Theories included ocean warming and changing chemical ocean compositions.

These trap large amounts of the greenhouse gas methane, which ocean warming has the potential to release.

Since 1950 alone, algal biomass decreased by around 40%, probably in response to ocean warming - and the decline has gathered pace in recent years.

Phytoplankton concentrations in surface waters were estimated to have decreased by about 40% since 1950 alone, at a rate of around 1% per year, possibly in response to ocean warming.