Evidence of the decline of ocean surface phytoplankton since the turn of the century

5th August 2010

You may have seen the press coverage of the paper by Boyce and colleagues (Nature 466: 591-596). The paper relates a clever study where scientists have examined chlorophyll data obtained through various methods, including very primitive ways of assessing the transparency of the sea, to figure out how levels of primary production have changed since 1899. For those of you who find that confusing let me explain. On land, the primary producers are the plants that we are all familiar with, grass, trees, shrubs and so on. These plants use the sun’s energy to trap (fix) carbon from atmospheric carbon dioxide and they use this to synthesise organic molecules (photosynthesis). These form the basis of the energy that drives life and the molecules that form structures, enzymes and so on. Animals graze plants and this energy and material is used by them to grow, and these animals in turn are eaten by other animals (predators) and so on, forming successive layers or trophic levels of food chains, many of which go to form foodwebs. In the oceans, carbon is fixed through the same process of photosynthesis but in this case most of the work is done by tiny cells, phytoplankton, that live in the surface layers of the ocean (upper 150m depth). The phytoplankton are grazed by zooplankton, tiny animals that also live in the surface layers of the ocean, and these in turn are preyed on by larger animals, including fish and so on up to the large marine predators we are familiar with such as sharks and tuna. Some very large animals also graze on zooplankton, such as baleen whales short-circuiting lengthy food chains which lose a lot of the energy of primary production at each step.

Whilst people are familiar with the process of photosynthesis on land, and they are aware that plants can be used as food (cereals like wheat etc), they may not be aware that about half of the production on Earth takes place in the oceans. We do not directly consume phytoplankton but we do eat fish, that ultimately live off the phytoplankton, and which feed many millions of people and provide a livelihood for many others. However, photosynthesis also has other critical effects that are important to maintaining the balance of the Earth’s life support system. These include the production of oxygen, a by-product of photosynthesis, and the draw-down of CO₂ from the atmosphere into the deep ocean. It is the latter that has meant that the oceans have taken up nearly half of humankind’s CO₂ emissions. The oceans are providing us with food, with the air we breathe and they clean up the atmosphere through the process of photosynthetic primary production.

The paper by Boyce and colleagues (2010) demonstrates that the concentrations of phytoplankton in the surface layers of the ocean have declined in 8 out of 10 of the major ocean regions of the world. This is occurring mainly through an effect of global warming. When the surface layers of the ocean are heated up they have a lower tendency to mix with the cooler deeper layers, a process called stratification (layering) of the surface of the oceans. The phytoplankton only live in the surface waters because they require sunlight to photosynthesise and therefore to live. However, they also require nutrients other than carbon to synthesise organic molecules, particularly nitrogen and phosphorus but also other nutrients such as sulphur and iron. As the phytoplankton grow in surface water layers they use up these nutrients and their growth becomes limited. The phytoplankton cells undergo a form of starvation. Tropical oceans are naturally stratified, whilst in temperate seas stratification occurs during the summer but storms during the winter mix up the surface layers of the oceans with deeper, nutrient rich water, annually replenishing the nutrients phytoplankton need to grow. Global warming has increased the level of stratification throughout the oceans and thus has caused a decrease in global primary production by phytoplankton through reducing the supply of nutrients from deep water to the oceans surface.

The simple fact is that almost all life in the oceans, with the exception of chemosynthetic communities that are based on chemical energy, depend on surface production by phytoplankton. Even the deepest living organisms live off the rain of dead cells and occasional carcasses descending from the surface layers of the ocean. Much of this material arrives in the deep sea in the form of marine snow, aggregates of dead phytoplankton cells, cast off skins of zooplankton, faecal material etc. The reduction in surface primary production will have impacted almost other marine life in the oceans, even reducing the production of the fish that we eat. That this has been happening for so long is disturbing. It seems that even by the turn of the twentieth century production was declining as a result of industrial activity, a long time before we became aware of climate change.