Brown Rivers Actually a Good Thing?
by Jeremy Elton Jacquot, Los Angeles 
on 11.22.07
Image courtesy of Tracy O
Despite conventional wisdom, it now seems as though those clear, blue waters we've come to cherish for so long may not be so pristine after all. A new study published in the journal Nature has indicated that rivers and lakes that are turning brown may actually be returning to a more natural state.
According to Don Monteith, co-author and professor at University College London, the staining is a result of more dissolved organic carbon entering their waters and not, as has often been suggested of late, an effect of global warming or increased land-use. Monteith attributes this to a reduction in the acidity of soils, a consequence of a large-scale decline in acid rains.
Acid rain occurs when the burning of fossil fuels causes emissions of sulfur and nitrogen to react with water in the atmosphere. "The solubility of organic carbon is pH-dependent, so the more acidic a soil gets, the less soluble a number of these organic compounds are," said Monteith.
Because this phenomenon has decreased in recent decades, a result of more stringent regulation, rivers and lakes have been able to return to a more pre-industrialized state, characterized by less acidic soils and, therefore, more carbon run-off. But is this discoloration also a sign of deteriorating water quality? That is unclear, explains Monteith:
"The problem is that people have been living with the impact of acid rain for so long that no-one alive today really has an idea of what the waters were like before acid rain took hold . . . One impact is the distribution of sunlight in lakes. In some aquatic ecosystems, plants will not be able to grow as deeply as they did before, because light will be attenuated in the upper levels of the lake."
One issue of concern for Monteith and his colleagues is that toxins, especially heavy metals such as mercury and copper, are known to bind well with organic molecules. A further avenue of research, he suggested, should be to investigate the implications of the cycling of these noxious substances. "We believe that there should be a lot more work going into the consequences of the potential changes in the [carbon] cycle - we don't have any real idea as to the fate of this (dissolved organic carbon)."
Via ::BBC News: Brown rivers are 'more natural' (news website)
See also: ::Ask TreeHugger: Why Is My Water Brown?, ::"Brown Clouds" Over India Just as Bad as Greenhouse Gases, ::Chinese Water: A Picture is Worth...
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This work represents a very narrow view of the issue of apparent color of fresh waters. Operant factors include incident sky light, leaf fall directly into the water, fall decay of aquatic vegetation, and plankton blooms. Also, litoral zone turbidity is a function of wind and hence wave energy that directly stirs up and suspends the sediments. If a water level is receding, as in the Great Lakes, then sediments that earlier were out of reach of wave driven eddy currents are now more likely to be re-suspended with the winds of November. In short, there is much more to be said on this topic that these authors either are unfamiliar with or have purposefully overlooked to simply their points. They could start with a basic Limnology text.