Studies of lake ecosystems have often treated benthic and pelagic habitats as discrete food chains with parallel, separated compartments. A limited number of studies on the importance of fish feeding for benthic‐pelagic coupling have been conducted with a food‐web perspective of lakes. We hypothesize that pelagic fish couple benthic and pelagic food webs by feeding on chironomid pupae ascending from the sediment to the surface. Over a two‐year period, an analysis of the stomach contents of Eurasian perch (Perca fluviatilis L.) was combined with simultaneous catches of chironomids in near‐bottom pupal traps and floating emergence traps exposed in the littoral and profundal region of a deep temperate reservoir. Chironomid larvae rarely served as prey for perch. While chironomid pupae from the littoral region were of minor importance as perch prey, we determined a substantial vertical trophic biomass transfer from the profundal to the pelagic food web for the pupation peak of profundal chironomids from May through June. During this period, chironomid pupae represented approximately 20–48% of the ingested biomass of all size classes of perch (70–340 mm). The pupae of only three large profundal species out of 84 chironomid taxa detected in the reservoir represented approximately 80% of the chironomids consumed by perch, indicating a key role for these taxa in benthic‐pelagic coupling. The strong influences of species, habitat and size on the consumption of chironomid pupae by the pelagic perch provide the importance of detecting key benthic species that substantially contribute to the pelagic food web. The phenology of profundal species results in a seasonal shift in the feeding behavior of pelagic fish, which thereby influences the trophic dynamics of lake ecosystems. Therefore, we argue for a more integrated view of lake ecosystems that recognizes not only the littoral but also the complementarity of profundal and pelagic production pathways crucial for ecosystem functioning.

Detritus is an important energy source of stream food webs. Being a mix of allochthonous and autochthonous sources, it is often unknown, which components contribute to the growth of stream organisms. This study focussed on the comparison of two different detritus types (riparian detritus and stream detritus) with respect to food quality and effects on growth as a fitness parameter of juvenile freshwater pearl mussels (FPM). We performed feeding experiments with juvenile FPM under laboratory conditions using the two detritus types from four different natural sources each. Food quality was determined by analysing the fatty acid composition. Stream detritus (conditioned to stream environment including autochthonous microbes) resulted in significantly higher growth rates of juvenile FPM than predominately terrestrial-based riparian detritus indicating higher food quality. Significantly positive correlations were found between mussel growth and different groups of polyunsaturated fatty acids (PUFA). This suggests that especially trace substances such as long-chained n-3 PUFAs and a high ratio of n-3 to n-6 PUFAs enhance the food quality of stream detritus for juvenile FPM. These results highlight the importance of instream conditioning of detritus for the food mix in headwater streams and the importance of PUFAs for the development of juvenile FPM.

To predict the coherence in local responses to large-scale climatic forcing among aquatic systems, we developed a generalized approach to compare long-term data of dimictic water bodies based on phenomenologically defined hydrographic events. These climate-sensitive phases (inverse stratification, spring overturn, early thermal stratification, summer stagnation) were classified in a dual code (cold/warm) based on threshold temperatures. Accounting for a latitudinal gradient in seasonal timing of phases derived from gradients in cumulative irradiation (2.2 days per degree latitude), we found a high spatial and temporal coherence in warm–cold patterns for six lakes (84 %) and the Baltic Sea (78 %), even when using the same thresholds for all sites. Similarity to CW-codes for the North Sea still was up to 72 %. The approach allows prediction of phase-specific warming trends and resulting instantaneous or time-delayed ecological responses. Exemplarily, we show that warming during early thermal stratification controls food-web-mediated effects on key species during summer.

Freshwater pearl mussels (FPMs, Margaritifera margaritifera, Linnaeus, 1758) are endangered and particularly vulnerable to climate change. To create effective conservation strategies, we studied their thermal tolerance and the impact of elevated water temperatures on growth and survival. Our experiments included field mesocosm studies in five FPM-streams in the Vogtland region (Germany) (2016 to 2020), as well as laboratory experiments at temperatures ranging from 1 to 26 °C. Growth of juvenile FPMs increased significantly within a temperature gradient from 12 to 21 °C. In the streams, maximum growth was 8.9 µm/d in surface water and 6.5 µm/d in the interstitial. The upper thermal tolerance for the mussels ranged from 22.1 to 22.9 °C, resulting in low survival during hot summer periods in 2018 and 2019. Warming during winter (+5 °C) did not significantly affect growth and survival, but survival during winter increased with the pre-overwintering shell length. Exceeding a shell length of about 1100 µm in December indicating gill development corelated to 50% survival. Shell length in December is primarily controlled by growth depending on water temperatures during summer. These findings define the thermal niche of juvenile FPMs (average summer temperatures of 14.5–21 °C) and have implications for water management, conservation strategies, and site selection for releasing captive-breeding mussels.