News | December 17, 2020

Fertilizer Runoff In Streams And Rivers Can Have Cascading Effects, Analysis Shows

A stream in Costa Rica

Fertilizer pollution can have significant ripple effects in the food webs of streams and rivers, according to a new analysis of global data. The researchers also found some detection methods could miss pollution in certain types of streams.

The analysis, published in Biological Reviews, combined the results of 184 studies drawn from 885 individual experiments around the globe that investigated the effects of adding nitrogen and phosphorus, the main components of fertilizer, in streams and rivers. While the analysis only included studies where scientists added nitrogen and phosphorus experimentally, nitrogen and phosphorus pollution can run off from farms into streams, lakes, and rivers – as well as from wastewater discharge. At high levels, fertilizer pollution can cause harmful algal blooms and can lead to fish kills.

“Overall, we found that high levels of nutrients affect streams and rivers everywhere,” said the study’s lead author Marcelo Ardón, associate professor of forestry and environmental resources at North Carolina State University. “Wherever we looked, we saw increases in the abundance and biomass of organisms that live in streams, and also the speeding up of processes that happen in streams – how fast algae grow, how fast leaves decompose, and how fast organisms grow that feed on them.”

Across the studies, the researchers saw that nitrogen and phosphorus led to increased growth across the food web, such as in algae, the insects that eat the algae and the fish that eat the insects. In shaded streams where algae doesn’t grow, they reported nitrogen and phosphorus sped decomposition of leaves and boosted growth of organisms that feed on them.

“We saw an average 48 percent increase overall in biomass abundance and activity in all levels of the food web,” Ardón said. “We also found that the food webs responded most strongly when both nitrogen and phosphorus were added together.”

While experts already use the presence of a specific type of chlorophyll – chlorophyll a – in water to detect algae growth, researchers said using that method could miss pollution in waterways where algae do not grow, and where decomposition of leaves or other plant matter is the primary source of food for other organisms.

“The food webs in those streams don’t depend on algae – the trees shade out the algae,” Ardón said. “The streams there depend on leaves that fall in and decompose, which is what the insects, such as caddisflies and stoneflies, are eating. In those detrital-based streams, we found similar responses to increases in nitrogen and phosphorus as has been found in algae.”

Another finding was that factors such as light, temperature, and baseline concentrations of nitrogen and phosphorus impacted the response to increases in the two nutrients.

“All of those things will determine how much of a response you get to increased nitrogen and phosphorus,” said study co-author Ryan Utz of Chatham University.

The findings have implications for environmental policy, Ardón said.

“The EPA has been asking states to come up with ways to reduce runoff of nitrogen and phosphorus into streams, because we know they can cause these really big problems,” said Ardón. “We know that at a big scale, and we don’t really know the details. A lot of states that are coming up with criteria to reduce the amount of nutrients in the water focus only on algal responses. Our study suggests regulators should expand their view.”

The study, “Experimental nitrogen and phosphorus enrichment stimulates multiple trophic levels of algal and detrital-based food webs: a global meta-analysis from streams and rivers,” was published Dec. 17 in Biological Reviews. The study was authored by Marcelo Ardón, Lydia H. Zeglin, Ryan M. Utz, Scott D. Cooper, Walter K. Dodds, Rebecca J. Bixby, Ayesha S. Burdett, Jennifer Follstad Shah, Natalie A. Griffiths, Tamara K. Harms, Sherri L. Johnson, Jeremy B. Jones, John S. Kominoski, William H. McDowell, Amy D. Rosemond, Matt T. Trentman, David Van Horn and Amelia Ward. The study was funded by the National Science Foundation under grant DEB-0832653 through the Long Term Ecological Research Network Office. Individual authors were funded by the National Science Foundation under grant DEB-1713502, and the Department of Energy’s Office of Science, Biological and Environmental Research.

Anthropogenic increases in nitrogen (N) and phosphorus (P) concentrations can strongly influence the structure and function of ecosystems. Even though lotic ecosystems receive cumulative inputs of nutrients applied to and deposited on land, no comprehensive assessment has quantified nutrient-enrichment effects within streams and rivers. We conducted a meta-analysis of published studies that experimentally increased concentrations of N and/or P in streams and rivers to examine how enrichment alters ecosystem structure (state: primary producer and consumer biomass and abundance) and function (rate: primary production, leaf breakdown rates, metabolism) at multiple trophic levels (primary producer, microbial heterotroph, primary and secondary consumers, and integrated ecosystem). Our synthesis included 184 studies, 885 experiments, and 3497 biotic responses to nutrient enrichment. We documented widespread increases in organismal biomass and abundance (mean response = +48%) and rates of ecosystem processes (+54%) to enrichment across multiple trophic levels, with no large differences in responses among trophic levels or between autotrophic or heterotrophic food-web pathways. Responses to nutrient enrichment varied with the nutrient added (N, P, or both) depending on rate versus state variable and experiment type, and were greater in flume and whole-stream experiments than in experiments using nutrient-diffusing substrata. Generally, nutrient-enrichment effects also increased with water temperature and light, and decreased under elevated ambient concentrations of inorganic N and/or P. Overall, increased concentrations of N and/or P altered multiple food-web pathways and trophic levels in lotic ecosystems. Our results indicate that preservation or restoration of biodiversity and ecosystem functions of streams and rivers requires management of nutrient inputs and consideration of multiple trophic pathways.

Source: North Carolina State University