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Saturday, September 15, 2012
Human Impact on Danube and black Sea
It is all about deforestation. Trees produce and sustain deep soils
and generally constrain water removal to slow seepage. When the
trees are removed, the working soil shrinks in thickness releasing
clays in particular and other non organic material. Field drainage
then combines to remove the fines and the rivers soon become silt
Thus we have a serious jump in delta building and available mineral
content as is observed.
It is good to see it so carefully mapped out for the Black Sea.
Similar conditions hold for all human occupied valleys. This is the
best argument for formally managing woodlands and associated rivers
closely and intensely. Fields girded by forest margins will simply
release their silt load into the margins to be captured generally and
allow the rivers to run quite clear enhancoing their natural
Human Impact Felt
on Black Sea Long Before Industrial Era
by Staff Writers
Cape Cod MA (SPX)
Sep 06, 2012
A map of the Black
Sea's drainage basin. The Danube's watershed is delimited by a
continuous white line, its course is highlighted in blue, and the
Danube delta is shown in green. The Danube drains most of the Central
and Eastern Europe to dominate the freshwater and sediment budget of
the Black Sea. A sediment core shown by a white circle was used to
reconstruct the salinity and ecosystem composition in the Black Sea
over the last 7000 years. (Figure courtesy Liviu Giosan, Woods Hole
Oceanographic Institution; Stefan Constantinescu, University of
When WHOI geologist
Liviu Giosan first reconstructed the history of how the Danube River
built its delta, he was presented with a puzzle. In the delta's early
stages of development, the river deposited its sediment within a
protected bay. As the delta expanded onto the Black Sea shelf in the
late Holocene and was exposed to greater waves and currents, rather
than seeing the decline in sediment storage that he expected, Giosan
found the opposite.
The delta continued to
grow. In fact, it has tripled its storage rate.
If an increase in
river runoff was responsible for the unusual rapid build up of
sediment in the delta, says Giosan, the question is, "Was this
extraordinary event in the Danube delta felt in the entire Black Sea
basin? And if so, what caused it?"
In answering those
questions, Giosan and an international team of collaborators
including environmental engineers, modelers, paleogeographers, and
paleobiologists pieced together a unique history of the region that
ultimately provides evidence for a transformative impact of humans on
the Black Sea over hundreds, if not thousands of years. The study was
published on August 30 in Scientific Reports, a new online journal of
the Nature Publishing Group.
The largest and
longest river in the European Union, the Danube is the source of over
60 percent of the freshwater running into the Black Sea, and
therefore is a dominant factor in the biogeochemistry of this basin.
Because the Black Sea is nearly enclosed, changes in its hinterland
provoked by climate or people should be readily reflected there.
The research team
reconstructed a 9000-year record of the delta's growth, and through
various scientific techniques, developed and mapped against it a
record of changes in the Black Sea's salinity, nutrients, and
relative abundance of its ecosystem's major phytoplankton groups.
Finally, they also examined the history of land use in the greater
To reconstruct the
salinity, the WHOI team analyzed sediments containing highly
resistant organic compounds called alkenones, which are uniquely
produced by Emiliania huxleyi - the same photosynthetic organism
oceanographers study to determine past sea surface temperatures. By
examining the ratio of two hydrogen isotopes in the alkenones, they
were able to map the salinity trend in the Black Sea over the last
"One of the
isotopes, deuterium, is not very common in nature," explains
WHOI marine paleoecologist Marco Coolen, "and it doesn't
evaporate as easily as other isotopes. Higher ratios of deuterium are
indicative of higher salinity."
Salinity began to rise
in the Black Sea about 9000 years ago, when the ocean invaded the
previously freshwater lake through the Straits of Bosporus, and
continued to increase until approximately 3000 years ago, when the
levels approached normal ocean values.
"But the trend
since then is counter-intuitive," says Giosan. "The entire
basin freshened, especially in the last 1,500 years." The likely
explanation points to a increase in the river input combined with a
reduced evaporation as the climate cooled at the time, he adds.
an influx of nutrient-rich river water would affect the composition
of the ecosystem, and would be reflected by the ecological history
of major phytoplankton groups in the Black Sea, such as diatoms and
traditionally conduct paleo studies of phytoplankton by using a
microscope to count the fossil skeletons found in sediment cores.
But, this method is limited because some phytoplankton leave no
fossils, so, instead, Coolen looked for sedimentary genetic remains
of the past inhabitants of the Black Sea water column.
offers the best opportunity to learn the past ecology of the Black
Sea," says Coolen. "Calcareous and organic-walled dinocysts
are frequently used to reconstruct past environmental conditions, but
90 percent of the dinoflagellate species do not produce such
diagnostic resting stages, yet their DNA remains in
the fossil record."
His analysis found
that major phytoplankton groups in the Black Sea changed drastically
over the last millennium. Beginning about 500 years ago, a rise in
diatoms suggests there was more silicate available in the Black Sea.
The team at WHOI also found an increase in the abundance of
dinoflagellate DNA beginning about 650 years ago.
Both of these changes
are consistent with increased nutrient availability that would occur
as a result of human activity - the clearing of forests and increased
agriculture. Interestingly, a significant drop in diatom DNA occurred
in recent times when more recent human activity - the damming of the
rivers - entered the scene.
The final piece of the
picture comes from a model developed by Jed Kaplan, a scientist at
Ecole Polytechnique Federale de Lausanne in Switzerland. Kaplan
looked at the expansion of lands cleared for agriculture in the
Danube watershed as population grew and farming technology became
more advanced over the last 7000 years ago.
His findings agree
very well with recently published studies of the relative abundance
of grass pollen over tree pollen and the appearance of charcoal in
the sediment record from clearing large swaths of trees by burning.
All of these land signals point to human deforestation and increased
The model suggests
that deforestation of the lower basin of the Danube rapidly
accelerated over the last millennium and especially in the last 500
years. In contrast in the western sector of the watershed, farther
from the Black Sea, deforestation was already substantial as the
Roman Empire expanded there.
started by looking at how Danube River built its delta in the Black
Sea," says Giosan. "But together these indicators tell a
story of changing land use that ultimately changed the ecosystem of
an entire sea."
The ability to define
baselines and separate natural variability from human-induced changes
is essential for understanding biogeochemical cycles and managing
marine ecosystems, which ultimately depends on the detection and
attribution of long term environmental trends, the authors write.
changed that baseline a long time ago in the Black Sea" says
Giosan. "And they probably fertilized other coastal seas with
nutrients inadvertently stripped from soils much earlier than we
the Danube delta remains an extraordinarily lush, vibrant habitat,
home to hundreds of species of birds and fish.
"This is a unique piece of land - the Danube delta - blessed,
productive, and important for countless forms of life," adds
Giosan. "What is amazing to me is that we owe much of this
biodiversity to our own history of changing Nature."