Increasing dissolved silica trends in the Rhine River: an effect of recovery from high P loads?

Abstract

The development of river dams and further human activities (causing increased nitrogen (N) and phosphorus (P) nutrient loads), are responsible for a decline in dissolved silica concentrations (DSi) in many river systems. Here, the impact of the reduction of N- and P-concentrations on DSi is examined for the Rhine River. During the last decade of the twentieth century, annual average DSi concentrations increased by ~70% in the Rhine at Bimmen/Lobith, whereas nitrate (NO3) and phosphate (PO4) concentrations decreased by approximately one third. Accordingly, decadal changes in nutrient elemental ratios shifted the river system from DSi-limitation to P-limitation. Specifically, a seasonal DSi-concentration increase is observed from May to December for the Rhine River (with exception of June). Observed increases in DSi concentration are probably due to improvements in water-basin and land-use management, specifically a reduction in point-source P discharge, leading to P-limiting conditions for diatom growth. Data of the warm season suggest that as the system is moving through the transition from P-excess to P-limitation conditions, P-limitation according to the elemental ratio DSi/total phosphorus (TP) is occurring later than for the ratio DSi/PO4-P. Latter ratio will be buffered around ~16:1 during growing season. Reduction of N fertilization is less relevant, as N-limitation with respect to DSi is not achieved, even at the end of the analyzed period, but N-limitation may be reached in the future. Analysis of discharge–DSi relationship supports the hypothesis that DSi increase is affected by increasing P-limitation during the warm period and not only due to hydrological reasons. Results suggest, however, that the influence of hydrological parameters needs to be addressed in research for DSi concentration changes due to changed nutrient loads. Despite an overall increase in water temperature of 3°C over a 50-year period, no correlation with temperature was found for the last two decades of the twentieth century, for which DSi-data were available. In conclusion, in case of eutrophied river systems with excess of P, P-reduction may lead to an increase of DSi concentrations under certain conditions. This in turn is expected to impact not only DSi-sensitive coastal-zone ecosystems impacted by eutrophication but the carbon cycle as well.