Elevation, aspect, and local environment jointly determine diatom and macroinvertebrate diversity in the Cangshan Mountain, Southwest China

Freshwater ecosystems in high-mountain regions are subject to emerging threats such as global warming and expanding human activities. Stream diatoms and macroinvertebrates form an essential component of freshwater ecosystems in high-mountain regions. Although these organisms are sensitive to environmental changes, knowledge regarding their elevational diversity patterns remains limited. Opposite aspects (e.g., north vs south; west vs east) usually receive different amounts of solar radiation and precipitation, leading to distinct in-stream characteristics such as discharge, flow regime, and water temperature. Despite the suggested strong influence of aspect on biodiversity patterns in mountains, its effect on stream diatoms and macroinvertebrates has been largely overlooked. The aims of our study were to 1) investigate whether macroinvertebrate and diatom taxon richness follows the same pattern along an elevational gradient; 2) test the effect of aspect on the elevational diversity (i.e., taxon richness and assemblage dissimilarity) patterns of macroinvertebrate and diatom assemblages; and 3) examine the relative importance of elevation, aspect, and the local environment (e.g., in-stream physicochemical variables) in shaping macroinvertebrate and diatom assemblages. We investigated macroinvertebrate and diatom assemblages in six nearly parallel streams (three streams on the east aspect and three on the west) in the Hengduan Mountains region. We found that the taxon richness of both macroinvertebrates and diatoms showed a monotonic increase with elevation (1623-2905 m a.s.l.) when aspect was not accounted for. When aspect was taken into consideration, macroinvertebrate taxon richness still showed a monotonically increasing elevational pattern on both the east and west aspects, but with significantly different model slopes, while a monotonical pattern for diatoms only remained on the west aspect. In addition, taxon richness of macroinvertebrates may also follow a potential unimodal pattern. The distance-decay relationships followed the same patterns as taxon richness, suggesting that the influence of aspect on diatom and macroinvertebrate assemblages was not negligible. Our results suggested that the diversity of both diatoms and macroinvertebrates was jointly determined by elevation, aspect and local environment. Compared to macroinvertebrates, the diatom assemblages were more strongly influenced by the local environment. To gain a better understanding of the underlying mechanisms driving elevational patterns of stream biodiversity in high-mountain regions, comparative studies that involve multiple organisms, streams, and mountains across a large elevational range are needed.