Hydrobiologia 506–509: 497–501, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 497 Is roach herbivory preventing re-colonization of submerged macrophytes in a shallow lake? Sabine Körner 1 & Tony Dugdale 2 1 Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Müggelseedamm 301, 12587 Berlin, Germany E-mail: koerner@igb-berlin.de 2 National Institute of Water and Atmospheric Research, Hamilton, New Zealand Key words: exclosures, fish herbivory, Potamogeton pectinatus, restoration, Rutilus rutilus Abstract 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 erythroph- thalmus L.) have been described to hamper this process in temperate lakes. Lake Müggelsee 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 1 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. Introduction Re-establishing a stable submerged vegetation is con- sidered an important tool to restore shallow eutrophic lakes (Scheffer, 1998). Poor conditions for macro- phyte re-colonization, however, result from enhanced turbidity, enhanced sediment re-suspension and graz- ing by herbivorous birds (e.g. Jeppesen et al., 1991; Lauridsen et al., 1993). Fish have been found to regu- late macrophyte biomass by grazing as well. Amongst temperate freshwater fishes, this is mainly attributed to rudd (Scardinius erythrophthalmus L.) (e.g. Hans- son et al., 1987; Van Donk & Otte, 1996; Nurminen et al., 2003), bream (Abramis brama L.) and com- mon carp (Cyprinus carpio L.) (e.g. Crivelli, 1983). The dominant fish species next to bream in eutrophic lakes in temperate regions is roach (Rutilus rutilus L.) (Persson et al., 1988). Although this omnivor- ous cyprinid fish can be a main plant consumer (e.g. Goldspink, 1979; Prejs, 1984; Brabrand, 1985; Horp- pila et al., 2000), only Hofer & Niederholzer (1978) described an impact on macrophyte growth and its in- fluence on re-colonizing macrophytes has never been examined. In Lake Müggelsee submerged macrophytes dis- appeared due to increasing eutrophication at the end