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In many spring-fed rivers, benthic macroalgae and periphytic algae are increasing and, in some cases, replacing rooted vascular plants, which are critical to ecosystem function. While most research has focused on the role of nutrients in driving this change, in-channel hydrodynamics also control vascular plant and algal abundances and their interactions. Understanding relationships between hydrology and primary producers is essential for developing ecologically relevant flow regulations. We investigated the relationship between flow velocity and primary producer abundance in spring-fed rivers using observational data from 16 springs to determine critical velocity thresholds for periphyton, macroalgae, and vascular plants. We also used flow suppression experiments to quantify periphyton growth rates and test for hysteretic behavior. Results suggest a critical velocity of 0.22 m/s (95% CI: 0.13-0.28 m/s) for periphyton but no specific thresholds for macroalgae or vascular plants. Experimental and theoretical results supported these findings and suggest periphyton establishment is not hysteretic.

Plain Language Summary Spring-fed rivers are ecologically, socially, and economically important ecosystems. In many springs, algae has been increasing-and in some cases replacing -submerged plants, with negative ecosystem impacts. Restoring springs requires an improved understanding of the drivers of plant and algae abundance. In this study, we investigated the relationship between the speed of flowing water and the abundance of plant and algae using two approaches. First, we used observations of water speed and plant and algal abundances from 16 springs to determine if there was a critical flow speed above which plants and algae decreased. Second, we performed field experiments where we artificially reduced water speed, allowing algae to grow on submerged plants, and then quantified its growth and removal rates. The observational study showed that algae growing on plants (periphyton) is reduced at flow speeds above similar to 0.22 m/s. In contrast, submerged plants and algae growing on the river bottom did not have identifiable thresholds. The experimental study and theoretical calculations agreed with these results and identified the mechanism for this threshold. Finally, we found that increased algae abundance returns to its previous state when flow is restored, suggesting that restoring water flows can also help reduce algal levels in impacted springs.