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too much nitorgen
International Ecology Congress report
IE 2002 - Day 5 - Saturday 17 August 2002
Food webs spinning out of proportion
Investigator: Eugene Turner
Saturday Aug 17th, 2002
by Bea Perks
Marine food web structures are tightly controlled by changes in the
relative level of dissolved inorganic nutrients, says Eugene Turner,
director of the Coastal Ecology Institute at Louisiana State University.
His latest unpublished data suggest that nitrate load is dangerously
high in many of the world's largest rivers.
Marine food web structures are regulated by ratios, rather than absolute
concentrations, of dissolved inorganic nutrients, says Turner. "There's
a proportionality to life," he said.
For example, it is not the concentration of dissolved inorganic nitrate
(DIN) that determines food-web structure, so much as the ratio of DIN
concentration to dissolved inorganic phosphate (DIP) concentration,
Turner explained. An increase in nitrate load alone could prove
disastrous, he added.
Diatoms - microscopic photosynthetic algae that make up the primary
level of marine food chains - need a DIN:DIP ratio of below 16:1, the
so-called "Redfield ratio", Turner noted.
In order to form their protective shells, diatoms also require dissolved
silicate (DSi) to be present at more than an atomic ratio 1:1 with DIN.
Previous work by Turner's group in Louisiana showed that decreases in
the DSi:DIN ratio in the Mississippi river have altered the composition
of food webs, triggering changes that generate harmful algal blooms
instead of beneficial diatom populations.
Turner has recently analyzed data on the concentration of dissolved
inorganic nutrients, including nitrate, phosphate and silicate, in all
the world's largest rivers. The data are gleaned from a global database
representing 37% of the Earth's watershed area and half its population;
the findings are shortly to be published in Biogeochemistry.
The average total nitrogen concentration varies over three orders of
magnitude among the world river watersheds, Turner reports, and is
primarily dependent on variations in dissolved nitrate concentration.
There is a direct relationship between the ratio of DIN:DIP, he adds.
When nitrate levels rise above a certain point, the Redfield ratio is
exceeded, implying that the phosphorus limits diatom growth.
Silicate levels are much less variable between rivers than are nitrate
levels, suggesting that DSi levels are controlled by physical factors
(i.e. run-off from the surrounding land), rather than anthropogenic
factors, Turner says.
Since DSi levels are relatively stable, the DSi:DIN ratio is controlled
by nitrogen loading, Turner suggests. "Increased nitrogen loading is
thus driving the world's largest rivers towards a higher DIN:DIP ratio
and a lower DSi:DIN ratio," he said.
More than 7% of the world's population lives in watersheds that have
switched to a DSi:DIN ratio approaching 1:1, and a further 20% live in
watersheds with ratios close to 2:1. Predictions based on current rates
of industrial development suggest that these percentages will increase,
says Turner, particularly in China, East Asia and the Baltic states.
Since his earlier data from the Mississippi river showed that a DSi:DIN
ratio approaching 1:1 could trigger the appearance of noxious algal
blooms, these latest global data are a grave cause for concern, he says.
"We should be measuring silica, and in most places we're not," Turner
said. "We need to pay attention to the switch [in DSi:DIN ratio]."
By "just kind of poking around," Turner said, he has discovered that the
ratios are "way below" the "magic" 1:1 point. The ratios have been
dropping in France, in the Baltic, and in the US, Turner noted. "It's
probably happened in Korea but they just haven't been measuring it."