Anthropogenic and natural impacts on the Si-flows in the Danube catchment and the Western Black Sea Christoph Lampert, Jos van Gils, Matthias Zessner, Helmut Kroiss

anthropogenic sources of dissolved Silica washing powders: sodium silicate (water glass) as inhibitor of corrosion of stainless steel and aluminium water supply: application of water glass as a corrosion inhibitor For the Danube: > 95% of the dissolved Si in the river stems directly from geogenic sources

Annual fluctuations of Si-concentrations decrease during summer is due to the activities of diatoms mean: 62 µM/l

No impact of flow conditions on the Si-concentration in winter

Impact of flow conditions on the Si- concentration in summer r 2 = 0,80 r 2 = 0,6

Causes of impact High flow conditions are related to increased sediment transport resulting in an increase of the turbidity and low light transmissivity dissolution from the sediment transport pore water has a higher Si-concentration, sediments are eroded (lower retention time in the river)

Increase of Si-discharge in the last two decades? mean 346 kt Si/a mean 206 km3/a

Decrease of Si-discharges in the last four decades based on marine data? Si-SiO 4 (µM) (b) Mean annual Si-concentrations at Constanta near shore [RMRI, 2002] Building of Iron Gate I

Impact of Danube discharge on Si-conc. Mean annual Si-concentrations at Constanta near shore [RMRI, 2002]

Impact of Danube discharge on Si conc. in the western Black Sea years with high Danube discharges generally lead to high Si-concentrations until the beginning of the 70ies. low discharges in October, November or December cause relative low Si- concentrations in the following year flood events in early spring (March, April, May) have less impact on the Si-concentration than flood events in summer time (June - October)

P-emissions and river loads (Danube discharges to the Black Sea) in the Danube catchment (adopted from Behrendt et al., 2004) and waste water scenarios for future development.

P- and Si-concentrations at Constanta nearshore Since the beginning of the 70ies P was not the limiting factor any more at Constanta nearshore

P-concentrations at the Danube (Sulina) and Constanta nearshore

diatom blooms in Romanian marine waters : 3 blooms of only one diatom species (Skeletonema costatum); cell density of 10-18*10 6 cells/l, period : 16 diatom blooms: 8 blooms of Skeletonema costatum with 10-90*10 6 cells/l. In addition in total 8 blooms of other diatom species like Skeletonema subsalsum, Cyclotella caspia, Chaetoceros similes and Cerataulina pelagica were recorded. [Cociasu et al. 1996] The number of diatom blooms and the cell density of the blooms have increased dramatically.

Relation Danube discharge - salinity in some years until 1975 a dependency of the salinity on the Si-concentration

Conclusions Anthropogenic sources of Si are negligible. the Silica emissions into the Black Sea vary depending on the water discharge – there is no indication of an increase of the Si-conc. within the last two decades The discharge and the period (summer/ winter) of discharge influence the Si- concentration in marine waters as long as N or P limit the growth of Diatoms The Iron Gate did not decrease the loads of dissolved Silica

Conclusions Water currents in the Black Sea vary considerably interannually. Therefore the dilution of Danube waters differs. Low salinity corresponded to high Si-concentrations until the begin of the 70ies. The reduction of Si in the Western Black Sea since the middle of the 70ies at Constanta is caused by the increased amounts of nutrients discharged via the Danube as well as the Navodari industrial complex.