Global warming of the surface layers of the oceans reduces the upward transport of nutrients into the surface layers. This generates chaos among the plankton, according to a recent study of the University of Amsterdam and the Centrum voor Wiskunde en Informatica (CWI) in the Netherlands, and the University of Hawaii (USA). In Nature of 19 January 2006 the scientists present advanced computer simulations, which predict that plankton growth will show strong fluctuations when the supply of nutrients is reduced. This may have a negative impact on the production of the oceans and on uptake of the greenhouse gas carbon dioxide into the oceans.
Plankton plays a key role in the oceans. It forms the basis of the marine food web. Moreover, phytoplankton (microscopically small algae) consumes the greenhouse gas carbon dioxide during photosynthesis. Uptake of carbon dioxide by phytoplankton across the vast expanses of the oceans reduces the rising carbon dioxide levels in the atmosphere. Phytoplankton growth depends on light and on nutrients such as nitrogen and phosphorus. These nutrients are supplied from deeper ocean layers, and are slowly mixed upwards. In large parts of the oceans, phytoplankton is not concentrated near the surface, but at about 100 meters depth. Phytoplankton grows well at this depth, because there is a sufficient supply of light from above and a sufficient supply of nutrients from below.
Stratification of ocean waters
However, warm surface layers reduce mixing of the ocean waters. When you often go out for a swim, you may have noticed that surface water layers are heated up by the sun, while deeper water layers are often much colder. Warm water floats on cold water. This vertical stratification of the water column is widespread in the oceans. A larger temperature difference between two water layers implies less mixing of chemicals between these water layers. Global warming of the surface layers of the oceans, owing to climate change, strengthens the stratification and thereby reduces the upward mixing of nutrients.
What are the implications for phytoplankton growth? This question was investigated by Prof.dr. Jef Huisman of the Institute for Biodiversity and Ecosystem Dynamics (IBED) of the University of Amsterdam and drs. Nga Pham Thi and dr. Ben Sommeijer of the Centrum voor Wiskunde en Informatica (CWI), the national research centre for mathematics and computer science in The Netherlands. They developed advanced computer simulations, to study how reduced upward mixing of nutrients affects the growth of marine plankton. Surprisingly, the computer simulations predict that plankton populations will show strong fluctuations when vertical mixing of nutrients is reduced. This model prediction was rather unexpected, because it contradicts conventional wisdom that deep plankton in the oceans would represent a very stable system.
Hawaii
Therefore, the scientists compared their model predictions with data from long-term time series of plankton in the subtropical Pacific Ocean, carried out by Prof.dr. David Karl of the University of Hawaii. The subtropical Pacific Ocean is strongly stratified, with a low supply of nutrients into the surface layers. Phytoplankton in the subtropical Pacific indeed exhibits complex population fluctuations, consistent with the computer predictions. Reduced stability of the plankton, caused by global warming of the oceans, may result into a decline of oceanic production and reduced sequestration of the greenhouse gas carbon dioxide into the oceans.
Authors:
Prof.dr. Jef Huisman, Institute for Biodiversity and Ecosystem Dynamics (IBED), University of Amsterdam, The Netherlands
Drs. Nga Pham Thi & Dr. Ben Sommeijer, Centrum voor Wiskunde en Informatica (CWI), Amsterdam, The Netherlands
Prof.dr. David M. Karl, School of Ocean and Earth Sciences and Technology, University of Hawaii, USA.
Illustrations of the plankton oscillations can be found at the Nature website in pdf and HTML
