What Is An Ocean Dead Zone
But what is an ocean dead zone and how does a cow affect the ocean so detrimentally?
An ocean dead zone is an area of ocean, usually coastal which are hypoxic, this means that the oxygen that is usually present in those areas, has been removed, making that area dead and unfit for life to continue.
And without oxygen in that area, the sealife that exist and live there either end up dying or moving out from the area if they are mobile and lucky enough to do so.
Added Nutrients From Farming Are The Main Cause
There are many factors that can contribute and create these areas, but mostly it is a case of nutrients (from manure) getting in to a water source that leads to the ocean and the algae that is naturally present grows larger, more rapidly or more algae is created because of these nutrients and in this process of decomposition uses the oxygen that is present around them.
Ocean dead zones can also appear in some areas naturally, without the input of humans creating them.
Mostly though, and the largest ocean dead zones we have ever seen, are because of our input.
This is what NASA have to say about it.
However, these zones may grow larger with the additional nutrient inputs from agricultural runoff. 
This process happens naturally all over the oceans, but in smaller and more manageable sizes that the sea can work with, usually there is a balance and the dead zones are small or don't last long.
Dead zones occur around the world, but primarily near areas where heavy agricultural and industrial activity spill nutrients into the water and compromise its quality accordingly. 
But with the nutrients that run off of farms in the form of manure, this process can become explosive and ever expanding, covering hundreds of square miles of ocean.
Nutrient pollution is one of America's most widespread, costly and challenging environmental problems. 
The primary sources of excess nitrogen and phosphorus are:
- Agriculture: The nitrogen and phosphorus in animal manure and chemical fertilizers are necessary to grow crops. 
The statement above comes directly from the environment agency of the USA.
A minor detail at the end of their statement that animal manure is required to grow crops is wrong, we have veganic farming proving that.
In this article by George Monbiot about dairy farming and river pollution, you can easily see the issues that are caused and understand that when a river is polluted from farming run-off, such as manure, the oceans and coastal areas they run in to also become polluted and die off, creating ocean dead zones.
However, the majority of dead zones worldwide are a direct result of farm animal agricultural practices. [3}
It is the improper disposal of nutrient-rich animal waste that is the leading cause of ocean dead zones. The US Environmental Protection Agency estimates roughly 335 million tonnes of manure (measured in dry weight) is produced by livestock in the United States alone. Coupled with excess fertilizer run-off from the production of animal feeds such as corn and soy, the by-products of animal agriculture are a deadly combination for our oceans. 
At the bottom of this article we have included citations to statements we have made above.
As you can see, with the statements we have added above, this isn't a local phenomenon or one that happens infrequently.
They are continually happening all over the world, with dead zones getting larger and larger.
In this documentary by George Monbiot he demonstrates and shows how animal manure which runs in to the river systems are the leading cause of river pollution, which in turn makes its way in to our oceans.