Where do harmful algal blooms occur globally and why have HAB increased dramatically in the last few decades?

In Australia, our major river system, the Murray-Darling, has had numerous algal blooms over the last decade, due to increased fertiliser run-off from farm land and decreased water flow due to drought conditions. The fertiliser causes high nitrogen conditions, which depletes oxygen in the water and this is compounded by the fact that the water level is low and slow moving.