Dissolved organic nitrogen (DON) plays important biogeochemical and ecological roles in aquatic systems. However, little is known about its importance in small tropical lakes and reservoirs. We hypothesized that (a) DON is higher in these systems than dissolved inorganic N (DIN), and that (b) relative contributions of DON and DIN to the total dissolved nitrogen (TDN) pool decrease and increase, respectively, with trophic status. To test these hypotheses, we measured DON and DIN in six lakes and two reservoirs at the Atlantic forest‐Cerrado savanna transition in Southeast Brazil, in the rainy and the dry season. With one exception in each season, all oligo‐ and mesotrophic systems had DON:DIN ratios >1. DON:DIN ratios were negatively correlated with total phosphorus (TP). This pattern was due to high DIN, mainly ammonium, in eutrophic and hypertrophic systems. DON:DIN ratios <1 occurred at TP higher than 103 and 104 µg L−1 in the rainy and dry season, respectively. We found a general dominance of DON in near‐natural lentic systems at the Atlantic forest‐Cerrado savanna transition, which was more pronounced than that reported for other regions. Human alterations to the TDN pool may affect biological communities and ecosystem functioning of these systems.
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.
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