They are a result of all the extra nutrition where a river meets the sea, and this happens whether the source is from agricultural or natural vegetation, though obviously agricultural runoff is generally more intensely nutritious..
What we humans call garbage, is often vital nutrition and habitat for animals. In the middle of the ocean, in places once devoid of life, vegetation and fish thrive among floating garbage. It may not sound pretty to us, or mesh with our Little Mermaid visions of life under the sea, but the real wildlife is glad of it.
All this is not to say that humans are not guilty of any negative environmental impacts, staggering quantities of birds, large migrating species and birds of prey, are killed by pulmonary barotrauma- having their lungs sucked out by vacuums created by wind turbines, vastly more than the gulf oil spill killed - and every single year
Some comments are so totally devoid of wisdom that they don't deserve reply>