The global ocean supports a vast array of species, from the humble barnacle to the baleen whale. Anthropogenic climate change could threaten this biodiversity. Rising ocean temperatures have led to the bleaching and collapse of coral reefs. Declines in Southern Ocean sea ice seem to have deprived Antarctic krill — a food source for seals, penguins and whales — of a winter retreat. And it now transpires that ocean energy gains have taken their toll on a slightly less emotive, but no less important, group of organisms.

Phytoplankton — miniature photosynthetic organisms that float in the ocean — form the foundation of the marine food web. Collectively, phytoplankton capture large amounts of energy from the sun and convert it into organic matter, which supports all manner of marine life. A survey of global ocean transparency and chlorophyll measurements — spanning 1899 to the present day — revealed that phytoplankton levels have declined by approximately 1% per year over the past century (Nature 466, 591–596; 2010). The decrease was greatest in high-latitude and equatorial waters, and in open ocean areas far removed from the coast.

Rising sea surface temperatures may be responsible for the decline, according to an analysis of key climate variables. As the upper ocean warms, surface waters will mix less vigorously with the nutrient-rich deep ocean. This, in turn, could cut off phytoplankton food supplies.

But it's not just phytoplankton nutrient uptake that is influenced by sea surface temperatures. A separate analysis, of more than 11,000 marine organisms, suggests that patterns of diversity are shaped by sea surface temperatures (Nature 466, 1098–1101; 2010). The study is part of the Census of Marine Life, a ten-year initiative designed to document the diversity, distribution and abundance of life in the oceans. The complete census is due to be published in October and could provide a useful baseline for examining the impact of future physical change, and the loss of organisms such as phytoplankton, on marine ecosystems.

Apart from supporting much of marine life, phytoplankton help sequester carbon dioxide from the atmosphere and generate large quantities of oxygen. Thus the decrease in their abundance does not bode well for the planet. However, without a more mechanistic understanding — which cannot be derived from records of ocean clarity and chlorophyll content alone — it is impossible to say whether the phytoplankton decline can or should be extrapolated into the future.

But the findings are a wake-up call. Global ocean phytoplankton levels and physiology need to be monitored closely over the next few decades if unpleasant surprises are to be avoided. Ocean-colour satellites offer the best and most informative approach for assessing changes in phytoplankton biomass. Funding for these missions must be secured.