It's a fairly common thing. The ocean is full of algae, most of which is beneficial but some is dangerous to other forms of life. Think of how a field might be full of "useful" plants like wild herbs or shrubs that produce berries; or the field might be taken over by poison ivy. Sudden changes in the ocean (such as hurricanes that drop high amounts of fresh rain water which then mixes vigorously with ocean water due to high winds thus significantly lowering the salinity of the ocean surface) can cause the algae to reproduce at a greater pace than normal. It could be the "good" algae but sometimes it's the harmful algae.
Regular algae provides food for fish and converts CO2 into oxygen like other plants. The toxic algaes can poison fish, especially selfish, and make them unsafe to eat for both humans and other carnivorous sea life. Both however have a severe negative impact of creating ocean dead zones. The conditions which stimulated the algae are temporary so when the ocean ecosystem goes back to normal the over population of algae dies off and begins decomposing. The microbes which cause decomposition use a lot of oxygen in the process, in this case they take that oxygen from the ocean water and create patches of ocean that don't have enough oxygen to sustain fish and other life.
This process happens a lot but mostly goes unnoticed unless it's happening near the coastline.
