No abstract

No abstract

No abstract

Flocculation and removal of bacteria were observed during two separate aluminum sulfate (alum) treatments for removal of phosphorus from a eutrophic recreational lake. In addition, die-off and release of bacteria from alum floc were studied in columns under laboratory conditions. Membrane filtration and spread plates were used to determine concentrations of indicator species and total cultivatable bacteria, respectively. During the alum treatment of the lake, 90% of the fecal coliform (FC) population and ca. 70% of the fecal streptococci population were removed from the water column within 72 h. Numbers of FC in the floc on the lake bottom exceeded 2,400/100 ml at 120 h compared with the pretreatment concentration of 30 FC/100 ml. Inactivation of FC in the floc proceeded at a rate of 200 FC/100 ml per 24 h. In a second alum application to the lake, 95% of the total culturable bacterial population was removed from the water column. In a laboratory column study of survival and release rates, over 90% of an Escherichia coli suspension was concentrated in a floc formed at the bottom. E. coli was not released from the floc. The numbers of and survival of E. coli in the floc suggest the probable concentration of other enteric organisms, including pathogens. Thus, the floc poses a potential human health risk if ingested by swimmers or if others use the lake as a potable water source.

Wintertime toxic blooms of the blue-green alga Anabaena flos-aquae have been documented since 1989 in American Lake, a 446-ha lake near Tacoma, Washington. The toxic episodes were unusual in that algal toxicity occurred during the winter and this lake is considered to be only moderately productive. A year-long study was conducted to determine the environmental factors associated with toxic conditions. Toxic blue-green algal blooms in American Lake were associated with increased nutrient (especially phosphorus) availability following winter turnover. Phosphorus released from lake sediments under conditions of low oxygen during lake stratification fueled winter blooms of algae, including the toxic Anabaena flos-aquae strain. The lake's extremely low iron content was insufficient to completely remove this released phosphorus from the water column during lake turnover.