Assessing land-use impacts on a 5th-order tropical river using multiple environmental indicators

Land-use change, such as agricultural expansion and intensification, and urbanisation, affects river ecosystem health and threatens riverine biological communities in a multifactorial and interactive way. In this study, we compiled the results of several studies carried out in a 5th-order tropical river, the Rio das Mortes in the upper Rio Grande basin (Southeast Brazil). We analysed if a combination of different environmental indicators, such as river water quality, sediment structure, habitat structural integrity, biochemistry of suspended organic matter, and the composition and distribution of the benthic invertebrate community, provides a complementary and more complete assessment of land-use impacts from headwaters to the river mouth than single indicators. Water quality exhibited longitudinal changes along the studied river, especially during the dry season, corresponding to the urbanisation pattern in the river catchment with considerable urbanisation already in the upper catchment, and several urban centres along the river's course. The predominance of saturated fatty acids and bacterial fatty acids in the river water's suspended organic matter at urbanised river segments showed that the biochemistry of suspended organic matter, an important resource for the river's biological community, was a sensitive indicator of catchment urbanisation. In contrast, river sediment structure and habitat integrity showed local impacts, primarily in mid-catchment urbanised river segments, with notable positive effects of local conservation efforts and natural differences in sediment structure. Chironomidae and Oligochaeta were the dominant groups in the river's macroinvertebrate community. Their spatial distribution was mainly determined by sediment structure and river habitat integrity, and thereby, by local impacts on river structure. We argue that integrated assessment approaches rarely applied to larger tropical rivers, combining local structural, habitat and community characteristics with large-scale land use and water quality patterns, are important to understand and manage land-use stress in these systems.