PMC

Seasonal changes in periphyton fecal coliforms (A) and estimated E. coli concentrations (B) at the DBC (•), LRII (○), and BBL (▾) sites. Error bars indicate standard deviations of measurements.

Partial dendrogram of unique E. coli strains isolated from epilithic periphyton at the DBC site. E. coli isolates with an HFERP fingerprint similarity of ≥92% were considered to be clones of the same strain. Four such E. coli strains, indicated by letters, were isolated from periphyton at the DBC site. The arrow indicates the 92% HFERP DNA fingerprint similarity value.

Sampling sites in the western tip of Lake Superior and the Duluth-Superior harbor. (A) Locations of the DBC, LRII, and BBL sites. (B) DBC is located within the Duluth-Superior harbor (46°46.16′N, 092°05.40′W). (C) LRII is located along the north shore of Lake Superior at the outer urban limits of Duluth (46°50.23′N, 092°00.15′W). (D) BBL is located along the north shore of Lake Superior about 25 km from downtown Duluth (46°55.06′N, 091°51.15′W).

(A) Sources of E. coli in periphyton communities from the DBC, LRII, and BBL sites between July 2004 and July 2005. UP are unidentified E. coli strains that are unique to periphyton. NU are unidentified and nonunique periphyton E. coli strains. The distributions of source groups that could be identified are shown in the pie charts on the right, and the percentage in each source group is indicated. (B) Sources of E. coli in water overlying the periphyton communities at the three study sites during the same sample period. Numbers in each graph indicate percentages of each source group.

Changes in E. coli NRR27 abundances in laboratory microcosms attached to periphyton-covered (▾) and bare control (•) rocks and in water overlying these rocks (periphyton-covered ▿ and bare control ○ rocks). The water in each microcosm was inoculated with E. coli NRR27 (open symbols to left of dashed line) 2 days before the experiment began and then replaced with 0.2-μm-filtered, sterile lake water at the start of the experiment (dashed line). Error bars represent standard deviations. Linear regressions indicated that the loss of E. coli from bare control rocks was almost twice as fast as that from periphyton-covered rocks. All microcosms were vigorously agitated at the end of the experiment (20 days) to simulate wave action, and E. coli concentrations in the overlying water were measured again (see arrows).