When fish don't have enough oxygen, they face issues similar to those experienced by mountaineers at high altitudes. Even a small drop in oxygen levels can slow them down, impact their reproduction, and stunt their growth.
Oxygen is as vital to sea life as it is to land animals, but breathing underwater is more strenuous: fish use much more energy to absorb oxygen compared to humans. Larger fish, like cod and haddock, expend even more effort to breathe.
Warming oceans exacerbate this issue. Warmer water not only holds less oxygen, but warmer fish require more oxygen to survive. This puts deep-sea fish at greater risk.
So why is the deep ocean losing its oxygen? As organic matter sinks into the depths, it decomposes, consuming oxygen in the process. In the spring, the ocean can form layers that limit mixing, which isolates the deep water from the oxygen-rich atmosphere. This separation, combined with the oxygen-consuming decomposition of organic material, leads to deep-sea oxygen depletion, or deoxygenation. This process has a significant impact on large deep-water fish and other marine life, raising concerns about the future of ocean ecosystems as global warming continues to impact our planet's climate.
This is why low oxygen levels in the deep sea are a concern, and climate change is contributing to this problem.
However, new research published in Nature Communications reveals that summer storms, combined with tidal movements, help churn and oxygenate the ocean. This process ensures a healthier environment in the deeper coastal waters around the UK and beyond.
Oxygen is just as essential for marine life as it is for land animals, but breathing underwater is far more energy-intensive. Fish need to expend considerably more energy to extract oxygen, especially the larger species like cod and haddock.
As ocean temperatures rise, this problem worsens. Warmer water holds less oxygen, and warmer fish require more oxygen for respiration, putting them at greater risk. This makes large, deep-sea fish particularly vulnerable.
The deep ocean is losing oxygen due to several factors. As organic matter sinks, it decomposes, consuming dissolved oxygen. Additionally, in spring, the ocean can form layers that don't mix much, isolating the deeper layers from the atmosphere, which is the main source of oxygen. This lack of mixing, combined with the decomposition of sunken organic matter, leads to reduced oxygen levels in the deep sea, a process known as deoxygenation.
As climate change progresses, the effects on deep-water oxygen levels are likely to worsen. Warmer water not only retains less oxygen, but also prolongs the periods of deep-water isolation. This, coupled with stronger stratification, creates a significant barrier to oxygen mixing into deeper layers, potentially leading to more severe deoxygenation during the summer.
Recent research suggests that the process of mixing oxygen into deeper waters can slow the rate of oxygen depletion by about half. This mixing is influenced by a combination of summer storms and tidal forces, which stir up nutrients in deep water, fostering the growth of phytoplankton—tiny plants that produce oxygen. As this oxygen is mixed into deeper waters, it helps alleviate the oxygen deficit.
However, this research also highlights the need to understand how warming seas and extreme weather events, such as heatwaves, affect ocean mixing. For instance, the heatwaves experienced in stratified seas around the UK in June 2023 had an unknown impact on oxygen levels.
These findings have significant implications for the proposed development of floating wind farms in seasonally stratified seas, such as the Celtic Sea and the northern North Sea. These wind farms typically feature large ballasts, counterweights, and cables beneath the surface, which can help promote ocean mixing. This enhanced mixing may encourage phytoplankton growth and support the redistribution of oxygen from the surface to deeper waters during the summer.
Thus, these results suggest that ocean mixing should be a key consideration in designing turbine foundations and planning new wind farms, ensuring that they contribute to maintaining healthy oxygen levels in the ocean.
