Work

Impact of internal tides on chlorophyll a distribution and primary production off the Amazon shelf from glider measurements and satellite observations

Year: 2025

Type: article

Abstract: . The ocean region off the Amazon shelf including shelf-break presents a hotspot for Internal Tides (ITs) generation, yet its impact on phytoplankton distribution remains poorly understood. These baroclinic waves, generated by tidal interactions with topography, could modulate nutrient availability and primary production both by mixing and advection. While previous studies have extensively examined the physical characteristics and dynamics of ITs, their biological implications, particularly in nutrient-limited environments,remain underexplored. To address this question, we analysed a 26-day glider mission deployed in September–October 2021 sampling hydrographic and optical properties (chlorophyll-a) at high resolution along an IT pathway, satellite chlorophyll-a and altimetry data to assess mesoscale interactions. Chlorophyll-a dynamics were analysed under varying IT intensities, comparing strong (HT) and weak (LT) internal tide conditions. Results reveal that ITs drive vertical displacements of the Deep chlorophyll Maximum (DCM) from 15 to 45 meters, accompanied by a remarkable 50% expansion in its thickness during HT events. This expansion is observed with a dilution of the chlorophyll-a maximum concentration within the DCM depth. Turbulent cross-isopycnal exchanges driven by tides redistribute chlorophyll-a into adjacent layers above and below the DCM. At the surface, turbulent fluxes contribute to 38% of the chlorophyll-a supply, which directly influences primary production. Notably, the total chlorophyll-a content in the water column increases by 14-29% during high internal tide phases, reflecting a net enhancement of primary productivity. This increase results from the combined effect of vertical mixing and stimulated biological activity in the surface layer. These findings highlight the role of ITs as a key driver of chlorophyll-a distribution and short-term biological variability, reshaping the vertical chlorophyll-a profile and regulating primary productivity and potentially carbon cycling in oligotrophic oceanic systems.

Source: Ocean science

DOI: 10.5194/os-21-2873-2025