Nature Geoscience 9, 219–221 (2016); published online 11 January 2016; corrected after print 16 August 2016.

In the version of this Letter originally published, it was incorrectly stated in Figure 1 that Chlorophyll-a concentrations were recorded on 12 January 2013, when the date should have been 1 December 2013. This has been corrected in the online versions of the paper.