Rainfall washes fertilisers and pesticides out of farmed soil and into rivers and streams. Industries discharge waste water directly into rivers or the ocean. Sewage is discharged directly into the ocean.
The terrestrial runoff is warmer than the ocean, therefore it is less dense, which means it forms a ‘barrier layer’ of polluted water at the surface.
The barrier layer restricts resupply of oxygen to the deeper, saltier waters of the ocean, causing the beginnings of oxygen depletion. The pollutants in the terrestrial runoff contain high levels of nitrates and phosphates which encourage rapid growth of ocean plants and algae.