Study identifies the cascading effects of fertilizer pollution

In a new study from NC State, researchers are describing the cascading effects of fertilizer pollution throughout the food webs of rivers and streams. The experts also identified some methods of detecting fertilizer runoff that may not be effective in certain types of streams.

The team analyzed data from 184 studies which involved 885 individual experiments conducted all over the world to investigate the effects of fertilizer runoff in rivers and streams.

The research was specifically focused on elevated levels of nitrogen and phosphorus, the main two components of fertilizer, which can kill fish and cause harmful algal blooms.

“Overall, we found that high levels of nutrients affect streams and rivers everywhere,” said study lead author Professor Marcelo Ardón.

“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.”

The studies revealed that nitrogen and phosphorus led to increased growth in algae, the insects that eat the algae, and the fish that eat the insects. In shady streams where there were no algae, nitrogen and phosphorus accelerated the decomposition of leaves and boosted the growth of the 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,”said Professor Ardón. “We also found that the food webs responded most strongly when both nitrogen and phosphorus were added together.”

Experts look for the presence of a specific type of chlorophyll in water to detect algae growth. According to the researchers, this method could miss pollution in waterways where algae do not grow, and where the decomposition of leaves or other plant matter is a primary food source.

“The food webs in those streams don’t depend on algae – the trees shade out the algae,” explained Professor Ardón. “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.”

The analysis also pinpointed certain factors that influenced the food web’s response to fertilizer pollution. These factors include light, temperature, and baseline concentrations of nitrogen and phosphorus. 

“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 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,” concluded Professor 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 was published in the journal Biological Reviews.

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By Chrissy Sexton, Earth.com Staff Writer