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Note to editors: Alison Appling can be reached for additional comment at (919) 964-0196 or alison.appling@gmail.com. Jim Heffernan can be reached at (919) 681-4193 or james.heffernan@duke.edu

DURHAM, N.C. – Scientists have long searched for an accurate way to measure the growth rates, nutrient metabolism and other physiological traits of microscopic aquatic organisms, which often behave differently in the lab than in the field. Over the last decade, they’ve also struggled to interpret the increasing amounts of near-continuous water quality data being made available by new sensor technology.

A new study by 91�������� researchers may hold answers to both problems.  

The study, published in the September issue of The American Naturalist, finds that subtle differences in how nutrient concentrations vary over time in a stream, lake or ocean could be used to measure basic physiological traits of the hard-to-study organisms.

Put simply: we can learn a lot about ecosystems and organisms by simply watching nutrients come and go.

“Our model suggests that from a one-time experimental addition of a nutrient, or by studying natural pulses of resources such as sunlight, researchers may be able determine the growth rates of these organisms,” says postdoctoral associate Alison Appling, who led the study. “We might also be able to learn about the organisms’ abilities to acquire and store nutrients for later use; to take up nutrients opportunistically; or to change their internal composition to better match the nutrient content of their organic or inorganic food supplies.

“This provides a new conceptual tool for understanding links between patterns of fine-scale nutrient variation and physiological traits, and for interpreting the wealth of data on nutrient variation that is now pouring in from these new water quality sensors,” she says.

To conduct the study, Appling, now a postdoctoral associate at the University of New Hampshire, built on standard models of nutrient metabolism that have been used to understand competition and productivity of phytoplankton, but used them to examine how temporal variation in inputs of one resource – light, nitrogen, or phosphorous – influences temporal variation in the export of others.

Her model shows that the presence or absence of diel nutrient variation or responses to resource pulses may be an indicator of nutrient limitation. It also shows that variation in physiological traits can modify the temporal patterns of uptake in response to single or repeated nutrient pulses, and that differences in the physiology of nitrogen and phosphorus metabolic pathways can de-couple their uptake in time.

These findings can help scientists address the tricky problem of identifying the limiting nutrient in an ecosystem, says Jim Heffernan, assistant professor of ecosystem ecology and ecohydrology, who co-authored the study.

“Traditional techniques for measuring nutrient limitation can be time-consuming and difficult to apply accurately, but this study suggests that the limiting resource can be diagnosed with nutrient sensors by simply observing whether the potentially limiting nutrient varies synchronously or independently from other nutrients,” he says. “It’s potentially an elegant solution to a difficult problem.”

Identifying a limiting nutrient is important, he explains, because a limiting nutrient is one whose addition could stimulate faster growth, possibly even to the point of eutrophication – i.e., toxic algae blooms – or make it easier for non-native species to invade. 

The new model applies to aquatic ecosystems, but down the road it may prove applicable for investigating physiology and limitation in terrestrial ecosystems, as well.  

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CITATION: “Nutrient Limitation and Physiology Mediate the Fine-scale (De)coupling of Biogeochemical Cycles,” A.P. Appling and J.B. Heffernan, published September 2014 in The American Naturalist, http://www.jstor.org/stable/10.1086/677282