[mellavora]

maybe you have never heard of agricultural run-off? You might want to read up on i.e. 'dead zones'.

Here is an interesting article from 2018 https://science.sciencemag.org/content/359/6371/eaam7240 Short form " Ocean dead zones with zero oxygen have quadrupled in size since 1950, partly due to Rising nutrient loads"

Longer quote: "Over the past 50 years ... Open-ocean oxygen-minimum zones (OMZs) have expanded by an area about the size of the European Union ... and the volume of water completely devoid of oxygen (anoxic) has more than quadrupled over the same period ... "Agricultural production has greatly increased ..., resulting in a 10-fold increase in global fertilizer use over the same period (47). Nitrogen discharges from rivers to coastal waters increased by 43% in just 30 years from 1970 to 2000 (48), "