Blue Planet II has shown many sequences of the bounty of life in our oceans: the feasting Mobular rays sequence (episode 1); a plankton bloom causing a “Manta cyclone” (episode 3); a shark finding the perfect location at Darwin’s arch to give birth because of the abundant food supply (episode 4); the plankton bloom in the seasonal seas (episode 5), and so on.
The main reason for all of these amazing sights are microscopic algae: phytoplankton. Phytoplankton bloom by utilising available nutrients and harnessing the energy from sunlight. The sunlight powers the process by which organisms and plants use the inorganic form of carbon (carbon dioxide), water and other dissolved nutrients in seawater to form organic compounds (such as lipids (a type of fat), proteins and sugars), they produce oxygen as a by-product. The whole process is called photosynthesis and it happens in terrestrial plants too. This marine photosynthesis forms the basis for nearly all marine food chains and it is called primary production. Primary producers include marine algae (such as phytoplankton, a kelp forest), sea grasses and bacteria (such as blue green algae).
Plankton blooms from the topics to the poles depend on the availability of sunlight and nutrients
There are two physical limitations for oceanic primary productivity: sunlight and nutrients. In the tropical region, even though sunlight is available year around, primary productivity is limited due to a lack of nutrients in the surface waters. As soon as all the available essential nutrients for growth have been utilised in the surface ocean, the plankton bloom ceases. These waters are vertically stable (we say stratified) because solar heating warms the sea surface and this prevents further supply of nutrients from deeper waters, unless there is mixing by eddies, turbulence and storms that can break this barrier.
In contrast, the polar regions have nutrients in plenty due to stronger winds mixing the waters, but sunlight is limited to the summer. This summer warmth also releases nutrients from melting sea ice so polar waters produce some of the largest seasonal plankton blooms.
In temperate latitudes, the primary productivity peaks twice: in the spring, and in the autumn when there is sufficient sunlight and nutrients.
This general pattern of global primary productivity can be supplemented with additional short-lived (for days or weeks) plankton blooms caused by storms and hurricanes supplying essential nutrients into the surface waters.
Marine photosynthesis also regulates atmospheric carbon dioxide
One-third of the carbon dioxide emitted by burning fossil fuels is captured by marine photosynthesis. The captured carbon, organic and inorganic (in calcifying plankton) along with nitrogen, phosphorous and other chemicals (bounded in the organic matter produced from photosynthesis), from the surface ocean is vertically transported. Most of the organic matter decays and components are recycled as subsurface nutrients, which can become available through mixing into the surface waters for more primary production. A small proportion reaches the deep providing food for the benthic organisms in the abyss, and an even smaller proportion gets buried in the sediments locking carbon away for millions of years. Although small, the latter is responsible for regulating the climate on Earth.
Additionally, a by-product of marine photosynthesis provides us with oxygen, a vital ingredient for sustaining majority of life forms on Earth. The marine photosynthesis accounts for approximately half of the oxygen produced on Earth and the other half comes from all the terrestrial plants combined.
The future of phytoplankton growth
The rising sea surface temperatures causing stratification, variable nutrient availability and changing seawater chemistry is likely to impact the growth of primary producers. Some research shows that the warming oceans will slow down the phytoplankton growth, and some computer models present unexpected results in the Southern Ocean, indicating primary productivity increasing in some regions and decreasing in others. We do need more observational data to fully understand the future of phytoplankton productivity in rapidly changing oceans.
If you would like to learn more about our oceans, the physical and chemical properties of seawater, the causes for the variability in seawater properties and ocean circulation, see our courses S206 Environmental Science and S309 Earth processes.