Environmental News Phosphate: How low can it go?

L imnologists at Canada's Trent University have found that conventional chemical and

radiochemical techniques grossly overestimate concentrations of phosphate, an essential nutrient for aquatic microorganisms. In a study of 56 Canadian lakes pub­lished in the July 6 issue of Nature (see figure at right), Jeff Hudson and colleagues find that phosphate concentrations can be 100 to 1000 times lower than estimated with current conventional techniques.

This finding suggests mat some of the fundamental assumptions underpinning models of nutrient dynamics in aquatic systems need to be revised, according to David Karl, an oceanographer and micro­biologist at the University of Ha­waii-Honolulu.

Karl coauthored a paper re­cently published in Science that reports picomolar-level phos­phate concentrations, 100 times lower than previous measure­ments, in the Sargasso Sea in the North Atlantic, Karl says. The group used an innovative chemi­cal method for the measure­ments. (Wu et al., Science 2000, 289, 759-762).

Bom papers point out that in temperate lakes and some parts of the ocean, dissolved bioavailable phosphorus, in the form of phos­phate (P03~4), limits the growth of plankton, the first link in the aquatic food web, because it is the scarcest essential nutrient.

Models that attempt to explain how plankton participate in other biological and geochemical cycles rely, in part, on an understanding of phosphate cycling.

These new findings suggest that much remains to be learned. "It is incomprehensible to biolo­gists that microorganisms can get by on so little phosphate," Karl says. "We don't understand how the organisms can do this. We don't even know which organisms

are the main players in phos­phate cycling."

"Most of what we know about aquatic microorganisms is based on laboratory studies," Karl adds. "But we are starting to find out that the microorganisms that do well in the lab are not the most important in nature," Karl explains.

Finding such low phosphate concentrations in lakes and in some parts of the ocean should inspire scientists to conduct a new host of creative experiments and eventually lead to a better understanding of nutrient dy­namics, Karl predicts.

Hudson, who is affiliated with the Dorset Environmental Science Centre of Trent University in Ontario, and colleagues investigated a di­verse group of lakes from Canada's Rocky Mountains, Interior Plains, and Precambrian Shield. They sam­pled crystal-clear, nutrient-poor lakes and nutrient-rich lakes with abundant plankton. The character­istics of the studied lakes also ranged from soft to hard water, from shal­low to deep water, and included many different kinds of fish.

As a result, the researchers be­lieve their findings could be ap­plicable to any and all phospho­rus-limited temperate lakes, says Hudson.

The scientists estimate that phosphate concentrations in these lakes range from 27 to 885 picomo-lar (10~12) pM. So eager for phos­phate are the microorganisms and so efficient are they at taking it up that the total pool of available phosphate in these lakes is ex­changed in 10 minutes or less.

These observations indicate that phosphate concentrations limit the growth of microorgan­isms in these lakes.

In addition, the results illus­trate that biological processes within the food web meet the phosphorus requirements of the plankton. After microorganisms take in phosphate, they recycle it through excretion and other pro­cesses, which include sloppy eat­ing, when one microorganism bites into another but does not eat it all, leaving intracellular ma­terial—and phosphate—to reen­ter the water, Hudson says.

Lake phosphate estimates drop Phosphate concentrations determined by the new steady-state method are 2 to 3 orders of magnitude less than concentrations estimated by the traditional soluble reactive phosphorus method (SRP). Nutrient content increases from left to right for 14 Canadian lakes from 0.06 u.M total phosphorus to 2.6 u.M total phosphorus.

Source: Nature 2000,405,54-56.

© 2000 American Chemical Society 3 7 0 A • SEPTEMBER 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS

Until recently, external sources, such as atmospheric deposition and soil runoff, were thought to play a much more sig­nificant role in setting phosphate concentrations.

"This paper provides a rigor­ous analysis of the discrepancy between the chemically measured phosphate and the amount in solution measured using chemi­cal mass balances," says David Lean, who holds the National Sci­ence and Engineering Research Council Chair of Ecotoxicology at the University of Ottawa in Can­ada.

Since the late 1950s, limnolo-gists have known that chemically determined values of phosphate were too high, but many monitor­ing programs continue to mea­sure "phosphate" in this manner. Most scientists just did the best they could with the measure­ments available, Lean explains.

"The kinetic analysis used here provides valuable insights into phosphorus dynamics in lakes and illustrates that we still have a lot to learn about the limiting nu­trient phosphorus and the control of microbial metabolism in fresh waters," Lean says.

Hudson and co-workers in­vented a new method, which is based on steady-state mass bal­ances, to get around the problems of direct chemical measurement of phosphate. They measured the up­take and release of radiolabeled phosphate and used these rates to calculate the concentration of dis­solved phosphate.

"Our results indicate that these food webs remain incredibly pro­ductive with incredibly small amounts of phosphate present," says Hudson. A few previous studies support the low estimates obtained with the steady-state bioassay. —REBECCA RENNER

Scotchgard ban highlights unknowns Fluorinated organic surfactants are in the spotlight following a May decision by chemical manufacturer 3M Corp. to remove Scotchgard, its highly popular stain repellant, and other products that contain the surfactants from the market. One compound used to produce Scotchgard, perfluoro-octane sul­fonate (PFOS), led 3M to take this dramatic step after researchers funded by 3M found PFOS in hu­man blood and in animal tissue samples from many parts of the world where it is not manufactured.

PFOS has been found to accu­mulate in birds and mammals, not through fats as other persis­tent organic compounds do, but through a nonlipid mechanism involving the blood and liver, ac­cording to Michigan State Univer­sity zoologist John Giesy, who has analyzed archived animal tissue samples for 3M. Animal tests also indicate that PFOS causes death in adult monkeys, and rat off­

spring that received high doses of PFOS in utero. Research papers reporting these findings are ex­pected to be published soon.

Researchers and regulators worldwide are furiously investigat­ing the environmental fate, trans­port, and impact of PFOS, as well as many other fluorinated surfac­tants in use.

Questions surrounding these issues currently outweigh the an­swers. "Fluorine is being intro­duced into all manner of chemicals at an unprecedented rate," says University of Toronto chemist Scott Mabury, who is one of the few nonindustry scientists studying these compounds, thanks to fund­ing from the Canadian govern­ment. "But what we know about fluorine chemicals in the environ­ment is less than what we knew about chlorine chemicals in the 1950s," he warns.

The products containing PFOS include coatings for textiles, pa-

Government Watch

Combustion emissions To combat acidification, eutroph-ication, and ground-level ozone, the European Union's (EU) coun­cil of environment ministers has agreed on emission limits for large electric power plants. If the EU Parliament approves the June 22 directive as crafted, it would set nationwide emission standards for S02, NO ,̂ volatile or­ganic compounds, and ammonia (NH3). These standards would be met by 2010, but mem­ber countries could set tougher limits.

The environment ministers negotiated for a year on this directive because some countries did not want to set emission limits on currently op­erating plants. But discussions picked up following the sugges­tion to devise nationwide stan­dards, or national emission ceil­ings. Under this approach, combined emissions from all power plants in one country must not exceed the emission ceiling. This would make it pos­sible for a country to choose which plants will need to cut air pollution. Plants that produce 50 MW of power or more, including those licensed before the old 1987 EU directive, would have to meet the proposal's limits.

Science at U.S. EPA A high-level science and tech­nology czar, with a six-year ten­ure, should be installed at the U.S. EPA to keep the regulatory agency from adjusting its sci­ence to fit current policy, a com­mittee of the National Research Council (NRC) concludes. In a new report, "Strengthening Sci­ence at the U.S. EPA: Research Management and Peer Review Practices", the fourth and final in a series, the committee con­cedes that its 1995 recommenda­tion to designate the director of

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SEPTEMBER 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 3 7 1 A