Oceans are losing oxygen, just as they did 94 million years ago

[Global] A recent study sheds light on why Earth’s oceans have been losing oxygen in recent decades. Globally, scientists can measure oxygen levels in the ocean and evidence points to a 2% decrease in oxygen levels in the past 50 years.

Although this decrease in ocean oxygen levels seems small, there is a ripple effect that can alter ecosystems. This, combined with estimates that oxygen levels will continue to decrease by one to seven percent by 2100, leaves scientists concerned about what lies ahead.

The decrease we see in our ocean’s oxygen will stress ecosystems and alter habits of fish and mammals. For example, larger fish that require more oxygen will be more limited in the areas they can go to eat and reproduce.

What Is Causing Oxygen Levels To Decrease?

A number of factors are likely influencing the decrease in oxygen levels in oceans. However, the key culprits are likely an increase in fertilizer use and a warming planet.

Humanity has drastically increased the amount of fertilizer we use to produce ever higher crop yields. However, when that fertilizer runs off into our waterways it eventually makes its way to the ocean. When that happens, microbial communities and algae all of a sudden receive a massive delivery of food. Bacteria and algae colonies exponentially increase to feed on the newly delivered food and when that happens they respire, same as humans. As in, they take in oxygen and release carbon dioxide, removing dissolved oxygen from the ocean.

You have likely heard of dead zones in the Gulf of Mexico for instance. This is a scenario where enough nutrients are brought to the Gulf of Mexico that you get algae and bacteria blooms, plummeting the available oxygen in the water. This, in turn, kills nearby fish that require oxygen to breathe. In case you missed it, we just had one of the largest Gulf of Mexico dead zones in history.

Mississippi River sediment plume into the Gulf of Mexico

The second culprit to declining oxygen levels is a warming planet. This one is fairly straight forward. Basic principals of chemistry and physics dictate that colder liquid can hold more dissolved gas than a warmer liquid. You can witness this by opening two Coke cans, then leaving one out on your counter and the other in the fridge. Come back the next day and I bet the one in the fridge has more dissolved carbon dioxide (carbonation).


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