Does diel variation in oxygen influence taxonomic and functional diversity of stream macroinvertebrates?

The dissolved oxygen diel cycle is an inherent feature of stream ecosystems, and the amplitude in this diel fluctuation is thought to strongly influence the composition of stream communities. For instance, extremely low dissolved oxygen saturation (DDO) values have well-documented effects on stream macroinvertebrates. Less is known, however, about the effects of subtler differences in the diel cycle of dissolved oxygen on stream assemblages. In this study, we examined if the diel variation in DDO in streams is associated with changes in the taxonomic and trait composition of macroinvertebrate assemblages. We examined assemblages in 3 drainages of the Great Basin of the USA covering a total elevational range of 1394 m. We described DDO in terms of minimum and maximum dissolved oxygen saturation values as well as the skewness and kurtosis of the distribution of these values in a diel cycle. We partitioned the variation in the extent to which DDO and other environmental factors were associated with trait and taxonomic compositions. We examined how DDO may filter species occurrence and consequently influence both assemblage taxonomic structure and the size of the trait space occupied by an assemblage. We examined this filtering effect across different stream habitat-types and by separately considering both functional and habitat aspects of the ecological niche. Our results show that the shared explained variance associated with DDO was greater when partitioning variance in assemblage trait structure than taxonomic structure. The association between assemblage structure and DDO differed for taxonomic- and functional-based descriptions of assemblages. No loss in overall taxonomic richness occurred along the DDO gradient, but the functional trait space decreased in association with lower DDO values and high skewness. Our results are consistent with the hypothesis that prolonged exposure to low DDO selects against oxygen-sensitive species and, thus, influences trait composition in macroinvertebrate assemblages. The reduction of trait space, which leads to functional homogenization, was less extreme in assemblages occupying fast-flow habitats than in assemblages in low-flow habitats. Functional homogenization was mainly associated with losses in the functional niche along the DDO gradient, whereas habitat traits were less affected.