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Chemical Geology Highlights Importance of Diel Processes in Rivers and Lakes

View of the Big Hole River in southwestern Montana with mountains in the background
Big Hole River in southwestern Montana. USGSN scientists and their university colleagues have shown that photosynthesis by aquatic plants causes large diel (daily) cycles in pH, dissolved oxygen concentrations, and the isotopes of oxygen and carbon. In addition, the aquatic plants are thought to cause large changes in streamflow as oxygen production during photosynthesis stiffens the plants and thereby retards downstream flow during the day.

Many natural chemical processes in rivers and lakes respond to the daily (diel) changes in sunlight intensity and exhibit a day-night, or 24 hour, cycle. Understanding these diel processes, as well as their sometimes dramatic affects on stream chemistry, is critical for regulatory agencies and the greater scientific community. Scientists have increased focus on the mechanisms responsible for these diel fluctuations in recent years. New studies are showing that combinations of physical, chemical, and biological processes are involved and that a wide range of chemical constituents can be affected.

U.S. Geological Survey (USGS) scientist David Nimick and his colleague Christopher Gammons from Montana Tech of The University of Montana edited a special issue of Chemical Geology that highlights the latest work on understanding diel processes in a variety of settings. The special issue is composed primarily of papers resulting from presentations at a topical session held at the annual meeting of the Geological Society of America in October 2009. The topical session brought together hydrologists, biologists, geochemists, and ecologists to discuss field studies, laboratory experiments, theoretical modeling, and measurement techniques related to diel cycling. The papers in the special issue are varied and include a comprehensive review article as well as articles reporting new findings on diel changes in concentrations of nutrients, organic carbon, algal pigments, dissolved inorganic carbon, trace elements, and mercury.

USGS scientists have been studying processes responsible for diel cycling of trace metals in both acidic (low pH) and alkaline (high pH) streams since the 1980s. More recently, studies have expanded to include diel cycling of other chemical constituents. The topical session and the special issue of Chemical Geology have brought together diverse researchers who have studied many different aspects of diel cycling phenomena found in rivers and lakes. Expanded knowledge of diel cycling may lead to better predictions of how aquatic ecosystems might react to changing conditions of contaminant loading, eutrophication, climate change, drought, industrialization, development, and other variables.

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