Abstract. Eutrophication of water bodies has been a problem causing severe degradation of water quality in cities. To gain mechanistic understanding of the temporal dynamics of nitrogen and phosphorus in a groundwater fed low-lying urban polder, we applied high frequency monitoring in Geuzenveld, a polder in the city of Amsterdam. The high frequency monitoring equipment was installed at the pumping station where water leaves the polder. From 2016 March to 2017 June, total phosphorus (TP), ammonium (NH4), turbidity, electrical conductivity (EC), and water temperature were measured at intervals smaller than 20 minutes. This paper discusses the results at three time scales: annual scale, rain event scale, and single pumping event scale. Mixing of upwelling groundwater and runoff was the dominant hydrological process and governed the temporal pattern of the EC, while N and P fluxes from the polder were also significantly regulated by primary production and iron transformations. The mixing of groundwater and runoff water governed water quality through variation of the intensity and duration of the events. For NH4, the dominant form of N in surface water originating from groundwater seepage, we observed low concentrations during the algae growing season, while concentrations were governed by mixing of groundwater and precipitation inputs in the late autumn and winter. The depletion of dissolved NH4 in spring suggests uptake by primary producers, consistent with high chlorophyll-a, O2, and suspended solids during this period. Total P and turbidity were high during winter, due to the release of reduced iron and P from anoxic sediment to the water column. Rapid Fe2+ oxidation in the water column is the major cause of turbidity. In the other seasons, P is retained in the sediment by iron oxides. Nitrogen is exported from the polder to the downstream water bodies throughout the whole year, mostly in the form of NH4, but as organic N in spring. P leaves the polder mainly during winter, primarily associated with Fe(OH)3 colloids and as dissolved P. Based on this new understanding of the dynamics of N and P in this low lying urban catchment, it is possible to formulate management strategies that can effectively control and reduce eutrophication situation in urban polders and receiving downstream waters.
High-frequency monitoring reveals seasonal and event-scale water quality variation in a temporally frozen river
