Marine Phytoplankton Declining: Striking Global Changes at the Base of the Marine Food Web Linked to Rising Ocean Temperatures
ScienceDaily (July 28, 2010) — A new article published in the 29 July issue of the journal Nature reveals for the first time that microscopic marine algae known as “phytoplankton” have been declining globally over the 20th century. Phytoplankton forms the basis of the marine food chain and sustains diverse assemblages of species ranging from tiny zooplankton to large marine mammals, seabirds, and fish. Says lead author Daniel Boyce, “Phytoplankton is the fuel on which marine ecosystems run. A decline of phytoplankton affects everything up the food chain, including humans.”
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Using an unprecedented collection of historical and recent oceanographic data, a team from Canada’s Dalhousie University documented phytoplankton declines of about 1% of the global average per year. This trend is particularly well documented in the Northern Hemisphere and after 1950, and would translate into a decline of approximately 40% since 1950. The scientists found that long-term phytoplankton declines were negatively correlated with rising sea surface temperatures and changing oceanographic conditions.
The goal of the three-year analysis was to resolve one of the most pressing issues in oceanography, namely to answer the seemingly simple question of whether the ocean is becoming more (or less) „green’ with algae. Previous analyses had been limited to more recent satellite data (consistently available since 1997) and have yielded variable results. To extend the record into the past, the authors analysed a unique compilation of historical measurements of ocean transparency going back to the very beginning of quantitative oceanography in the late 1800s, and combined these with additional samples of phytoplankton pigment (chlorophyll) from ocean-going research vessels. The end result was a database of just under half a million observations which enabled the scientists to estimate phytoplankton trends over the entire globe going back to the year 1899.
The scientists report that most phytoplankton declines occurred in polar and tropical regions and in the open oceans where most phytoplankton production occurs. Rising sea surface temperatures were negatively correlated with phytoplankton growth over most of the globe, especially close to the equator. Phytoplankton need both sunlight and nutrients to grow; warm oceans are strongly stratified, which limits the amount of nutrients that are delivered from deeper waters to the surface ocean. Rising temperatures may contribute to making the tropical oceans even more stratified, leading to increasing nutrient limitation and phytoplankton declines. The scientists also found that large-scale climate fluctuations, such as the El-Niño Southern Oscillation (ENSO), affect phytoplankton on a year-to-year basis, by changing short-term oceanographic conditions.
The findings contribute to a growing body of scientific evidence indicating that global warming is altering the fundamentals of marine ecosystems. Says co-author Marlon Lewis, “Climate-driven phytoplankton declines are another important dimension of global change in the oceans, which are already stressed by the effects of fishing and pollution. Better observational tools and scientific understanding are needed to enable accurate forecasts of the future health of the ocean.” Explains co-author Boris Worm, “Phytoplankton are a critical part of our planetary life support system. They produce half of the oxygen we breathe, draw down surface CO2, and ultimately support all of our fisheries. An ocean with less phytoplankton will function differently, and this has to be accounted for in our management efforts.”
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Marine Biodiversity Strongly Linked to Ocean Temperature
ScienceDaily (July 29, 2010) — In an unprecedented effort that will be published online on the 28th of July by the international journal Nature, a team of scientists mapped and analyzed global biodiversity patterns for over 11,000 marine species ranging from tiny zooplankton to sharks and whales. The researchers found striking similarities among the distribution patterns, with temperature strongly linked to biodiversity for all thirteen groups studied. These results imply that future changes in ocean temperature, such as those due to climate change, may greatly affect the distribution of life in the sea.
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The scientists also found a high overlap between areas of high human impact and hotspots of marine diversity.
Much research has been conducted on diversity patterns on land, but our knowledge of the distribution of marine life has been more limited. This has changed through the decade-long efforts of the Census of Marine Life, upon which the current paper builds. The authors synthesized global diversity patterns for major species groups including corals, fishes, whales, seals, sharks, mangroves, seagrasses, and zooplankton. In the process, the global diversity of all coastal fish species has been mapped for the first time.
The researchers were interested in whether there are consistent “biodiversity hotspots” — areas of especially high numbers of species for many different types of marine organisms simultaneously. They found that the distribution of marine life showed two fundamental patterns: coastal species such as corals and coastal fishes tended to peak in diversity around Southeast Asia, whereas open-ocean creatures such as tunas and whales showed much broader hotspots across the mid-latitude oceans.
The scientists also tested whether these global patterns could be consistently explained by one or more environmental factors. Temperature was the only factor found to be linked with the distribution of all species groups, with the availability of habitat also playing a role.
Says lead author Derek Tittensor of Dalhousie University, “it was striking how consistently temperature was linked with marine diversity. This relationship suggests that ocean warming, such as that due to climate change, may rearrange the distribution of oceanic life.” Co-author Walter Jetz of Yale University notes “while we are increasingly aware of global gradients in diversity and their associated environmental factors, our knowledge of patterns in the ocean has lagged behind that of patterns on land. Our study attempts to help overcome this disparity.”
The study also assessed the overlap between hotspots of marine diversity and human impacts, i.e. the combined effects of fishing, habitat alteration, climate change and pollution. Human impacts were found to be particularly concentrated in areas of high diversity, suggesting the potential for severe species losses in these regions. Says co-author Camilo Mora of Dalhousie University, “the combined effects of exploitation, habitat alteration, pollution and climate change are threatening the diversity of life in the global ocean. Our research provides further evidence that limiting ocean warming and other human impacts will be particularly important in securing these hotspots of marine biodiversity into the future.”
Co-author Boris Worm of Dalhousie University also highlights the need to maintain biodiversity in the face of these impacts: “biodiversity and the functioning of ecosystems are often tightly coupled, with highly diverse ecosystems providing more goods and services that benefit human beings, as well as being more resilient in the face of disturbance, than less diverse ecosystems. The observed concentration of human impacts in our richest marine areas is a worrying indication of our growing footprint in the oceans.”
Many of the data used for this study come from the Ocean Biogeographic Information System, (OBIS) a public database created by the Census of Marine Life. Says Edward Vanden Berghe of Rutgers University, co-author of the paper and executive director of OBIS: “with OBIS we’ve created a framework for sharing and re-using data, which makes this type of global, all-encompassing science possible.”
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Signs of Reversal of Arctic Cooling: Rapid Temperature Rise in the Coldest Region of Mainland Europe
ScienceDaily (July 29, 2010) — Parts of the Arctic have cooled over the past century, but temperatures have been rising steeply since 1990. This is the finding of a summer temperature reconstruction for the past 400 years produced on the base of tree rings from regions beyond the Arctic Circle.
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German and Russian researchers analysed tree growth using ring width of pine from Russia’s Kola Peninsula and compared their findings with similar studies from other parts of the Arctic. For the past 400 years since AD 1600, the reconstructed summer temperature on Kola in the months of July and August has varied between 10.4°C (1709) and 14.7°C (1957), with a mean of 12.2°C. Afterwards, after a cooling phase, a ongoing warming can be observed from 1990 onwards.
Researchers from the Institute of Geography in Moscow, Hohenheim University and the Helmholtz Centre for Environmental Research (UFZ) report in journal Arctic, Antarctic and Alpine Research: “The data indicate that solar activity may have been one of the major driving factors of summer temperatures, but this has been overlaid by other factors since 1990.”
The researchers used for this study wood samples from a total of 69 Scots pines (Pinus sylvestris) from the Khibiny Mountains on the Kola Peninsula, situated between the Arctic Circle and the ocean port of Murmansk, not far from the Finnish border. The investigated region is a transition zone between Scandinavia, which is strongly affected by the gulf stream resp. North Atlantic Current, and the continental regions Eurasia. This makes the region particularly interesting for climatological studies.
Kola has a cold-temperate climate with long, moderately cold winters and cool, humid summers. In this part of the Arctic, the mean temperature fluctuates between -12°C in January and +13°C in July, with a growing season of just 60 to 80 days. The northern taiga vegetation is dominated by spruce, pine and birch. The samples came from three locations in the Khibiny Mountains close to recent altitudinal timberline at altitudes of between 250 and 450 m above sea level. The geographical northern timberline lies approximately 100 km further north.
In earlier studies, researchers led by Tatjana Böttger from the UFZ were able to show that pine forests on the Kola Peninsula expanded between 7000 and 3500 years ago to about 50 km north of their present-day limit.
However, for this study, they used trees from the altitudinal timberline, since they respond very sensitively to temperature fluctuations and provide particularly useful information, as demonstrated by US researchers in November 2009 in the journal PNAS when they used a long-lived species of pine in California and Nevada to show that these trees had grown particularly fast over the last 50 of the past 3500 years because of higher temperatures.
In the Tree-Ring-Laboratory at the University of Hohenheim in Stuttgart the German researchers measured the width of the individual tree rings. The calibration of these data with the help of meteorological records for the last 127 years and the interpretation of results occurred together with Russian Academy of Sciences in Moscow and the Helmholtz Centre for Environmental Research in Halle. “Besides of temperature, growth is also strongly influenced by non-climatic factors like light, nutrients, water supply and competition from other trees. So it is vital to isolate these trends to obtain a climate signal as pure as possible,” explains Yury M. Kononov from the Russian Academy of Sciences in Moscow.
Following the summer temperature reconstruction on the Kola Peninsula, the researchers compared their results with similar tree-ring studies from Swedish Lapland and from the Yamal and Taimyr Peninsulas in Russian Siberia, which had been published in Holocene in 2002. The reconstructed summer temperatures of the last four centuries from Lapland and the Kola and Taimyr Peninsulas are similar in that all three data series display a temperature peak in the middle of the twentieth century, followed by a cooling of one or two degrees. Only the data series from the Yamal Peninsula differed, reaching its peak later, around 1990. What stands out in the data from the Kola Peninsula is that the highest temperatures were found in the period around 1935 and 1955, and that by 1990 the curve had fallen to the 1870 level, which corresponds to the start of the Industrial Age.
Since 1990, however, temperatures have increased again evidently. What is conspicuous about the new data is that the reconstructed minimum temperatures coincide exactly with times of low solar activity. The researchers therefore assume that in the past, solar activity was a significant factor contributing to summer temperature fluctuations in the Arctic. However, this correlation is only visible until 1970, after which time other — possibly regional — factors gain the upper hand. “One thing is certain: this part of the Arctic warmed up after the end of the Little Ice Age around 250 years ago, cooled down from the middle of the last century and has been warming up again since 1990,” says Dr Tatjana Böttger, a paleoclimatologist at the UFZ.
In September 2009, another international team presented model calculations showing that the Arctic had gradually cooled down by around 0.2 °C per thousand years over the last two millennia to the start of the Industrial Age. They attributed this to a gradual decline in solar radiation in the summer. However, the last decade was the warmest of the Common Era and was 1.4 °C above the forecasts, report Darrell S. Kaufman and his colleagues in Science. The new data produced by Kononov, Friedrich and Böttger support the thesis that solar activity seems to be a significant factor influencing summer temperatures in the Arctic, but that its influence has weakened considerably over the past few decades.
phill Parsons
