Confluences act as biogeochemical hot spots in Mediterranean stream networks

Headwater streams are critical for global biogeochemical cycles, transporting and retaining large amounts of carbon (C), nitrogen (N) and phosphorus (P). However, global element fluxes within headwater fluvial networks remain poorly constrained, partially due to the extreme spatial variability in water chemistry. Here, we assessed how confluences shape C, N and P concentrations and major biogeochemical fluxes along Mediterranean fluvial networks. We hypothesized that confluences act as biogeochemical hotspots because lateral inflows can supply limiting resources to the receiving streams. To test this hypothesis, we conducted synoptic surveys in fall 2024 across three Mediterranean headwater fluvial networks within the Tordera basin (Catalonia). We measured organic and inorganic C, N and P concentrations every 50 meters along the mainstem as well as in major lateral inflows, including permanent tributaries, intermittent tributaries and preferential groundwater flowpaths. Further, we performed laboratory incubations to assess changes in heterotrophic activity, C degradation and nutrient uptake between sites located upstream and downstream of major confluences. Preliminary results show that C:N:P ratios varied across streams (from 483:2:1 to 818:58:1), suggesting that stream biota was limited by either N, P or both. Further, confluences shaped element concentrations along the mainstem by either diluting element concentrations (mixing effect) or delivering limiting nutrients that enhanced biogeochemical activity (reactor effect). Overall, these findings underscore the role of confluences as biogeochemical hotspots and highlight their importance for regulating water chemistry and element fluxes within stream networks.