Ocean acidification and poison seas

CO2 and phosphate availability control the toxicity of the harmful bloom dinoflagellate Karlodinium veneficum

I have commented here before that additional CO2 in the ocean may make some algae grow better, but my guess was that algal blooms are not necessarily something you want to encourage. The article above indicates that this is true, especially for toxin producing algae:

Growth rates or toxicity of K. veneficum could increase substantially in the future with high CO₂ levels in the ocean, depending on P availability, and so interactions between rising CO₂ and eutrophication could cause major shifts in present day patterns of harmful algal toxin production. These results suggest that over the coming decades, rising CO₂ could substantially increase karlotoxin damage to food webs in the often P-limited estuaries where Karlodinium blooms occur.

Eutrophication is additional nutrients in the water, typically from coastal run off. In other words, it would appear that coastal areas near cities that already have occasional toxic algal blooms can expect it to get worse with more CO2 in future.

Here's a good Australian article by a Tasmanian professor on the expected increase in harmful algal blooms. The conclusion:

We can expect: (1) Range expansion of warm-water species at the expense of coldwater species which are driven pole wards; (2) Changes in the abundance and seasonal window of growth of selected HAB species; (3) Earlier timing of peak production of some phytoplankton; (4) Knock-on effects for marine food webs, notably when individual zooplankton and fish grazers are differentially impacted by climate change (“match-mismatch” disturbances). Some harmful algal bloom phenomena (e.g. toxic dinoflagellates benefiting from land runoff and/or water column stratification, tropical benthic dinoflagellates responding to coral reef disturbance) may become worse, while others may diminish in areas currently impacted. The greatest problems for human society will be caused by being unprepared for significant range extensions or the increase of algal biotoxin problems in currently poorly monitored areas, thus calling for increased vigilance in seafood biotoxin monitoring programmes. Predicting the impact of climate change on algal blooms presents a formidable challenge!