Plain language summary Oxygen is a key requirement for respiration by marine living organisms. Warming of the atmosphere and the ocean surface reduces the oxygenation of offshore waters. Similarly, the extra load of nutrients from agriculture or waste waters modifies algal production, particularly in coastal regions, often resulting in oxygen-depleted waters. Specific reactions affecting the ionic forms of nitrogen also occur within oxygen-depleted waters, which also impact the nitrogen cycle by generating nitrite, which is poisonous for marine organisms, and nitrous oxide, a powerful greenhouse gas. We took measurements at sea to show that a poorly studied coastal sector of the North Atlantic Ocean, the Senegalese continental shelf, can be episodically subjected to complete depletion of subsurface oxygen (anoxia) as well as high nitrite concentrations, constituting the first report of anoxia for this oceanic region. We also show that this anoxia is likely the consequence of the decay of a bloom of diatoms, a group of microalgae common in this type of ecosystem that initially developed in shallow waters and transported offshore by anomalous currents associated with low-wind conditions.