Biogenic silica accumulation in the sediments of Iron Gate I Reservoir on the Danube River

Abstract.

Damming of rivers can result in severe downstream effects such as changing sediment and nutrient fluxes that potentially affect coastal ecosystems. Closing of the Iron Gates Dams in the lower Danube River was linked to a decrease in dissolved silica flux to the Black Sea of 600,000 t yr⁻1. A recent study on the Iron Gate I, however, indicated a dissolved silica removal within the reservoir of only 16,000 t yr⁻1. Such an order of magnitude difference between actual budgets and earlier estimates is unlikely to be caused by changes in hydrological or biogeochemical conditions. In order to separate annual variations and downstream effects of damming, we analyzed the sedimentary records of biogenic silica using dated sediments. Results confirm the detailed budgets of dissolved silica. In 2001, a total biogenic silica accumulation in the sediments of the Iron Gate I Reservoir of 19,000 t Si yr⁻1 was determined and represents the highest retention over the past 20 years. The accumulation of biogenic silica in the Iron Gate I Reservoir was compared with data from the coastal Black Sea. Biogenic silica in the sediments of the coastal Black Sea start decreasing before Iron Gate I Dam was completed in 1971. In conclusion, construction of the largest impoundment on the Danube River, the Iron Gate I Reservoir, was not solely responsible for decreasing the silica loads downstream at the coastal Black Sea.