Study: Excess nutrients affect stream's ecosystem
University of Alabama professor Johnathan Benstead co-authors article in the journal Science
Published: Thursday, March 5, 2015 at 10:00 p.m.
Last Modified: Thursday, March 5, 2015 at 10:40 p.m.
A study published in the journal Science co-authored by a University of Alabama associate professor reports that nutrient pollution accelerates the breakdown of forest detritus in streams, affecting the food webs in the ecosystems.
“The significance is ... what we are seeing in our experiments is what we are seeing over large parts of the world because of human pollution,” said co-author Jonathan Benstead, an associate professor of biological sciences at UA.
The paper, based on nine years of work between two experiments in forest streams in North Carolina, was published in today’s edition of Science. The work was funded by the National Science Foundation and led by researchers at the University of Georgia.
Forest leaf litter in many streams and rivers is the most important source of energy, Benstead said. Many of the streams are well-shaded by the forest canopy above, limiting photosynthesis and algae growth, another major energy source in streams.
“There are always microbes and animals that make their living by breaking the leaf litter down. That is a very important process in the streams,” Benstead said.
When a leaf falls into a stream, it is quickly colonized by fungi and bacteria that begin the process of breaking it down.
“The microbes actually make it good food for the invertebrates,” Benstead said.
The invertebrates, in turn, become food for other life in the stream.
In the process, the carbon from the litter is exhaled as carbon dioxide by organisms or eventually transported downstream by the current. Carbon in the form of leaf
litter also gets stored longer term in sediments in the streambed, where it continues to break down, Benstead said.
“The carbon goes somewhere. Either it goes into the atmosphere as carbon dioxide or it goes downstream,” Benstead said.
The rate by which the process occurs is limited by the amount of nitrogen and phosphorus present in the stream. More nitrogen and phosphorus in the system means the process of breaking down the leaf litter is accelerated. The experiments by Benstead and his colleagues sought to mimic the excess nitrogen and phosphorus caused by human impact, such as agriculture and other land uses.
“We were frankly shocked at how quickly leaves disappeared when we added nutrients,” said Amy D. Rosemond, associate professor in the University of Georgia Odum School of Ecology and the study’s lead author. “By summer, the streams looked unnaturally bare.”
Benstead said the acceleration means less carbon is captured locally in the stream, and it is dispersed into the atmosphere and farther downstream faster.
“If you consider leaf litter is the base of the food web, the leaf litter is what all of the insects are feeding on, anything that affects the insects is going to affect things higher in the food web,” Benstead said.
Benstead used the example of insect species that feed on the material during their aquatic larval stages. Some may take a year or two to develop, but the accelerated breakdown of the leaf litter means their food source is exhausted quicker. The acceleration means the material, once reduced to smaller particles, is also transported downstream faster rather than remaining in the streambed near where it entered.
In natural conditions, the fall leaf fitter remains in the streams until the following summer. But with an accelerated process, it might disappear by late spring, Benstead said.
“The pattern of available food for these things will change greatly if everything disappears faster,” Benstead said.
In the team’s first experiment observing two streams from 1990 to 2005, one was left unaltered and the other had nitrogen and phosphorus added at a set ratio. Researchers observed significant changes in the community of insects. Some species declined, and others did very well under the conditions of nutrient enrichment, Benstead said.
“It’s unlikely that everything will crash, but what you will see is a shift in the populations,” he said.
The researchers observed a species of caddis fly that did very well under enriched conditions as other species suffered, Benstead said.
“That is a nice example of how the effects of nutrient enrichment really can be quite unpredictable,” he said.
The larval stage of the fly protects itself by building a case of small stones and sticks, making it difficult for predators to eat.
“The predators began to suffer because the caddis fly became so dominant,” Benstead said, noting that other species declined.
The researchers conducted their experiments at the Coweeta Hydrological Laboratory, a USDA Forest Service and NSF Long Term Ecological Research site in North Carolina.
The team chose the streams in North Carolina, among other reasons, because they were naturally low in nutrients because of low human impact.
In addition to the 2005 study, the researchers observed five streams from 2010 to 2013 to record the effects of varying the ratios of phosphorus and nitrogen meant to simulate the effects of different land uses.
“We wanted to mimic really what is quite common around the world right now,” Benstead said.
Through its experiments, the team collected the equivalent of roughly 27 years of data on the stream, Benstead said.
“The acceleration of this processing of organic matter we documented by adding nutrients is occurring now all over the world because of human pollution,” Benstead said.
Benstead said the experiments in North Carolina are complete and that the researchers are working on other papers based on their findings.
Benstead and Rosemond’s co-authors are research coordinator Phillip M. Bumpers, graduate student David W.P. Manning and professors emeritus J. Bruce Wallace at UGA’s Odum School of Ecology, UA professor emeritus Keller Suberkropp, associate professor Vladislav Gulis of Coastal Carolina University and assistant professor John S. Kominoski of Florida International University.
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