Long-term trends in nitrogen and phosphorus concentrations in the Lower River Rhine
Van Dijk, G. M.; Stalnacke, P.; Grimvall, A.; Tonderski, A.; Sundblad, K.; Schäfer, A.
A long-term trend analysis was performed on flow-adjusted mean annual nitrogen and phosphorus concentrations in the Lower River Rhine at Lobith Station (German - Dutch border). The effect of seasonality on the relationship between flow and concentration was suppressed by separate analyses of data for each of the twelve months of the year and the use of fixed weights to combine monthly flow-adjusted concentration data into annual values. Statistical models based on simultaneous estimates of concentration - flow relationships and temporal trends explained a substantial part of the interannual variation in concentrations of ammonium, total nitrogen, phosphate and total phosphorus, but only a minor proportion of the interannual variation in nitrate concentrations. A dramatic drop in the concentration of ammonium in 1974 was accompanied by improved oxygen conditions in the Rhine, but a simultaneous fall in total nitrogen clearly indicated that processes other than in-stream nitrification were the main causes of the observed change in ammonium concentrations. The flow-adjusted total phosphorus concentration peaked in the early 1970s whereas the flow-adjusted phosphate reached its maximum value in 1981. The relatively weak concentration - flow relationships during the past decade indicate that diffuse sources contribute significantly to the present phosphorus load.