Both of the pictures shown above are of the same lake in Canada.  The picture on the right was taken after large amounts of nutrients (including nitrates) were added to the water.

The photo above shows another lake that was artificially divided into two parts; the part at the top of this photo was left in its natural state and the part at the bottom had large amounts of nutrients (including nitrates) added to it.  Note the difference in color due to algae growth.
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