Habitat Heterogeneity and Channelization of Small Rivers and Streams: Fish Community Responses to Ecological Gradients.

Physical habitat gradients in small rivers and streams profoundly influence aquatic community structure. These ecosystems are critical for biodiversity conservation, serving as refugia and nurseries for numerous species. Effective freshwater conservation necessitates tailored strategies addressing specific anthropogenic pressures and each habitat type's unique geomorphological and hydrological characteristics. This study examined the response of fish communities to habitat heterogeneity and channelization in small rivers and streams within the Drawa River catchment, Pomerania, northwestern Poland. The interaction of morphohydrochemical characteristics created distinct spatial patterns within the catchment. Critical physical habitat characteristics significantly influencing fish community structure were identified through Canonical Correspondence Analysis (CCA). Aquatic macrophyte coverage showed a negative relationship with both fish biomass and abundance in the catchment. Habitat diversity, mediated by physical factors and their interactions, significantly influences the fish community in the catchment. More abundant species have a greater impact on beta diversity patterns, while sites with fewer fish often have unique fish communities. This study underscores the importance of prioritizing rivers with simplified substrates and anthropogenic modifications for conservation and restoration efforts. Furthermore, sites lacking fish populations warrant consideration in conservation planning. Optimal reconstruction strategies to mitigate anthropogenic impacts may include enhancing substrate complexity, removing obstructions, constructing fish passages, and reconfiguring channels. In-stream habitat improvement emerges as the primary strategy for protecting side channels in temperate lowland catchments.