Carbon, Nitrogen, and Phosphorus Fluxes in Sixty Tropical Brazilian Rivers: Current Status, Stoichiometry and Trends

Public data from sixty tropical rivers were used to explore variations in C, N, and P and the potential impact of anthropogenic activities over a decade on freshwater bodies. The results showed the Total Organic Carbon (TOC) varying between 2.7–73.0 mg C l⁻¹; whereas Total Nitrogen observations showed concentrations between 0.9 and 32.0 mg N l⁻¹ and Total Phosphorus showed a higher frequency of data in the range of 0.02–0.4 mg P l⁻¹. The annual total C load (TC) was estimated at 32.7 Tg C yr⁻¹, where dissolved inorganic carbon (DIC) accounted for 69% of TC and dissolved organic carbon (DOC) contributed 22%. The stoichiometry showed P depletion relative to C and/or N in 39 of the sixty rivers (65%). This was further supported by the fact that most catchments had TOC values > 50% (C/N/P = 100%). A significant trend was found for yearly fluxes of TOC for the period 2008–2018 (Mann–Kendall test; p = 0.0006; α = 0.05). Land-use and cover at period 2008–2018 indicated a trend of increasing anthropized area of 4%, whereas the natural area decreased by 3.1%. The organic load trend analysis showed 85% of municipalities with a positive trend, this high rate in the remaining organic load is indicative of urban and agricultural in the region.