The effect of nutrient transport downstream on food web stability in an experimental freshwater meta-ecosystem

Recent spatial nutrient transport theory suggests that accumulation of nutrients downstream in riverine systems can amplify the magnitude of phytoplankton and zooplankton blooms and/or lead to competitive replacement of phytoplankton by less edible species, such as cyanobacteria. We used an experimentally controlled three-node network of freshwater mesocosms with green algae, cyanobacteria, and Daphnia magna to test these hypotheses. Nutrients and detritus accumulated significantly downstream, reaching maximum values in the terminal nodes, resulting in small increases in abundance for green algae and D. magna populations. Stability analysis from the Lotka–Volterra competition model for green algae and cyanobacteria did not provide evidence of complete competitive exclusion, but cyanobacteria projected to equilibrate at densities 50% higher than those of green algae in all nodes. Our results support the theoretical prediction that unidirectional flow in riverine systems contributes to accumulation of nutrients downstream and increased heterotrophic bacterial activity, but these changes were of insufficient magnitude to produce variance- or mean-driven destabilization of food web relationships downstream in our experimental system.