The study estimated half of it comes from fishing nets (i.e. commercial fishing) and 20% from the Tōhoku tsunami. As to where the remaining 30% comes from I will leave to the scientists, FNN experts notwithstanding.
Enforcing treaties on open-sea commercial fishing is challenging, to say the least. Fishing gear is lost; whether accidentally or on purpose, it should fall on that industry to monitor, report, and fund the clean-up for it's portion of the mess.
Save the rhetoric. The studies are necessary, not only to identify the scope of the problem, but also to fend off the critics who would say that without the studies, any claims about this problem would be baseless and anecdotal (in their cynical, disingenuous minds).
It's not a kelp paddy. It's a big floating pile of trash. Composed mainly of materials used to kill fish. It is not an ecosystem; it is environmental pollution within an ecosystem. If you have any evidence to back up your bet of the ecological benefits of this pollution, by all means present it.