Environment Counts | Impact of climate change on phytoplankton
Author: Geoff Zeiss – Published At: 2012-03-25 18:04 – (1095 Reads)
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Oceanic phytoplankton contribute about half of the world’s net primary production (NPP) of organic compounds from carbon dioxide, equivalent to all the world’s land plants combined. This article reports global ocean NPP changes detected from space between 1997 and 2005. The period is dominated by an initial increase in NPP, followed by a prolonged decrease. These trends are tightly coupled to climate variability. From the observed reductions in ocean productivity during the recent post-1999 warming period, the article projects the impact of future climate change on NPP and the dependent food chain. Nature 444, 752-755 (7 December 2006)
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Phytoplankton, which live in the top 100 meters of the world’s oceans and convert 45 to 50 billion tonnes of inorganic carbon (CO2) into their cells every year. For comparison all the world’s land plants incorporate 52 billion tonnes of inorganic carbon every year.
Much of the phytoplankton are consumed by the marine food chain in the upper 100 meters of the ocean. About 15% of this organic material drifts down to the lower depths of the ocean below 500 meters. About 0.1% of this is deposited on the ocean floor and over millions of years becomes the petroleum that we use as a source of energy and other products. As a result phytoplankton determine the composition of the deep sea. The best know example is the ratio of nitrogen to phosphorous atoms which in the deep sea is 16, the same ratio as in phytoplankton.
Nutrients(nitrogen, phosphate, iron), carbon dioxide, and sunlight are required to convert by phytoplankton to produce carbon compounds and oxygen. The availability of nutrients is regulated by physical processes of ocean circulation, mixed-layer dynamics, upwelling, and atmospheric dust deposition. In fact it was phytoplankton that 2.4 billion years ago created Earth’s oxygen rich atmosphere, about 2 billion years before there were land plants (the oldest land plants are about 425 million years old).
In this study the researchers compared satellite measurements of ocean chlorophyll with global climate temperature between 1997 and 2005. As the climate warmed the upper layer of the ocean warmed, as measured by sea surface temperatures (SST), and the mixing between the upper layer and the colder layers below was reduced. This reduced the nutrients in the upper layer which reduced phytoplankton growth and decreased pumping of organic compounds to the deep seas. The period is dominated by an initial increase in NPP of 1,930 teragrams of carbon a year (Tg C/yr), followed by a prolonged decrease averaging 190 Tg C/yr. The result is less CO2 removal from the atmosphere, reduced fisheries, and altered ecosystems.
Nature 444, 752-755 (7 December 2006)
