Effects of Reservoirs on Nutrient Concentrations and Ratios along the Longitudinal Gradient of Danube River Basin

Abstract:

Reservoirs reduce water flow and increase the retention time which can provide conditions to increase primary production, sedimentation and nutrient retention. As a consequence, nutrient ratios and fluxes of nitrogen (N), phosphorus (P), and silica (Si) may be altered which in turn affects the identity of limiting nutrients and the dynamics of primary production in downstream ecosystems. Residence time as well as the position of reservoirs along the longitudinal gradient (headwaters vs. mouth) may affect these processes. The Danube River Basin is one example where reservoirs have likely altered nutrient stoichiometry along the longitudinal gradient. It has a dam every 17 Km in the upper 1000 km of the river along with a very large dam complex (Iron Gates Dam) 117- Km from the mouth. There has been there has been an observed decline in Si flux, which may have led to changes in phytoplankton community structure in the Black Sea, but for which the causes for this decline are not yet clear. The purpose of this study is to examine the effects of reservoirs from headwaters to the mouth on nutrient stoichiometry in the Danube Basin.

Data on dissolved Si, N, and P concentrations from 1996 to 2012 were analyzed from 40 monitoring stations from the TransNational Monitoring Network (TNMN), which are located in the main stem of the Danube. Time series analysis is used to compare nutrient concentrations and ratios both through seasons and through the 15 year time-period. The monitoring stations are located above and below reservoirs in order to analyze the effect of reservoirs on nutrient ratios and fluxes. Preliminary results show that relationship of dissolved inorganic N (DIN): soluble reactive P (SRP) range from 207 to 76, while DIN:Si ratio ranges from 1.89 to 0.2 from the headwaters to the mouth.