Is roach herbivory preventing re-colonization of submerged macrophytes in a shallow lake?

Re-establishing a stable submerged vegetation is considered an important tool to restore shallow eutrophic lakes. Enhanced turbidity and sediment re-suspension as well as grazing by waterfowl and rudd (Scardinius erythrophthalmus L.) have been described to hamper this process in temperate lakes. Lake Muggelsee lost its submerged vegetation following increasing eutrophication in 1970. Re-colonization, mainly with Potamogeton pectinatus L., started in 1990 after reduction of external nutrient loads. Ten years later, plant biomass and colonization depth remained low. Exclosure experiments revealed that fishes contributed to the reduction of P. pectinatus biomass, although rudd were not abundant. Underwater videos showed that the abundant species perch (Perca fluviatilis L.) and roach (Rutilus rutilus L.) plucked on the plants approximately 2 and I times per hour and that bream (Abramis brama L.) mechanically stressed the plants by sucking the surrounding sediment. In laboratory experiments 1+ roach consumed P. pectinatus at a maximum rate of 1.4 mg dw plant g(-1) fw fish d(-1) when discontinuously fed or not fed with zooplankton. When continuously fed with zooplankton, plant biomass was not significantly reduced, independently of the amount fed. Plant material, however, was not found to be a major food item in the guts of roach in the lake. These findings suggest that young roach and perch did not feed on the plants but started plucking leaves during their search for benthic invertebrates during periods of low zooplankton biomass. This prevented P pectinatus from growing to the surface to survive periods with low light availability in summer. The results imply that young roach and perch contribute to a delay in the re-establishment of submerged macrophytes, at least when plants are not growing at an optimum rate due to shading by periphyton and summer cyanobacteria blooms. In this case, biomanipulation of zooplanktivorous fish abundance will not only improve conditions for macrophyte re-colonization due to enhancement of grazing zooplankton but also due to reduced plant plucking.