What is the deep sea?
All around the world the ocean is divided into broad horizontal layers, or “zones”. The uppermost zone is called the euphotic zone (“euphotic” means “well lit” in Greek), and in this zone enough sunlight can get through to allow phytoplankton (microscopic algae) and other plants to photosynthesise. Photosynthesis is the process by which plants use sunlight to convert carbon dioxide and water into food and oxygen. This process is called primary production because it is normally the first (primary) step in the food chain (click here to find out about chemosynthetic production - another type of primary production that doesn’t use sunlight!). Other animals rely on this primary production and oxygen to survive, so the euphotic zone is often the most productive.
The bottom of the euphotic zone is the depth at which there is no more sunlight available for photosynthesis, i.e., where all the sunlight has been absorbed or reflected (it is formally described as the depth at which < 1 % sunlight can be measured). On average the depth of the euphotic zone is around 200 m, but this depends on how clear or cloudy the seawater is in a particular area. Seawater becomes cloudy if there are a lot of particles in it, which can be sediment or organic matter (plant or animal matter) or both. Sunbeams get reflected off these particles as they go through the water, so the more particles there are (the more cloudy it is), the less sunlight gets through, and the shallower the bottom of the euphotic zone is.
The dark deep sea
The deep sea is often described as beginning at the edge of the continental shelf. The depth of the continental shelf varies around the world, but on average it is around 200 m. This coincides with the average depth of the bottom of the euphotic zone, which means that the deep sea is also a dark sea. The average depth of the sea is around 4,000 m (almost 2.5 miles) and the deepest point in the Mariana Trench is about 11,000 m, so there is a lot of deep sea around the world! In fact, about 90 % of the world’s ocean is deep sea.
Not having any sunlight doesn’t mean that animals living in the deep sea don’t rely on it. Plant and animal matter from the euphotic zone sinks through the water column and eventually reaches the deep sea. This organic matter is known as “marine snow”, and most deep-sea animals rely on it as a food source. Early scientists used to think that because the deep sea was dark and didn’t have any plant life, that there wouldn’t be much living down there. Luckily, scientists continued exploring and eventually found out how rich and abundant life was in the depths.