What happens when you go deeper in the ocean and why does it happen?

There are four obvious changes that can be noted:

1. The pressure increases, because there is water above you pushing down, just as the air does on the surface of the land and sea. Even diving in shallow water, the pressure can be uncomfortable. Deeper down, it can be dangerous.
2. The visibility is reduced, because sunlight is absorbed or reflected in each layer of the ocean above you.
3. The temperature drops as you go deeper, because of less solar heat and because mixing does not occur below a certain point (except where volcanos or thermal vents release heat)
4. As a result of the first three: there are far fewer lifeforms deep in the ocean. With a higher pressure, colder temperature, and especially a lack of sunlight, the plant life is greatly reduced and with it the number of fish and other organisms that feed on plants. There are some thermal vents deep in the ocean where plants use heat instead of photosynthesis.