Scaling Patterns of Point and Diffuse Source Nutrient Loads and Response of Benthic and Pelagic Algal Communities in a Fractal River Network Perspective

Impacts of the anthropogenic nutrient pressures on impairment of river water quality and riverine ecosystem integrity are of increasing significance with rapid land use changes in many regions of the world. A deeper mechanistic understanding of cause-effect relations among anthropogenic nutrient loads from point and diffuse sources, resulting eutrophication risks, and cascading riverine ecosystems impairment is crucial to identify critical zones for monitoring programs and to design more effective management guidelines/strategies. Therefore, we employed a data-modeling synthesis approach to characterize geochemical and ecological impacts in a large, densely populated and agricultural river basin (Weser River; ~46,000 km²; ~8 million population) in Germany. In our high-resolution network modeling analyses we link: (1) spatial patterns and variability of wastewater discharge and nutrient (N and P) loads from heterogeneous distributions of multiple point-sources (~850 wastewater treatment plants; five class sizes) plus diffuse-source nutrient loads from heterogeneous land-use and land-cover over the entire river basin; (2) reach-scale variability of steady-state river discharge (mean- and low-flow) resulting from stochastic hydroclimatic forcing; and (3) nutrient- and light-limited population dynamics of pelagic and benthic algae, which drive the spatial patterns of algal concentration. We examine the emergent steady-state spatial patterns of in-stream P concentration and compare predicted and long-term monitoring data. We also examine simulated spatial patterns of benthic and pelagic algal biomass across the river network. Using these simulations, we derive the scaling properties of nutrient loads to the river network, the hydrological and biogeochemical outcomes (e.g., dilution for WWTP discharges, in-stream nutrient uptake), and algal biomass in fractal river networks. We also present the inter-relationships among the various scaling coefficients. Further we identify "hot spots" for eutrophication and algal blooms to help design management options to improve water quality and reduce ecological impairment.