“Streams and rivers have long been thought to be important as ‘nutrient filters’, but the data showing this was indirect and circumspect,” Pat Mulholland of Oak Ridge told environmentalresearchweb. “With the increasing concern about such issues as hypoxia [lack of oxygen] in the Gulf of Mexico and many other estuaries and coastal oceans, red tides etc, there is a need for studies to help identify how natural systems may be operating to reduce nutrient pollution and how this capacity is being degraded by human alteration of the landscape – e.g. channellizing or eliminating streams, eliminating riparian vegetation and the ability to store organic matter, or overloading streams with nutrients from over-application of fertilizers.”
Mulholland and colleagues followed the fate of nitrate containing the 15N isotope in streams and rivers in the US and Puerto Rico. In each of the 8 regions studied, three of the streams were bordered by agricultural lands, three were surrounded by urban areas and three by vegetation typical of the area. The study showed that streams are very important and effective sinks for nutrient pollutants – especially nitrogen – in the landscape, removing a considerable portion of the nutrients from water.
Rivers and streams are able to remove nitrogen in two ways – plants and microbes can store the nitrogen as a nutrient, and denitrifying bacteria in sediments can convert nitrates to molecular nitrogen and nitrous oxide, which then leave the river in gaseous form.
“It’s the entire stream network that is important in removal of pollution because of the serial nature of these networks – small streams flow into larger streams, and so on, until rivers eventually empty into the oceans,” said Mulholland. “The most effective removal of pollutants occurs when the entire network is viable and functioning, including the smallest streams which are about 80% of the total length of streams in the network and are the first to receive most of the pollutant loads from the land and groundwater.”
The researchers found that streams can be overloaded if pollutant inputs to them from the land are very high, reducing their effectiveness as pollutant filters. As a result, much larger amounts of pollutants reach the lakes, estuaries and coastal oceans downstream. “It is very important to maintain the ecological integrity of stream networks,” said Mulholland.
“Our work has very strong implications for some of the legislation now being considered in state and the federal government [in the US],” he added. “Some have argued that small streams should not be regulated in terms of the Clean Water Act, and our research shows that even the smallest streams are important for pollution control and removal from water.”
The team’s research also impacts on issues such as zoning and development, where streams are often destroyed, channelized, re-routed or degraded. “Our work indicates that streams and their riparian (floodplains and near-stream environment) should be protected because of their role as pollutant filters, protecting drinking water sources and ecological systems downstream,” said Mulholland. “Stream restoration ... may [also] be a way to reduce nutrient pollution.”
Now the researchers hope to conduct follow-up studies on nutrient cycling and removal in streams and rivers, focusing on seasonal variations and the long-term fate of the portion of the nutrients that are taken up by stream algae and bacteria. Many of the team are also involved in planning the stream component of the US National Ecological Observatory Network (NEON). An initiative of the US National Science Foundation, NEON is “intended to study the detailed response of ecosystems at many locations across the US to such large-scale changes as global climate change, land use change and changes in air pollution”.
The researchers reported their work in Nature.