Periphyton and benthivorous fish affect charophyte abundance and indicate hidden nutrient loading in oligo‐ and mesotrophic temperate hardwater lakes

Charophytes are a key group of submerged macrophytes in temperate hardwater lakes, but have declined or been replaced by angiosperms as a result of eutrophication for more than 100 years. At present, this process is continuing in many European lakes despite an overall reduced nutrient loading. In eutrophic lakes, light attenuation by periphyton is a major factor responsible for declining angiosperm abundance. For charophyte declines, the impact of benthivorous and herbivorous fish has been discussed often, yet periphyton shading has been largely neglected. In our study, we investigated 11 temperate oligo‐mesotrophic (total phosphorus [P] concentrations 9–22 μg/L) hardwater lakes along a gradient of charophyte abundances. Charophyte abundance was mapped using underwater cameras or scuba diving, and coverage was estimated in five classes. Periphyton biomass, nutrient stoichiometry and invertebrate grazer abundance were measured twice on artificial substrates exposed inside and outside fish exclosures for 4 weeks in July and August. Fish biomass was sampled once using multi‐mesh gill netting. We analysed the relationships between water chemistry (concentrations of nutrients and dissolved organic carbon), periphyton biomass and nutrient stoichiometry, periphytic invertebrate grazers and fish biomass to test (a) whether periphyton biomass was controlled bottom‐up by nutrient availability or top‐down by an omnivorous fish–invertebrate–grazer cascade and (b) whether charophyte abundance was affected by periphyton biomass, benthivorous and herbivorous fish. Multiple linear regressions revealed that charophyte abundance was significantly predicted by the biomasses of periphyton and benthivorous bream (Abramis brama L.), whereas herbivorous rudd (Scardinius erythrophthalmus L.) had no significant effect in the investigated lakes. Bream biomass and its proportion in the total fish biomass were linearly related to lake total P concentrations. Periphyton biomass in the tested lakes reached 0.1–21 g dry weight/m2 and primarily was controlled bottom‐up by P availability as indicated from stoichiometry and positive correlations with grazer abundance. Top‐down control was found in only three lakes, which had significantly lower periphyton biomass inside fish exclosures as compared to controls. Our data imply that periphyton shading and negative fish impacts on charophytes increase with P loading. Low charophyte abundances at moderate P concentrations in the lake water indi