Waters used in Experiment 1 differed in two major characteristics:
First, WARU water contained appreciable DOM while SNARC water
did not. Second, SNARC water contained higher levels of calcium and
magnesium (cations contributing to hardness) than WARU water. Either
parameter could have accounted for the large difference in fish
mortality rates seen in Experiment 1. Furthermore, the in vitro experiment
indicated that humic substances at the levels found in WARU
water were not directly toxic to F. columnare.
Dissolved organic matter in ground water consists of variable and
largely undefined mixtures of humic substances derived from contact
of the underground waterwith decaying or fully decayed organicmatter
(such as lignite, in the case of the Sparta Sand Aquifer). Tannins—a group
of polyphenolic compounds often found inwater in contactwith organic
matter—are known to have antimicrobial properties. Scalbert (1991)
reviewed the antimicrobial properties of tannins and described different
mechanisms which include: 1) inhibition of extracellular microbial enzymes,
2) deprivation of the substrates required for microbial growth,
or 3) direct action onmicrobialmetabolism through inhibition of oxidative
phosphorylation. Chung et al. (1995) investigated tannins as antimicrobial
agents for their inhibitory effects on selected aquatic