Trophic Cascade Effects in Freshwater Ecosystems

Abstract

Trophic cascades -- the indirect propagation of top predator effects through intermediate consumers to primary producers-- are among the most consequential ecological interactions in freshwater ecosystems, with practical implications for the management of eutrophication, harmful algal blooms, and water quality. This study quantified trophic cascade strength across 24 temperate lakes and rivers in Central and Northern Europe spanning a gradient from apex predator-present to apex predator-absent conditions, using paired measurements of: (i) piscivorous fish biomass (electrofishing surveys, 2019-2023); (ii) planktivorous and benthivorous fish assemblage composition and biomass; (iii) zooplankton community structure and mean body size; and (iv) phytoplankton biomass, chlorophyll-a, and cyanobacteria proportion. Mean zooplankton body size was 2.84 +/- 0.42 fold larger in lakes with high piscivore biomass (> 40 kg/ha) than in piscivore-depleted lakes (p < 0.001), consistent with the size-efficiency hypothesis of zooplankton grazer community regulation. Chlorophyll-a concentration was 3.84-fold lower and cyanobacteria proportion 58.4% lower in high-piscivore systems (p < 0.001 in both cases). Structural equation modelling confirmed a significant four-level cascade (piscivore biomass -> planktivore biomass -> zooplankton body size -> chlorophyll-a; path coefficients all significant at p < 0.01). Trophic cascade strength was significantly moderated by lake area (larger lakes = weaker cascade; r = -0.62, p < 0.001) and total phosphorus loading (higher P = weaker cascade; r = -0.58, p < 0.001). These findings support biomanipulation-- the deliberate restoration of piscivore communities -- as a cost-effective complementary tool to nutrient load reduction for eutrophication management in Central European lakes.