Tidal modulation of nitrate supply and chlorophyll-a in the Amazon shelf–offshore continuum

Abstract. The Amazon shelf–offshore continuum is a dynamic biogeochemical hotspot of the tropical Atlantic, where river
discharge, ocean circulation, and strong tides interact to shape nutrient and phytoplankton distributions. The Amazon plume
and regional circulation have been widely studied and are known to strongly influence nutrient availability and biological
productivity in this region. However, shelf-break tides remain an overlooked pathway linking physical energy to offshore fertilization, and their contribution to seasonal and intraseasonal biogeochemical variability remains unclear. Here, we
quantify how tidal dynamics, including internal tides, modulate nitrate supply and chlorophyll distributions from the
Amazon shelf to offshore waters. We use a high-resolution coupled physical–biogeochemical model (1/36°), evaluated
against climatological, satellite, and in situ observations. The model reproduces the main observed patterns of surface nitrate
and chlorophyll, as well as key vertical features such as the nitracline and the deep chlorophyll maximum. We show that tides strongly enhance upward nitrate transport, increasing surface nitrate by more than 50% over the northern shelf, along
the shelf break, and within the main internal-tide pathway. This tidally supplied nitrate fuels offshore phytoplankton growth,
increasing chlorophyll by about 15–50%, while reducing surface chlorophyll near the Amazon mouth by 30–
40%.Seasonally, surface chlorophyll and nitrate are higher over the Amazon shelf during April–June but lower offshore,
revealing a marked cross-shelf contrast. When the tidal contribution is isolated, a similar but weaker spatial structure
30 emerges, with a cone-shaped chlorophyll anomaly extending from the shelf break toward the offshore internal-tide
propagation region. Remarkably, tides account for about 63% of the total seasonal variability in surface nitrate, meaning that
tidal forcing alone explains more than half of the seasonal nutrient signal. At intraseasonal timescales, tides generate a clear
spring–neap rhythm of about 15 days in both nitrate and chlorophyll. This spring–neap tidal pulse propagates from the shelf
break toward offshore waters and is especially pronounced near the deep chlorophyll maximum, where oscillations of the
upper nitracline periodically modulate nitrate availability and drive a corresponding chlorophyll response., where
chlorophyll variability is nearly doubled when tides are included. The concurrent increase in nitrate variability indicates that
this spring–neap phytoplankton response is sustained by tidally driven nutrient supply.
These findings identify internal tides as a key biogeochemical driver of the Amazon shelf–offshore continuum, linking tidal
energy to nutrient injection, offshore fertilization, seasonal redistribution, and rhythmic ecosystem variability in the western
tropical Atlantic.