Plankton: doing more than just drifting through

Thimble jellies, Linuche unguiculata, one of the many types of zooplankton. Photo courtesy of Rich Galiano

Plankton is found all over the planet – from the polar-regions to the tropics, freshwater lakes to the sea. Plankton are mostly small organisms, drifting along with the currents and unable to swim against the waves. Unnoticed by most, these small creatures at the bottom of the food chain give life to most of what we see in the sea. Climate change around the world is having numerous impacts on the oceans, and a major concern is how plankton is being affected.

The importance of plankton

Planktonic photosynthesis accounts for roughly half of the primary productivity on earth and plays an important role in the ocean's carbon cycle. If you live by the coast this means that some of the oxygen that you breathe has come from plankton. As photosynthesis occurs in phytoplankton, carbon dioxide is incorporated into the cells and taken out of the environment. During this process more than 100 million tons of inorganic carbon is fixed each day around the world, reducing the amount of carbon dioxide in the atmosphere [1].

Diatoms under the microscope (x400), an important part of the photosynthetic phytoplankton.

This massive conversion of inorganic carbon into a useable form allows much of the life in the oceans to survive. Carbon gets converted into sugars that are stored in cells that are eaten by zooplankton, filter feeders, and baleen whales. Zooplankton are eaten by small fish species, which are eaten by salmon, tuna, seabirds, marine mammals and on throughout the food web. Phytoplankton is in such high demand that the entire phytoplankton biomass of the world's oceans is consumed by filter feeders, from barnacles to baleen whales, every 2 to 6 days![1]

Why study plankton?

By examining patterns in plankton distribution we can learn what affect climate change is having on marine ecosystems. Since plankton is not harvested or exploited like fish or intertidal organisms, adjustments in distribution and abundance can be attributed to changing environmental factors [2]. As plankton are indicators of healthy aquatic environments, long-term studies have been carried out on plankton since the 1930s with numerous research projects continuing today [2].

Warming of the world's oceans has already caused major shifts in plankton distribution and abundance on a global scale. From 1999 to 2004, when a Sea Surface Temperature (SST) change exceeding ± 0.15°C occurred in 74% of the globe's oceans, a decrease in plankton productivity in these areas was also observed [1], effectively decreasing the amount of available energy in the food chain.

We now know that the earth's oceans have been warming for the past 40 years [3], and the Intergovernmental Panel on Climate Change (IPCC) report in 2007 indicates that warming is going well below the surface to at least 3000 meters [4]. Warming waters will impact many phytoplankton species that live in the top layers in order to carry out photosynthesis, and studies show that when phytoplankton populations suffer, so do other species in the surrounding area [2].

Planktonic connections

Plankton bloom off the west coast of British Columbia and the Olympic Peninsula, an area known for strong upwelling that brings nutrients to the surface, supporting diverse and numerous ecosystems in the area. Satellite image from NASA in the public domain.

As water temperatures increase, plankton species normally found farther south are being observed in more northern waters that are no longer too cold for them to live in [2]. While some plankton is moving further north, many other species are being left behind. In the wake of low plankton densities around Scotland, drastic decreases in plankton-eating fish populations have resulted in vacant seabird nest sites due to starving birds. Hundreds of Black Guillemots have been reduced to just a few. Thousands of Arctic Tern nests sit empty. In recent summers, the largest colony of Skuas had only a few chicks [2]. Without the plankton from the oceans to feed the fish, these seabird populations who depend on fish as prey are being devastated.

Plankton populations in the Pacific Ocean also decrease during warm water El Nino/Southern Oscillation (ENSO) periods. During these events not only is the water warmer but there is a decrease in upwelling, the process that brings nutrient-rich water from the bottom to the surface. With fewer nutrients, phytoplankton are less productive, and this has repercussions on seabirds, marine mammals, and fish due to their reliance on this usually abundant food source. Global warming is predicted to increase the frequency of ENSO events, which may lead to less plankton being available [5].

The trouble with change

Warming water is not the only threat to plankton. As atmospheric carbon dioxide (CO₂) levels increase, the oceans absorb more of this gas. It's been estimated that since the year 1800, the oceans have taken up roughly 120 billion metric tons of human generated CO₂ [2]. Currently, the oceans are up-taking roughly 20-25 million tons each day – with no relief in sight [2].

As the oceans take in carbon dioxide the gas forms carbonic acid, lowering the pH of the ocean water and turning it dangerously acidic. This acidification is occurring at a rate 100 times faster than ever recorded, with some estimating that by the end of the 21^st century, the surface waters in some of the worlds oceans may not be able to support shell-bearing plankton [2]. The IPCC report released in February 2007 stated that even with significant CO₂ emission reductions, the oceans would still see a decrease in pH by about 0.14 units. Without a reduction in CO₂ emissions, pH is expected to decrease another 0.35 units, which is too low for some organisms to form shells [4].

Historical evidence shows that plankton does not recover easily from catastrophes. When a population crash occurred across the oceans 65 million years ago, it took approximately 3 million years for the plankton to recover [2]. How the plankton around the world will be affected in the long-term by abrupt climate change is difficult to predict. We do know that with less plankton, ecosystems from the poles to the tropics, and from freshwater to the salty seas will be negatively impacted.

Plankton lesson plan

A research diver and a large jellyfish, Cyanea capillata, and other smaller jellies in Barkley Sound, on the
west coast of Vancouver Island.

1. Behrenfeld, M.J., et al., Climate-driven trends in contemporary ocean productivity. Nature, 2006. 444 .

2. Dybas, C.L., On a collision course: oceans plankton and climate change. BioScience, 2006. 56 (8).

3. Barnett, T.P., et al., Penetration of the human-induced warming into the world's oceans. Science, 2005. 309 .

4. Alley, R., et al., Climate Change 2007: The Physical Science Basis, Summary for Policymakers. 2007, Intergovernmental Panel on Climate Change. p. 18.

5. Hansen, J., et al., Global temperature change. PNAS, 2006. 103 : p. 14288-14293.