Nitrification in the Amazon River plume

Abstract

The Amazon River delivers high concentrations of nitrate (NO3-, ~16 µM) to its estuary, shaping phytoplankton community assemblages in the estuary and up to 1500 km away from the river mouth. Yet NO3- production rates via nitrification, a central process of the nitrogen (N) cycle, have never been studied in this region. By combining nitrification rates and classical oceanographic field measurements with the help of a machine-learning model, we highlight for the first time the variability of nitrification rates along the Amazon River plume and discuss potential relationships with environmental variables. The highest nitrification rates observed at the river mouth (up to 302 nmol l-1 h-1) co-occurred with high turbidity, nitrite (NO2-) and phosphate (PO43-) concentrations, consistent with studies documenting high rates in turbid estuaries. Within less than 200 km from the river mouth, nitrification rates drop to minimum values in the plume and NO3- is depleted, likely consumed by phytoplankton when the light limitation constraint is lifted. In addition to climate change, the Amazon River catchment is subject to an array of anthropogenic impacts, such as deforestation, mining and damming. Under these pressures, the riverine discharge, turbidity, nutrient load and nitrification rates will likely be altered. This study gives a baseline for a central N-cycle pathway, also broadening our understanding of its control factors in the region, which is crucial for predicting the future of processes relying on the Amazon River discharge.