What are artificial fertilizers and what are they good for
Dead zones are caused by a variety of factors, but one of the most common is the use of artificial fertilizers. One of the most successful cleanups in the Black Sea was actually caused by stopping the usage of artificial fertilizers(this was actually not intentional, an economic collapse in the region led to the inability to use these farming techniques, but that's another story). So one question that immediately follows is "why not stop using artificial fertilizers full stop? The answer to that is somewhat complex, and the short of it is that it's almost certainly impractical to do so.
So what is artificial fertilizer anyways?
Artificial fertilizer is soil with a variety of additives designed to make it yield a better crop. The biggest and most notable(and most infamous) of these is nitrogen, which also happens to be what we are interested in. In the 1800's a process for extracting nitrogen was developed and leveraged to insert this nitrogen into the soil. This nitrogen has a lot of potential energy, which is good for fueling things such as plant growth or explosions (hence fertilizer bombs, in fact a good deal of the original research for extracting nitrogen was from military developments). Today a more modern process known as the Haber-Bosch process is used, but the idea remains the same, take dirt, add macronutrients for plants to eat, add nitrogen and voila, good fertilizer. These ingredients are found naturally in several places, most notably in manure, however not in quite as ideal quantities as in artificial fertilizers.
Only a small portion of the nitrogen actually goes into the plants, so the rest is runoff, which is where it goes into the ocean. In the ocean it has exactly the same effect that it does on land, it causes plants to grow really well. If these plants happen to be certain types of algae, then the air is consumed in water and dead zones occur. There is a certain irony that an excess of life causes death, but this an example of natures delicate balance, if one plant gets too greedy or successful, the whole system suffers.
Artificial fertilizer is one of the major achievements of the industrial revolution, and to this day is one of the most influential inventions ever made. This is also only a brief overview of the whole details, for example there are both organic and inorganic fertilizers, there are a variety of styles and recipes for making and a variety of ways they are used, however the fundamental goal of these fertilizers is to make stuff grow, and when they are turned into runoff, they are still active, and succeed in the ocean as well as they did on land, disrupting a delicate ecosystem.