There wasn't a real conclusion because the tests were always small. Governments became afraid of triggering toxic blooms and otherwise messing up coastal ecosystems and then being liable for damages, so they put a moratorium on testing [1]. Then in 2012 someone illegally tried it again anyway, with mixed results [2].

Basically it's like most geoengineering projects - everyone is too afraid of negative consequences to really try anything at scale, so instead we get endless inconclusive studies. Personally, I think we're going to pretty quickly get to the point where we need to make decisions about what negative consequences are acceptable to prevent complete environmental catastrophe.

[1] https://www.nature.com/articles/453704b

[2] http://www.planetexperts.com/two-years-russ-george-illegally...