Cause

Plankton grown by eutrophication died,it sink to the ocean floor.
Oxygen concentration in the sea is reduced in the process of decomposing dead plankton by bacteria.
As a result, hypoxic water will be produced.
In this, part of the photosynthetic bacteria,such as green sulfur bacteria bacteria get energy without oxygen is a (anaerobic bacteria) will generate hydrogen sulfide.
It is oxidized this rises, particles of sulfur oxides or sulfur is generated, this will be colloid, and drifting in seawater is due.
The cause of hypoxic water rises is convection.
(Rip current occurs near the surface of the water is carried away to the open sea by the strong wind, water mass of the bottom layer will have to rise to make up for it.)