Isolated by lakes: the influence of connectivity on benthic macroinvertebrate community structure in river- and lake-connected streams

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Habitat connectivity is a key spatial attribute influencing biodiversity patterns in river networks by controlling the extent of dispersal in stream metacommunities. Large lake basins within river networks may hinder upstream dispersal of benthic macroinvertebrates to inlet streams, but the effects on biodiversity patterns have not been explicitly explored. We studied the effects of lake-induced variation in connectivity and environmental factors on alpha and beta diversity, and the active aerial dispersal (AAD) trait of stream macroinvertebrate species. Our analysis included data from 19 river-connected (RC) tributary streams and 17 more isolated lake-connected (LC) tributaries, with sampling conducted within approximately 1.5 km upstream of the stream-river confluence or stream-lake transition zone. Generalized additive models (GAM) and partial effects plots were used to assess the associations and the relative importance of connectivity and environmental factors on the community attributes. Species richness and the relative abundance of AAD were positively associated with the connectivity of the sampling site but species richness was also related to environmental factors, whereas the rarefied species richness and exponential of Shannon diversity were solely related to environmental variability. Beta diversity was negatively associated with connectivity but also to variation in pH. The results suggest that environmental factors largely control macroinvertebrate community structure in RC and LC streams, but the connectivity of the site might influence stream macroinvertebrate metacommunity patterns in river networks by enhancing dispersal and the occurrence of rare species. However, large lake basins seem to have a limited barrier effect on stream macroinvertebrate metacommunities, though further research is needed to fully understand this influence.