Half a century of nutrient trends across spatial scales in the Rhine delta: overall improvements, but polder ditches lag behind

Many river catchments are exposed to eutrophication driven by anthropogenic activities. Despite substantial efforts to reduce nutrient enrichment in large river deltas, the water quality often remains poor and ecological improvements have stagnated. Here, we argue that the limited success of these efforts is partly due to a lack of insight into how anthropogenic landscapes drive eutrophication trends across different spatial scales. The aim of the present study was, therefore, to disentangle long-term trends of nutrients at scales ranging from the main river catchment to local water bodies in the Dutch lowland delta of the river Rhine. Specifically, changes in nutrient concentrations were compared between the main river scale, the sub-catchment large canals scale, the regional storage basin scale and two local scales: polder ditches and isolated lakes. Fifty years of monitoring the total nitrogen ([TN]) and total phosphorus ([TP]) concentrations in the Rhine delta revealed gradual decreases, yet stronger for [TP] than for [TN]. Despite significant improvements, the nutrient concentrations are nowadays still indicative of eutrophic to hypertrophic conditions, especially in the polder ditches at local scale. Moreover, the increase in N:P ratios in foremost the main river and isolated lakes will likely hamper ecological recovery. To conclude, the present study demonstrates an overall improvement of nutrient concentrations in the surface waters of the river Rhine delta over the last 50 years, and although polder ditches achieved the largest improvements they still lag behind. As land use at the local scale seems to be a dominant source of nutrient input, we suggest that implementing small-scale measures in the polder ditches could lead to large-scale improvements in the water quality of the Rhine delta.