In general, what causes an increase in taste and odors when natural waters lack dissolved oxygen?

An increase in taste and odors in natural waters lacking dissolved oxygen is primarily due to anaerobic decomposition. In conditions where dissolved oxygen levels are low, aerobic bacteria, which typically break down organic material, become less active. Instead, anaerobic bacteria take over, leading to the decomposition of organic matter without oxygen. This process often produces various byproducts, including volatile organic compounds and gases like hydrogen sulfide, which are known for their foul odors.

As these anaerobic bacteria break down material, the resultant changes in chemical composition can lead to noticeable alterations in the water's taste and smell. This phenomenon is particularly pronounced in stagnant waters or areas where organic pollution occurs, as the buildup of decaying material is enhanced in the absence of oxygen. Consequently, the distinctive and often unpleasant tastes and odors associated with such water are direct results of the metabolic processes of anaerobic microorganisms.