William M. Lewis, Jr.
Aquatic ecosystems, tropical fresh waters, biogeochemistry of inland waters, nutrient cycling, aquatic food chains.
Nitrogen takes a detour along the South Platte
In aquatic environments, ammonia (NH3) released from organic nitrogen (total oxidized nitrogen, TON) by microbes is oxidized to nitrate (NO3-) by the nitrification process of other microbes in the presence of oxygen through a series of steps involving nitrite (NO2-) as an intermediate (see figure). The result is an increase of nitrate and a decrease in ammonia and TON in the environment. In addition, nitrate, when entering aquatic sediments where oxygen is absent, is converted by microbes through the process of denitrification to N2 gas, an inert end product that corresponds to elimination of nitrogen pollution from the aquatic environment. These processes occur at very high rates in the South Platte River, which, like many other rivers that lie within the drainage of large population centers, receives substantial amounts of ammonia and nitrate from treated wastewater and agricultural practices. The two processes can be quantified based on mass balance of the input and output components for reactions as shown in figure.
Validity of mass balance calculations for nitrification and denitrification processes can be checked independently by calculation of mass balance for oxygen. The nitrification process, which produces NO3- as an end product, uses a predictable amount of oxygen. Thus, the oxygen mass balance, which incorporates all processes affecting dissolved oxygen, incorporates a prediction of ammonia loss caused by nitrification. Large inconsistencies between the computed oxygen loss caused by nitrification and the observed oxygen loss from the South Platte River indicates that another process is causing loss of ammonia. Assimilation of ammonia by algae is too small to account for this large difference, so the mass balance points to anaerobic ammonium oxidation (ANAMMOX), a process that is known to occur in aquatic environments but has never been quantified as a major factor in nitrogen mass balance for rivers. An estimation of the rates based on mass balance of oxygen and nitrogen components of the relevant processes suggests that ANAMMOX accounts for 50 percent of ammonia loss in the South Platte River below Denver. This information on the South Platte suggests that ecologists have not given sufficient attention to the importance of ANAMMOX, which is hidden within the related chemical transformations associated with the long-recognized processes of nitrification and denitrification in aquatic systems, and particularly those that carry large pollutant loads.
Honors and Awards
- Past President, American Society of Limnology and Oceanography
- Member of the National Research Council's Water Science and Technology Board, 1993-1999
- Recipient of the Naumann-Thienemann Medal of the International Society of Pure and Applied Limnology, 1999
- Sustained Achievement Award of the Renewable Natural Resources Foundation, 1996
- Chair, National Research Council Committee on Endangered and Threatened Fishes in the Klamath River Basin (2001-2003).