Nitrogen Assimilation and Nitrification in Surface Waters of the Amazon and Pará Estuaries

Under increasing anthropogenic impacts, the hydrology, weathering, turbidity, and biogeochemistry of the Amazon and Pará river catchments are changing, potentially affecting the concentration and speciation of nitrogen (N) entering the estuary, as well as the subsequent primary productivity. Primary production and several N‐cycling rates have been studied in the northern river plume, and new studies focusing on specific processes in the river mouth emerged. Yet, a general overview is still lacking. For the first time, we provide an overview of primary production along with nitrification, ammonium (NH 4 + ), nitrate (NO 3 − ), and amino acid uptake rates in the Amazon and Pará river mouths which exhibit different degrees of catchment land use. Our results indicate that the Amazon supplies tenfold more NH 4 + and NO 3 − , and its nitrification rates are four times higher than in the Pará River. The stable isotope composition of ambient water and NO 3 − confirmed that nitrification is a dominant pathway, with fluxes reaching 8.5 × 10 8 mol N d −1 in the Amazon. Nutrients are transported to regions with reduced turbidity and little nitrification, facilitating NH 4 + (0.2–2.4 × 10 8 mol N d −1 ) and carbon uptake (4.6 and 14.0 × 10 8 mol C d −1 in the Pará and the Amazon, respectively) by phytoplankton. The lower N‐cycling rates measured in the Pará relative to the Amazon River mouth likely originate from differences in river discharge and watershed characteristics.