Lakes reveal low phosphate concentrations

Like any other critters, organisms at the bottom of food chains need certain nutrients to thrive. One of those nutrients is phosphorus. Its availability, in the form of phosphate, in most lakes limits the growth of microorganisms including bacteria and phytoplankton.

Now, studies using a new technique indicate that common methods of measuring phosphate concentrations that scientists have used for decades have been routinely off by factors of 100 to 1,000. The latest analyses find phosphate at surprisingly low concentrations in 56 undisturbed lakes sampled throughout Canada.

“It’s one of the most important nutrients, particularly in lakes, because [a low concentration] restricts microorganisms,” says Jeff J. Hudson of the Dorset Environmental Sciences Centre and Trent University in Ontario. “It then restricts production of the rest of the food web.” Hudson and his colleagues publish their work in the July 6 Nature.

Too many nutrients-sometimes from fertilizer or urban runoff—can promote overgrowth of algae, which can block sunlight and deprive fish of oxygen. Microorganisms in the lakes that the team studied, however, appear to obtain most of their phosphate from recycling processes in the food web, says Hudson.

“This is an incredible report, if it’s true,” comments David M. Karl of the University of Hawaii in Honolulu. The indication, he says, is that “organisms can assimilate phosphorus while growing rapidly,” even at the dramatically lower phosphate concentrations the new studies indicate, he says.

The work holds global consequences as well, says Karl. Researchers increasingly are identifying phosphorus’ importance in oceans, he says. “We probably don’t fully understand the dynamics of something even as fundamental as nutrients in these aquatic systems,” says Karl.

Most scientists have recognized that previous methods, which try to directly measure phosphate concentration, overestimate it. So, Hudson and his colleagues tried a new, indirect measurement based on their findings that the rates at which the food web absorbs and releases phosphate appear to be the same in the lakes they sampled.

Since the researchers can measure the release rate, they can use an equation that relates the organisms’ phosphate uptake rate to the phosphate concentration in water. In their new analysis, the team determined how much phosphate the plankton released back to water by following the movement of trace amounts of radioactive phosphate through lake samples.

When Hudson’s group compared their results with other measurements of the same lakes, they found that the concentrations of phosphate revealed by the new technique were 0.1 percent of those determined with a common chemical test. They were 1 percent of those made with a more complicated method called the Rigler radiobioassay.

“This is a sobering report,” says Karl. “It means that a hundred years of research may be in jeopardy because we’ve built models on what we thought the concentrations were.”