The effect of lateral connectedness on the taxonomic and functional structure of fish communities in a lowland river floodplain

Abstract In river floodplains many conservation programs focus on the main river channel as the richest in species. Lateral floodplain waterbodies, which contribute largely to functional processes in river systems, often remain overlooked and exposed to anthropogenic pressures. Although the role of hydrological connectedness between lateral waterbodies and the main river on taxonomic composition of fish communities is well understood, effects on functional community composition is much less studied. Abundance data of fish communities were gathered from 152 electrofishing sites in the main channel and lateral floodplain waterbodies of the river Lippe (Germany), over 18 years. These data were used to compare taxonomic, functional, conservation and recreational fishing aspects along the floodplain lateral connectedness gradient. Fish species richness decreased along the lateral continuum from the main river channel to isolated floodplain waterbodies. In contrast, the relative abundance of endangered and also of non-native species increased along this gradient, highlighting the ecological and conservational importance of floodplain waterbodies. Species composition in floodplain waterbodies differed across the connectedness gradient showing distinct assemblages which were not merely subsets of the main channel. The variability of life-history and feeding strategists among classes of lateral connectedness confirmed the importance of each connectivity class in contributing to the overall floodplain functional diversity. This study highlights the need of preserving fish taxonomic and functional biodiversity across the floodplain as one integrated hydrosystem. Conservation and restoration measures should therefore extend to include the whole floodplain area and the complete spectrum of differently connected floodplain waterbodies in addition to the main channel of the river.