Stream confluences are ubiquitous interfaces in freshwater networks and
serve as junctions of previously independent landscapes. However, few
studies have investigated how confluences influence transport, mixing,
and fate of organic matter and inorganic nutrients at the scale of river
networks. To understand how network biogeochemical fluxes may be altered
by confluences, we conducted two sampling campaigns at five confluences
in summer and fall 2021 spanning the extent of a mixed land use stream
network. We sampled the confluence mainstem and tributary reaches as
well as throughout the mixing zone downstream. We predicted that
biologically reactive solutes would mix non-conservatively downstream of
confluences and that alterations to downstream biogeochemistry would be
driven by differences in chemistry and size of the tributary and
upstream reaches. In our study, confluences were geomorphically distinct
downstream compared to reaches upstream of the confluence. Dissolved
organic matter and nutrients mixed non-conservatively downstream of the
five confluences. Biogeochemical patterns downstream of confluences were
only partially explained by contributing reach chemistry and drainage
area. We found that the relationship between geomorphic variability,
water residence time, and microbial respiration differed between reaches
upstream and downstream of confluences. The lack of explanatory power
from network-scale drivers suggests that non-conservative mixing
downstream of confluences may be driven by biogeochemical processes
within the confluence mixing zone. The unique geomorphology,
non-conservative biogeochemistry, and ubiquity of confluences highlights
a need to account for the distinct functional role of confluences in
water resource management in freshwater networks.