuaries are scarce and widely underrepresented in global budgets, findings from this study are a valuable contribution, especially since the Elbe Estuary seemed to act contradictory to observed systems so far.