Habitat-specific fishing revealed distinct indicator species in German lowland lake fish communities

1. With the implementation of the European Water Framework Directive, the need for studies on European lake fish communities has increased to include lake type-specific fish community features. Although several standardized fish sampling methodologies are available, most previous fish community studies lack a simultaneous consideration of the littoral, benthic and pelagic habitats of lakes. 2. To compare habitat-specific fish communities, we sampled 67 lakes in the north-eastern German lowlands using Norden multimesh gillnets in the benthic and pelagic habitats, and electrofishing in the littoral zone. 3. Standardized catches and diversity of the fish community differed among the three habitats sampled. Species richness and Shannon diversity were higher in benthic and littoral habitats compared with pelagic habitats. Overall, the benthic habitat had the most homogeneous catches and contained the most diverse fish community. 4. Cluster analysis and subsequent indicator value analyses produced substantially different optimum cluster numbers for the three habitat-specific fish communities. Based on the significant differences in fish community composition among the habitats, a simultaneous consideration of numerical fish catches from all habitats was performed using standardized fish abundances. 5. The cluster analysis of the combined abundances resulted in three groups of lakes that were indicated by three fish species. Morphological descriptors (volume, area, maximum depth, mean depth) and descriptors related to the trophic situation (conductivity, total phosphorus) clearly distinguished the three lake groups. 6. All three habitats showed distinct characteristics with respect to either species diversity or relative species' abundances. Our results do not support a conceptual model for all lakes of a gradual succession of fish communities as a result of eutrophication. 7. Synthesis and applications. Only simultaneous consideration of all lake habitats will fulfil the requirements of the Water Framework Directive for evaluating the ecological integrity of lakes. A pre-separation into at least two community types according to lake morphology is necessary before the deviation of the present fish community relative to a reference state can be determined.