Is the Iron Gate I reservoir on the Danube River a sink for dissolved silica?

Abstract

Damming rivers changes sediment and nutrient cycles downstream of a dam in many direct and indirect ways. The Iron Gates I reservoir on the Yugoslavian-Romanian border is the largest impoundment by volume on the Danube River holding 3.2 billion m 3 of water. Silica retention within the reservoir in the form of diatom frustules was postulated to be as high as 600 kt year À1 in previous studies using indirect methods. This amount of dissolved silicate was not delivered to the coastal Black Sea, and presumably caused a shift in the phytoplankton community there, and subsequent drastic decline in fishery. We directly quantified the amount of dissolved silicate (DSi) entering and leaving the reservoir for 11 continuous months. The budget based on these data reveals two important facts: (1) only about 4% of incoming DSi was retained in the reservoir; (2) the DSi concentrations were relatively low in the rivers upstream of the reservoir compared to regional and global averages. Thus damming the Danube at the Iron Gates could not have caused the decline in DSi concentrations documented downstream of the impoundment. Rather, this change in DSi must have occurred in the headwaters of the Danube River. Potential reasons include the construction of many dams upstream of the Iron Gates, hydrologic changes resulting in lower groundwater levels, and clogging of the riverbed limiting groundwater-river exchange.

The influence of castanhão reservoir on nutrient and suspended matter transport during rainy season in the ephemeral Jaguaribe river (CE, Brazil)

“…The Iron Gate I reservoir presented statistically significant higher outflow concentrations of dissolved inorganic nitrogen (9% increases along the reservoir), orthophosphate (60% increase) and total nitrogen (12% increase) attributed to resuspension and anthropogenic emission. The same pattern was reported for total phosphorus and D-Si between inflow and outflow (Friedl et al, 2004;Teodoru and Wehrli, 2005). The Saguling reservoir also showed variable nitrogen and phosphorus outlet concentrations compared to inflow (Hart et al, 2002).…”

“…On the other hand, these reservoirs can also export suspended sediments (158% of the income) and ammonium (100 to 350% of the income) according to dam management (Jossette et al, 1999). Similarly, the management of Iron Gate I Reservoir retains 56% of the suspended matter income and 4% of D-Si, but also exports 18%, 60% and 13% of TN, SRP and TP, respectively (Friedl et al, 2004;Teodoru and Wehrli, 2005). Thus, the average retention capacity of the Castanhão for suspended matter and nutrients estimated in this study (89% for D-Si, 98% for SRP, 71% for NH 4 + , 87% for TN, 98% for TP, 97% for suspended matter) is higher than for registered reservoirs in the literature.…”

“…In the literature, alterations of nutrient fluxes by dams vary from small to strong retention capacity, although the same reservoir which traps nutrients, seasonally can act as a source (Jossette et al, 1999;Friedl et al 2004); Teodoru and Wehrli, 2005;Cook et al, 2010). This variability occurs because nutrient loadings are dominated by flows and thus dam management imposed by various reservoir purposes determines specific inflow and outflow rates and consequently the variability of nutrient loadings.…”

Effects of upstream hydropower operation on riverine particle transport and turbidity in downstream lakes

“…Some consequences of altered discharge in rivers are subtle and interfere with the biogeochemical cycle and hence only become evident at long timescales [Friedl and Wüest, 2002]. Notably, damming might interfere physicobiogeochemically by causing (1) water temperature changes resulting from hydropower operations [Preece and Jones, 2002;Meier et al, 2003;Bartholow et al, 2005], (2) nutrient retention [Humborg et al, 2000;Friedl et al, 2004], and (3) hydrological pattern changes and reductions in suspended particle loads [Vörösmarty et al, 2003;Teodoru and Wehrli, 2005;McGinnis et al, 2006].…”

Dissolved silicate retention and transport in cascade reservoirs in Karst area, Southwest China

“…Increased diatom production and its subsequent sedimentation are undoubtedly the key reasons responsible for the downstream DSi decline in dammed rivers. However, controversy still exists concerning the "silicon trapping capacity" of reservoirs, specifically from the research reports on Iron Gate I Reservoir on the Danube River (Humborg et al, 1997;Humborg et al, 2000;Friedl et al, 2004;Teodoru et al, 2006a;Teodoru et al, 2006b;Teodoru and Wehrli 2005). McGinnis et al (2006) also ascribed the side bays, not the reservoirs itself, as the important silicon trap (McGinnis et al, 2006).…”