The response of an ecosystem to pulsed events is likely state-dependent, with a regime shift potentially making an ecosystem more or less sensitive to pulses. Herein, we evaluate the response of Lake Erie’s ecosystem to tributary pulses that deliver sediments and nutrients during spring, contrasting two regimes caused by planned (i.e., phosphorus abatement programs) and unplanned (i.e., dreissenid mussel invasion) anthropogenic influences: a regime of high nutrient inputs and no dreissenids (1969-1986) and one of low nutrient inputs with dreissenid mussels present (1987-2007). Specifically, we explore the response of Lake Erie yellow perch (Perca flavescens), an ecologically and economically important species that resides in the middle of the food web, to pulsed inputs from the Maumee River, a large tributary that provides large sediment and nutrient subsidies to Lake Erie. Historical analyses demonstrate that yellow perch recruitment to the fishery was unrelated to the magnitude of the spring tributary pulses during the first regime; however, recruitment was strongly positively correlated (R2 = 0.89) to spring tributary inputs during the second regime with the legacy of river pulses lasting up to five years. We used a variety of approaches—field surveys, satellite imagery, otolith microchemical, experimental, and modeling—to determine the mechanisms underlying this river pulse – recruitment relationship during the second regime.
Our analyses demonstrate that pulsed Maumee River inputs promote yellow perch recruitment through formation of turbid, nutrient-rich open-lake plumes with plume size explaining 98% of observed recruitment variation. Otolith microchemical analyses conducted on juveniles surviving to the new year-class (a strong predictor of future recruitment to the fishery; R2 = 0.75) also show that the Maumee River plume confers a significant recruitment advantage to individuals that use it as larvae relative to larvae from non-plume areas. While zooplankton productivity was higher in the Maumee River plume than outside of it, larval yellow perch diet and growth rate information indicate that reduced predation risk offered by high turbidity in the plume (i.e., top-down effects) is more important than food resources (i.e., bottom-up effects) in explaining this differential recruitment pattern. Further, historical water clarity data reveal a strong reliance of water clarity to Maumee River pulses during the more recent regime, which was absent during the first regime. Our observed alteration of yellow perch recruitment mechanisms, exemplified by an increasing reliance on tributary pulses, illustrates how human-driven ecosystem change can have unexpected impacts on ecosystem processes.