Seasonal dynamics of eutrophication in human-impacted rivers

Abstract. Eutrophication – i.e., biomass overproduction due to nutrient enrichment – persists as a threat to riverine ecosystems despite achievements in lowering phosphorus (P) concentrations. This study explores seasonal patterns and drivers of chlorophyll a (Chl-a) linked with P availability across a selection of human-impacted rivers in Germany. We analyzed Chl-a and total phosphorus (TP) concentration measurements from 133 river sites and quantified their relationship using the degree of realized eutrophication (α_realized) – the ratio of measured to maximum Chl-a at a given TP. By applying k-means clustering on seasonal α_realized cycles, we identified five archetypal patterns. To understand the drivers of these patterns, we examined the seasonal dynamics of total nitrogen (TN), the TN:TP ratio, and fractions of reactive P and N. We further conducted a correlation analysis of α_realized and photosynthetically active radiation (PAR), water temperature, and discharge. In addition, we compared static river network and catchment characteristics between the clusters. We found that (1) constantly high α_realized was associated with close upstream lakes, along with nutrient concentrations suggesting co-control of Chl-a by P and N. (2) High α_realized in mostly lake-free rivers throughout spring and summer were associated with light control, as indicated by a high correlation with PAR. (3) Rivers with spring peaks and low summer α_realized were explained by summer Chl-a losses through grazing. Here, differences in spring peak timing and intensity could be related to differences in land use, hinting to riparian shading as a modulator of phytoplankton growth. Therefore, we find especially high risk of phytoplankton blooms downstream of lakes throughout the vegetation period, in long rivers without effective grazer control from mid-spring to early autumn, and in rivers with a lack of riparian shading during spring. Effective management may comprise dual management of P and N, especially for locations prone to summer blooms, and targeted riparian shading as an additional measure.