Question

The underwater environment in the world's oceans is rapidly changing. Recent years have seen drastic shifts in the ecosystem due to human activity and its impact on the environment. Many populations of fish that rely heavily upon oxygen are declining at extraordinary rates, whereas other ocean species that can survive in oceanic regions of oxygen-depletion are on the rise. What is the most likely explanation, based on scientific theory, for the decline in dissolved oxygen in the world's oceans? A. Carbon dioxide pollution has increased ocean acidity. B. A new species of predator shark preys on fish in oxygen-rich regions. C. The average temperature of the oceans is rapidly increasing. D. Increased rainfall has added water to oceans without adding more oxygen.

Short answer

The average temperature of the oceans is rapidly increasing (Option C).

Step-by-step solution

Understanding the Context

Recognize that the question is about determining the reason behind the decline in dissolved oxygen in the oceans, affecting fish populations.

Analyze Option A

Review the claim that carbon dioxide pollution has increased ocean acidity. Identify how it impacts marine life, but note it primarily affects calcium carbonate-producing organisms like coral rather than dissolved oxygen levels.

Analyze Option B

Consider the possibility that a new species of predator shark preys on fish in oxygen-rich regions. Note that while this could change fish populations locally, it wouldn't directly cause a decrease in dissolved oxygen across the oceans.

Analyze Option C

Examine the claim about the average temperature of the oceans rapidly increasing. Note that increased temperatures reduce oxygen solubility in water, leading to lower dissolved oxygen levels globally.

Analyze Option D

Evaluate the claim that increased rainfall has added water to oceans without adding more oxygen. Note this is an unlikely scenario on a large scale and wouldn’t substantially affect dissolved oxygen levels.

Draw a Conclusion

Based on scientific theory, connect increased ocean temperatures with decreased oxygen solubility, leading to a decline in dissolved oxygen. Option C is the most scientifically plausible explanation.

Key concepts

Ocean Temperature Increase

One of the primary reasons for the decline in dissolved oxygen in the oceans is the increase in global temperatures. When the Earth's temperature rises, so does the temperature of the ocean water.
This is largely due to human activities such as burning fossil fuels and deforestation, which release large amounts of greenhouse gases into the atmosphere.
The warmer the water, the less capable it is of holding oxygen. As a result, as the temperature increases, the amount of dissolved oxygen decreases. This phenomenon is a central reason behind the reduction in oxygen levels in our oceans.
With warmer waters, the stratification of the ocean layers becomes more pronounced. This means that the mixing of surface water, which gets oxygen from the atmosphere, with deeper layers occurs less frequently. Without this mixing, the deeper layers of the ocean receive less oxygen, further contributing to the decline in dissolved oxygen. This process disrupts the delicate balance of oceanic ecosystems and poses serious challenges for marine life.

Understanding the link between ocean temperature and oxygen levels is crucial for addressing the decline of marine biodiversity. Hence, mitigating global warming can play a significant role in preserving the health of our oceans.

Dissolved Oxygen Solubility

Dissolved oxygen refers to the amount of oxygen that is present in water, which is crucial for the survival of aquatic organisms.
The solubility of oxygen in water depends on several factors, with water temperature being one of the most significant.
As water temperature increases, its ability to hold oxygen decreases.
This is because warmer water molecules are more active and tend to release dissolved gases more readily.
In colder water, the molecules are less active, allowing them to hold onto the oxygen more effectively.
Consequently, the warming of ocean waters leads to a decrease in oxygen solubility, meaning less oxygen is available for marine organisms.

In addition to temperature, other factors such as salinity and pressure play roles in oxygen solubility. Higher salinity levels can further reduce the ability of water to hold oxygen.
As ocean temperatures rise, it exacerbates the problem by increasing the stratification of ocean layers, thus limiting the mixing of oxygen-rich surface waters with deeper, oxygen-poor layers.
This results in larger regions of low oxygen concentration, often referred to as hypoxic zones or 'dead zones,' where marine life struggles to survive.
Hence, maintaining a balanced level of dissolved oxygen is critical for supporting diverse and healthy marine ecosystems.

Marine Life Impact

The decline in dissolved oxygen levels has profound impacts on marine life. Many marine organisms rely on a specific range of oxygen concentration to survive. When oxygen levels drop, it puts a strain on these organisms, leading to a decrease in their populations.
Fish and other marine animals that need high levels of dissolved oxygen, such as tuna, swordfish, and certain crustaceans, find it difficult to survive in low oxygen conditions.
The reduced oxygen levels can lead to hypoxic conditions, which can cause mass die-offs and disrupt the food chain.
Marine species that have adapted to low-oxygen environments, such as certain jellyfish and microorganisms, may find these changing conditions more favorable, leading to a shift in the ecosystem dynamics.
These changes in marine biodiversity can have ripple effects throughout the ecosystem, impacting not just the species directly affected but also predators, prey, and the overall health of the ocean.

Additionally, the physiological stress caused by low oxygen conditions can make marine organisms more susceptible to diseases and reduce their reproductive success.
Some species may migrate to areas with higher oxygen levels, which can further alter the distribution patterns and interactions within marine ecosystems.
Protecting and restoring the health of our oceans is essential for the survival of a diverse range of marine species and for maintaining the balance of marine ecosystems.