Long-term phytoplankton decline

In the newest of what has become an increasingly alarming series of items demonstrably showing the decline of ocean life, Nature published online today an articled entitled “Global phytoplankton decline over the past century” (behind pay wall) by Daniel G. Boyce, Marlon R. Lewis and Boris Worm, all of Dalhouise University. The three compiled 100 years of data from measurements by Secchi disks since 1899 as well as observations from satellites since 1979 to show a long-term and significant deline in ocean phytoplankton. The Secchi disk was invented in the 1860s; it is a simple white disk, lowered into water until it cannot be observed. The depth of that point is recorded. The group analyzed 445,237 data points from observations using Secchi disks since 1899. They estimate that phytoplankton has declined on average about 1% per year, and since 1950 by 40%. The decline is most prnounced in the Northern Ocean (where the data are also more reliable). The Indian Ocean has, however, seen an increase in phytoplankton biomass, owing to fertilizer run-off from Indian rivers.

The decine of phytoplankton appears to be related to global warming. Yearly fluctuations “are strongly correlated with basin-scale climate indices, whereas long-term declining trends are related to increasing sea surface temperatures,” they say in their abstract.

Of course the decline of phytoplankton could act as an amplifier of climate change because the organisms represent such a high percentage of total global photosynthesis. A decline in photosynthesis reduces the amount of carbon removed from the atmosphere. This could lead to a vicious cycle, although lead author David Boyce told the BBC: “It’s tempting to say there will be further declines, but on the other hand there could be other drivers of change, so I don’t think that saying ‘temperature rise brings a phytoplankton decline’ is the end of the picture.”

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