Ocean Zones

1. Ocean Zones Part 3: Pelagic Zone Part 4: What Changes with Ocean Depth?

2. Pelagic Zone

3. Pelagic Zone • The pelagic zone is divided into five vertical ecological zones (Figure 3). The zones differ in temperature, water pressure, light, and availability of nutrients. Marine organisms have different adaptations for living in each of these zones.

4. Pelagic Zone: Epipelagic • The uppermost division of the pelagic zone is called the epipelagic zone (sunlight or euphotic zone), and ranges from the surface to 200 m (0 ft. – 656 ft.). • There is enough light in this zone for photosynthesis to occur, therefore plants and phytoplankton (photosynthetic plankton) are abundant. • Most of the ocean’s wildlife lives in this zone.

5. Pelagic Zone: Mesopelagic • The zone directly below the epipelagic zone is called the mesopelagic zone (twilight zone). • This zone ranges from 201 m to 1,000 m (657 ft. – 3,281 ft.). • While some sunlight reaches this zone, there is not enough light for photosynthesis to occur. • Animals in the mesopelagic zone are adapted to live in darkness, increased water pressure, and cold water.

6. Pelagic Zone: Bathypelagic • Below the mesopelagic zone is the bathypelagic zone (midnight zone), which extends from 1,001 m to 4,000 m (3,282 ft. – 13,124 ft.). • No light reaches the bathypelagic zone and the abundance and diversity of marine life decreases with depth. • Due to the lack of light, the bathypelagic zone lacks photosynthetic plants and primary productivity.

7. Pelagic Zone: Abyssopelagic • The next deepest layer is called the abyssopelagic zone (abyss) and it ranges from 4,001 m to 6,000 m (13,125 ft. – 19,686 ft.). • There is no light in the abyss and the temperature of the water is near freezing. • Very few organisms can live in these depths. • The abyssal zone is known for its high oxygen content and lack of nutrients. • Organisms tend to be translucent or red in color.

8. Pelagic Zone: Hadalpelagic • The last pelagic zone is the hadalpelagic zone (trenches), which extends from 6,001 m to the bottom of the deepest parts of the ocean. • The hadalpelagic zone includes deep water trenches and canyons. • The deepest point in the ocean is called the Mariana Trench, located in the Pacific Ocean. • The Mariana Trench reaches a depth of 10,911 m (35,797 ft.). That’s deeper than the height of Mount Everest at 29, 035 ft.!

9. What Changes with Ocean Depth?

10. Temperature • Most of the visible light is found in the epipelagic zone. Here the sun warms the surface of the water. • Wind and waves keeps the epipelagic zone mixed and allows the heated surface water to mix vertically into deeper waters. • The bottom of the mixing layer is called the thermocline, which is a layer of water where the temperature decreases rapidly with increasing depth. • The thermocline is located in the mesopelagic zone.

11. Temperature - Thermocline

12. Temperature • In contrast to the rapid temperature drop in the mesopelagic zone, the temperature in the bathypelagic zone remains constant. • Temperature is approximately 39 ºF (4 ºC). • The water temperature in the abyssopelagic zone and hadalpelagic zone remain constantly freezing.

13. Pressure • Pressure greatly increases as depth increases in the ocean. • Pressure is a force applied over an area. Since water has weight, it exerts pressure. • The pressure of water increases with depth because of the weight of the water pushing down from above.

14. Pressure • Pressure greatly increases as depth increases in the ocean. • Pressure is a force applied over an area. Since water has weight, it exerts pressure. • The pressure of water increases with depth because of the weight of the water pushing down from above.

15. Light • As depth increases, the quantity and quality of light decreases.

16. Questions?