In shallow coastal regions, these zones can be caused by runoff of excess fertilizers from farming. (sciencecodex.com)
Rains wash excess fertilizer from farms into interior waterways, which eventually empty into the ocean. (nrdc.org)
One of the benefits of constructing these artificial wetlands was thought to be in cleaning and filtering polluted water, including mitigating the effects of excess fertilizer runoff, which has been contributing to hypoxic zones in the ocean. (treehugger.com)