Composition and Flux of Dissolved and Particulate Carbon and Nitrogen in the Lower Tocantins River

The Tocantins River contributes similar to 5% of the total flux of water to the Amazon River plume in the Atlantic Ocean. Here, we evaluate monthly variability in the composition and abundance of carbon, nitrogen, and suspended sediment in the lower reaches of the Tocantins River from 2014 to 2016. Dissolved organic carbon concentrations generally increased during periods of high discharge and are similar to 1.5 times lower than average concentrations at the mouth of the Amazon River. Dissolved inorganic carbon similarly increased during periods of high discharge. Total dissolved nitrogen and individual nitrogen species followed a similar temporal pattern, increasing during highwater. NO3- predominated the dissolved inorganic nitrogen pool, followed by NH4+ , and NO2-, characteristic of environments with a relatively low anthropogenic impact. Dissolved fractions represented 92% of the total carbon exported and 78% of the total nitrogen exported. The suspended particulate sediment flux was 2.72 x 10(6) t yr(-1) , with fine suspended sediment dominating (71.3%). Concentrations of carbon relative to nitrogen indicate a primarily terrigenous source of organic matter and CO2 derived from in situ respiration of this material during the rainy season and a primarily algal/bacterial source of organic matter during the dry season. Considering past estimates of dissolved carbon and nitrogen fluxes from the Amazon River to the Atlantic Ocean, the Tocantins River contributes 3% and 3.7% to total fluxes to the Amazon River plume region, respectively. While this contribution is relatively small, it may be influenced by future changes to the basin's land use and hydrology. Plain Language Summary Rivers are important environments that connect the continents to the oceans. The Tocantins basin, which drains part of the Amazon and Cerrado biomes is a hydrographic region typically considered separate from the Amazon basin. This important, but often overlooked, region has experienced increasing anthropogenic pressure from agricultural and hydroelectric development, which can directly affect water quality, biogeochemical fluxes to the ocean, and coastal productivity, and nutrient cycling in the Amazon River plume. In this study, the composition and transport of carbon, nitrogen, and suspended sediment from the Tocantins River were quantified. We found that terrestrial ecosystems were the primary source of carbon and nitrogen to the river during the rainy season in contrast to aquatic sources during the dry season. Although carbon, nitrogen, and sediment fluxes were an order of magnitude lower than from the Amazon River, they may nonetheless influence productivity in the plume and changes to these fluxes due to continued anthropogenic perturbation remains uncertain.