A review of: Boyce, D. G., Lewis, M. R., and Worm, B. (2010) Global phytoplankton decline over the past century. Nature 466: 591-596
Recent research has suggested that phytoplankton numbers are plummeting. Marine phytoplankton are important marine organisms, strongly influential in major climatic process and biogeochemical cycles as well as being responsible for over half of global primary production. A decline in phytoplankton numbers does not just threaten ocean processes and diversity. Three billion people depend on seafood as part of their diet and nearly one tenth of the world’s population depend on the fisheries industry.
Perhaps the most obvious suggestion for this decline can be found in global warming. More than 90% of the heat retained by the Earth due to greenhouse gases end up in the sea. This causes surface waters to warm and as they do so, become less dense, preventing colder nutrient-filled waters to rise to the surface. This, in turn prevents the waters mixing and so, the phytoplankton in the warmer waters run out of nutrients required for growth. In this article, the authors compile and analyse in-situ chlorophyll (Chl) and ocean transparency measurements collected over the last century, to investigate the changes in phytoplankton biomass and to see whether this is in fact happening.
The authors found that in 59% of the data used, there was a decline in phytoplankton numbers. It was also observed that Chl declined more rapidly, the further the distance from land. They suggest this may be due to an increasing intensity of vertical stratification and ocean warming. The global oceans were then split into ten regions in which to evaluate regional trends and found that there was phytoplankton decline in 8 of the 10 regions. The largest declines were found in the South and equatorial Atlantic regions.
All in all, the analysis suggested that global Chl concentration and therefore phytoplankton biomass had declined over the last century, and has declined by almost 40% since the 1940s. Evidence from the analysis related primarily to climate variation, particularly to rising sea surface temperatures. These results are therefore consistent with the hypothesis that increasing ocean warming is changing the marine ecosystem with implications for both biogeochemical cycling and population numbers. The authors hope that this study provides an incentive for greater observation and remedial action.
Additional reference: Holmes, B. (2012) Too-blue oceans: The invisible famine. New Scientist 2859
3 comments:
Hi Rachelle
Interesting paper. The authors kind of contradict the conclusions made in my first blog in october. In that article they compared different environmental models and found that rising sea temperatures may cause a shift in phytoplankton species and actually increase levels photosynthesis. This will have a postive impact on the microbial loop and the rest of the marine food web. Did the authors in your study discuss changes in phytoplankton species?
Hope the revision is going well.
Sarmento, H. Montoya, J. Vazquez-Dominguez, E. Vaque, D and Gasol, J. (2010) Warming effects on marine microbial food web processes: how far can we go when it comes to predictions? Phil. Trans. R. Soc. B. 365,2137-2149
Hi Lee
They do don't they?! The authors don't look at changes in phytoplankton species in this article but look at chlorophyll data as an indicator of phytoplankton biomass to look at general fluctuations in phytoplankton numbers. This enabled them to analyse data over a long period of time. It is acknowledged that warmer conditions do help phytoplankton to grow faster but the authors suggest that it is the lack of upwelling from the nutrient-rich deeper waters, caused by the warming surface sea temperature, that are inhibiting phytoplankton growth.
Thanks, you too!
After reading your comments, I felt I had to read both papers, although both put forward some interesting points, such as an increase in photosynthesis levels due to an increase in sea temperature, with room and suggestions for further areas of study. A nice study to see would be a direct comparison of findings between the model approach found in Lees review and the chlorophyll data found in Rachelles to see if both backed up one another.
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