Dissolved organic carbon and chromophoric dissolved organic matter properties of rivers in the USA
Dissolved organic carbon (DOC) is present in all rivers, lakes, and oceans. When present in high concentrations, it imparts color to the water, making it look like tea. The DOC in freshwater lakes and rivers comes largely from dead leaves and pieces of wood present in soils, which are carried into the waterways either in dissolved form or as small pieces of leaves and wood that later break down once they reach the water. This source of organic matter becomes a form of food for microorganisms in rivers, lakes, and oceans, and hence plays an important role in the aquatic food chain and in the delivery of carbon from the land to the ocean. In some aquatic systems, such as estuaries, scientists are able to estimate the concentration of DOC by measuring the color of the water, which is called the Colored Dissolved Organic Matter (CDOM).
This CDOM measurement gives useful information not only about the aquatic environment and ecosystems, but also about the ways that the land-based sources of carbon are delivered to aquatic systems. Commonly, CDOM comes from naturally occurring decay products on land and in water. However, human activities, such as logging and agriculture, are also sources of CDOM in aquatic systems. In addition, if the water is stagnant, as in lakes and manmade reservoirs, the sun shining through the water can help break down the CDOM in a process called “photochemical degradation.” Fortunately, it is relatively easy and inexpensive to measure the CDOM content in small volumes of water by measuring the absorbance of certain wave lengths of light by the water, which is related to the color of the water.
To examine the circumstances under which measurement of CDOM accurately gives estimates of DOC concentration, Spencer and colleagues made the simple measurements of CDOM and some more difficult and complicated measurements of DOC concentration in water collected from 30 rivers across the United States. The rivers included a wide range of climate, watershed environments, and human influence. Overall, the authors found that CDOM measurements served as a good estimate of DOC concentration in 26 rivers, but not in 4 large rivers, namely, the Colorado, Columbia, Rio Grande, and St. Lawrence Rivers.
These four rivers are different from the other rivers; they either drain from the Great Lakes or have significant restriction within their watersheds, such as a dam or extensive levee building, or other modifications. Dams that slow down the river flow can affect the river chemistry in many ways, including giving more time for the sun to break down the CDOM by photochemical degradation. As a result, the amount of colored organic material in these four rivers may have decreased without a similar decrease in the DOC content. Based on their findings, the authors suggest that CDOM measurements are a useful way to investigate water quality in most rivers and to monitor delivery of DOC as carbon and food for microbes in coastal regions. However, where aquatic ecosystems have been greatly altered by human activity, precautions must be taken to make the correct measurements to understand how that human activity is affecting water quality, aquatic food chains, and the role of rivers in delivering carbon to the oceans.