Internal-tide nutrient supply to the euphotic layer: vertical advection versus turbulent diffusion off the Amazon shelf from glider observations

Abstract. Tropical oligotrophic surface waters are typically nutrient-limited, and internal tides have been proposed as a key mechanism supplying nutrients to the euphotic zone through two complementary physical pathways: vertical advection and turbulent mixing. However, separating and quantifying these two pathways from in situ observations, and assessing their respective ecosystem implications, remains challenging. Unlike previous studies, which have generally investigated these mechanisms separately, this study directly compares their respective contributions within a common observational framework. Here, autonomous glider observations collected during the AMAZOMIX campaign off the Amazon shelf were used to reconstruct nutrient profiles using CANYON-B neural network, identify internal-tide-driven isopycnal and nutrient-associated vertical displacements, and independently estimate turbulent dissipation rates validated against microstructure profiler (VMP) measurements. Vertical nutrient profiles revealed contrasting nutrient limitation regimes, with a nitrate deficit in the upper layer and a silicate deficit at depth. Our results demonstrate that internal-tide-induced vertical advection dominates the mean nutrient enrichment at the base of the euphotic layer (F_ADV = 2.3 ± 0.5 mmol NO₃ m⁻² d⁻¹ and 0.4 ± 0.1 mmol Si m⁻² d⁻¹), whereas turbulent diffusion provides smaller mean fluxes (F_DIFF = 1.07 mmol NO₃ m⁻² d⁻¹ and 0.14 mmol Si m⁻² d⁻¹) but remains essential because it represents an irreversible transport pathway and can dominate during episodic high-energy mixing events. Together, these processes modify not only nutrient availability but also nutrient stoichiometry, highlighting internal tides as an important regulator of ecosystem functioning in the western tropical Atlantic. These results suggest that internal tides act both as a nutrient-supply mechanism and as a potential community-structuring process, with implications for nitrate–silicate-dependent phytoplankton groups at the base of the euphotic layer.