The main way humans are contributing to hypoxia is excessive amounts of nitrogen and phosphorus put in the water from agriculture. Monocultures need massive amounts of fertilizer with high amounts of phosphorus and nitrogen to maintain crop productivity. When it rains these chemicals leech out to nearby rivers and creeks which then take the chemicals to wherever the rivers lead. This excessive amount of chemicals in the water causes cultural eurtrofication (a massive plant bloom or algael bloom) which takes oxygen out of the water, killing the marine life below.
These rivers and creeks lead to other places like the ocean after they leave the farm which is what farmers don't understand. Tragedy of the commons is alive and well and farmers have the mentality that it's the next guys problem but we need to change that way of thinking. Aside from agriculture sewage, industrial and vehicle emissions, and some natural factors are causing hypoxia as well.
Oceans Monthly/June, 2014 2