In large bodies of standing water (including the ocean and lakes), the water can be divided into zones based on the amount of sunlight it receives. There is enough sunlight for photosynthesis only in – at most – the top 200 meters of water. Water down to this depth is called the photic zone. Deeper water, where too little sunlight penetrates for photosynthesis, is called the aphotic zone.
Surface water
- Surface water dissolves oxygen from the air, so there is generally plenty of oxygen in the photic zone to support organisms.
- Water near shore usually contains more dissolved nutrients than water farther from the shore.
- This is because most dissolved nutrients enter a body of water from land, carried by runoff or rivers that empty into the body of water.
- When aquatic organisms die, they sink to the bottom, where decomposers release the nutrients they contain.
- As a result, deep water may contain more nutrients than surface water.
Deep ocean water
- Deep ocean water may be forced to the surface by currents in a process called ”’upwelling.”’
- When this happens, dissolved nutrients are brought to the surface from the deep ocean.
- The nutrients can support large populations of producers and consumers, including many species of fish.
- As a result, areas of upwelling are important for commercial fishing.
- With these variations in sunlight, oxygen, and nutrients, different parts of the ocean or a lake have different types and numbers of organisms.
- Therefore, life in a lake or the ocean is generally divided into zones.
- The zones correlate mainly with the amount of sunlight and nutrients available to producers. Figure below shows ocean zones. Lakes have similar zones.
Zones
- The littoral zoneis the shallow water near the shore. In the ocean, the littoral zone is also called the intertidal zone.
- The pelagic zone is the main body of open water farther out from shore. It is divided into additional zones based on water depth.
- In the ocean, the part of the pelagic zone over the continental shelf is called the neritic zone, and the rest of the pelagic zone is called the oceanic zone.
- The benthic zone is the bottom surface of a body of water. In the ocean, the benthic zone is divided into additional zones based on depth below sea level.
