system
Given the following post, respond as if you're commenting on Reddit:
human
Are there ocean dead zones where there is little or no life activity? If so why do they exist?
gpt
Absolutely. 

Consider a typical oceanic water column of about 3.5 km. The overwhelming majority of living critters are found within the photic zone, which ends at an average depth of about 200 m. Within this zone, all of the necessities of life are present: an energy source and nutriments. However, light from the sun cannot reach beyond this 200 m zone, so biological productivity decreases abruptly. Still 3.3 km to go.

Some critters will spend part of the day below the photic zone and have a daily vertical migration to get to rich food sources from the surface, before retreating to deeper waters where the likelyhood of predation is reduced. A lot of these daily migrators are plankton sized, and stay within a few hundred meter of the photic zone. 

Below this, life is scarce. It is not quite a dead zone in the formal sense of the word, but the combined presence of some of the more basic necessities of life (nutriments and an energy source) is not there. Consider this a biological desert. The main source of food is dead organic matter (mostly plankton) raining down from the surface. This may be degraded by bacteria on the way down, which may take a few days. Significant food resources, such as whale falls, are scarce and just "passing through".

Next stop is the ocean floor, 3.5 km down. Here is the final resting stop for that rain of organic matter from the surface. This allows for the (relative) concentration of this organic matter in a single locale. There is more life here than in the intervening 3 km between here and the surface. The food chain for the most part depends of this rain of dead plankton; this is the land of bottom feeders and detritivores, with a few predators which pick upon the benthos. 

Very locally, you'll find extraordinarily lush oases of life tightly clustered around hydrothermal vents. These are exceptional, and constitute a parrallel food chain where the input of energy is not from the sun (no matter how far removed) but from chemical energy transported by the Earths heat transfer system. A lot of these critters depend on symbiotic relationships with specialised bacteria and archea, and live off methane, sulphate, sulphuric acid, or some other energy source.

So, in retrospect, you'll find that most of the water column, from a few hundred meters of the photic zone to the ocean floor, is pretty much a biological desert with little biological productivity.