Pollutants cause quick growth, leave less for the future

Adding more nutrients to bodies of water leads to the rapid growth of organic matter plants and animals use for fuel, according to a new analysis of global rivers and streams.

But long-term exposure to elevated levels of nitrogen and phosphorus can cause unexpected changes and even declines in plants and animals, according to an analysis of 184 studies and 885 experiments where researchers added nitrogen and phosphorus to streams and rivers.

The analysis showed that production and consumption rates climbed. More algae grew. More leaves decomposed. More microbes ate up oxygen in the water. This happened consistently across the planet, especially where light was more prevalent and temperatures were higher.

Left unchecked, long-term nutrient pollution could create a runaway train effect. Fueled by the added nutrients, stored resources become rapidly consumed, leaving less and less for future use.

“The overwhelming majority of energy from plants is not directly consumed, but decomposed by microorganisms, so increasing rates of decomposition can result in less resources for longer-lived animals,” said John Kominoski, an FIU Institute of Environment ecologist and co-author of the study. “They can become resource diminished.”

Ramped up demand by nutrient-enriched microorganisms, Kominoski said, can quickly outstrip stored energy in ecosystems from years and years of plant growth. Plants and animals that do remain in nutrient-enriched aquatic ecosystems will likely be fewer. They will likely be smaller.

Fast-growing plants — think harmful algal blooms— produce oxygen but also a lot of fast-food-like energy that microbes rapidly consume along with oxygen in the water, contributing to fish kills like the one that affected Biscayne Bay earlier in the year.

As Kominoski discovered in an earlier study, U.S. streams and rivers already carry higher levels of nitrogen and phosphorus than is recommended.

The best way to keep this runaway train from even leaving the station is to curb run-off from agricultural fields, waste-water treatment plants, septic tanks and lawn clippings from entering our waterways, Kominoski said.

“Our aquatic systems are very sensitive to the injection of nitrogen and phosphorus,” he said.

The global analysis led by researchers at North Carolina State University was published in the journal Biological Reviews.

FIU is ranked No. 9 in the world for positive impact on life below water by The Times Higher Education Impact Rankings. The university ranked third in the United States.