Oxygen Deoxygenation

Question 1

Processes Controlling Dissolved Oxygen in Seawater

Answer 1

The solubility of atmospheric gases in seawater is influenced by temperature and pressure. Oxygen is produced through photosynthesis and consumed through respiration and organic matter decomposition.

Question 2

Oxygen Minimum Zones (OMZs)

Answer 2

OMZs are regions with naturally low oxygen concentrations, covering around 8% of the global ocean volume. They exist in various ocean basins, with the largest in the eastern tropical North Pacific.

Question 3

Link to Eutrophication and Warming

Answer 3

Deoxygenation is exacerbated by increased respiration due to eutrophication and warming-related alterations in circulation patterns. These changes lead to oxygen depletion, particularly in upwelling systems.

Question 4

Implications of Marine Deoxygenation

Answer 4

Marine deoxygenation alters biodiversity, species distributions, fishery resources, and promotes harmful algal blooms. It threatens ocean food provisioning ecosystem services.

Question 5

The ARGO Programme

Answer 5

The ARGO programme involves ~4000 floats reporting ocean data, including temperature, salinity, and now some with oxygen sensors. It provides valuable information for monitoring ocean conditions.

Question 6

Future Expectations

Answer 6

Ocean oxygen levels are projected to decline by 3-4% by 2100 due to climate change and increased nutrient discharges. This could have significant consequences for marine ecosystems and fisheries.

Question 7

How do oceans influence the concentration of atmospheric gases?

Answer 7

Oceans can act as both a source and a sink for atmospheric gases.

Question 8

What factors control the solubility of atmospheric gases in seawater?

Answer 8

The solubility of atmospheric gases in seawater is controlled by temperature and atmospheric pressure.

Question 9

How is oxygen produced in the ocean?

Answer 9

Oxygen is produced in the ocean by marine plants and microbes.

Question 10

What processes lead to the consumption of oxygen in the ocean?

Answer 10

Oxygen is consumed by organisms during respiration and organic matter decomposition.

Question 11

What is photosynthesis?

Answer 11

Photosynthesis is the process by which plants and some microorganisms use sunlight energy to produce glucose (sugars) from carbon dioxide and water.

Question 12

What is respiration?

Answer 12

Respiration is the process where organisms extract energy from organic matter using oxygen, producing carbon dioxide and water, which consumes oxygen in the process.

Question 13

Does photosynthesis augment atmospheric oxygen supply?

Answer 13

Yes, but the main source of atmospheric oxygen in surface waters is internal oceanic processes. However, photosynthesis and bubble trapping can lead to oxygen supersaturation in surface waters.

Question 14

Why does dissolved oxygen diminish with depth in seawater?

Answer 14

Dissolved oxygen decreases because airborne oxygen mainly dissolves at the surface, where oxygen-producing organisms are concentrated due to light, but respiration occurs throughout the water column.

Question 15

How does temperature affect the solubility of oxygen in water?

Answer 15

As temperature and salinity increase, the solubility of oxygen decreases, resulting in less oxygen being dissolved into water as it warms and becomes more salty.

Question 16

What defines Thermohaline Circulation or deep water circulation?

Answer 16

It involves the sinking of cold, oxygen-rich water at high latitudes, global transport without atmospheric exchange, and slower movement with minimal oxygen depletion due to respiration. Oxygen concentration in the Atlantic exceeds that in the Pacific for deep water

Question 17

How is oxygen distributed in seawater?

Answer 17

Surface layers have oxygen production, middle layers see high oxygen consumption, forming an Oxygen Minimum Zone, while bottom layers have less oxygen consumption and receive cold, oxygen-rich water from the deep.

Question 18

What is anoxia in ocean water?

Answer 18

Anoxia denotes areas lacking oxygen, often found in specific oceanic sea-floor regions within Oxygen Minimum Zones (OMZs). While most ocean water contains sufficient oxygen (>2mL per liter) to support life, anoxic water (<0.1mL per liter) cannot sustain life. Notably, exceptions to this rule include the discovery of animals living without oxygen in the Mediterranean in 2010.

Question 19

What are restricted basins and their conditions?

Answer 19

Restricted basins, like the Black Sea, deep-sea trenches (e.g., Cariaco), and fjords, can become anoxic due to oxidative decomposition of organic matter and stagnant waters, exhausting oxygen levels.

Question 20

Why are areas with low oxygen referred to as “dead zones”?

Answer 20

They are called “dead zones” because most marine life either dies or leaves the area, turning vibrant habitats into biological deserts.

Question 21

How does reduced oxygen supply exacerbate dead zones?

Answer 21

Reduced oxygen supply, linked to climate change and alteration of circulation patterns and winds, exacerbates dead zones.

Question 22

What are upwelling systems and why are they important?

Answer 22

Upwelling systems are very productive regions formed by ocean currents and seasonal winds that pull deeper, nutrient-rich but oxygen-poor water to the coast. They are commercially important fishing zones.

Question 23

Why are upwelling systems vulnerable to ocean deoxygenation?

Answer 23

Naturally oxygen-poor, upwelling systems are vulnerable to deoxygenation, causing coastal hypoxia, fish kills, habitat degradation, and reduced fishery catches. Climate change exacerbates these effects.

Question 24

What promotes shelf hypoxia in the Southern Benguela Upwelling System (SBUS)?

Answer 24

Nutrient trapping in the SBUS promotes shelf hypoxia, leading to severe seasonal hypoxic events that decimate regional fisheries

Question 25

What did a recent study reveal about nutrient regeneration and hypoxia in the Southern Benguela Upwelling System (SBUS)?

Answer 25

Over half of the on-shelf nutrients in SBUS are from in situ microbial respiration, with hypoxia severity at the shelf bottom correlating with regenerated nutrients

Question 26

What is the proposed nutrient-trapping mechanism in the Southern Benguela Upwelling System (SBUS)?

Answer 26

Deep nutrient-rich Subantarctic Mode Water (SAMW) gains nutrients from shelf sediment regeneration of particulate organic material (POM), which is produced using nutrients stripped from surface waters advecting offshore.