We used enzyme-linked immunosorbent assay to examine reservoir-mediated shifts in spring to fall exposure of aquatic organisms to the spring atrazine pulse over four years in a Midwestern stream-reservoir system. Peak atrazine concentrations in the major inflowing stream exceeded 10 lg ⁄ l in all four years. The reservoir had a beneficial effect in two of four years by diluting atrazine below the 10 lg ⁄ l threshold. However, during the other two years, exposure times above 10 lg ⁄ l were approximately doubled in the reservoir compared to the major inflowing stream. Thresholds of 3 and 5 lg ⁄ l were exceeded during all four years in the reservoir. The uplake and downlake reservoir sites were four to five times more likely to exceed these thresholds and aquatic organisms were subjected to longer exposure times above these thresholds compared to the inflowing stream. Release of elevated atrazine concentrations from the reservoir extended exposure times in the outflowing stream. This effect was most pronounced just below the dam. Aquatic organisms upstream of the reservoir were most likely to experience acute exposures whereas organisms within and immediately downstream of the reservoir were more likely to experience chronic exposures. The ubiquity of reservoirs and the annual spring herbicide flush highlight the importance of considering the presence and relative location of reservoirs when assessing risk to aquatic communities as well as locations of drinking water intakes.

Vertical pH profiles were recorded in sediment cores from four of the Turkey Lakes in the Algoma region of northern Ontario. In the three upper lakes of the chain, median down-core pH, which should be correlated with historic lake water pH, was below the median down-core pH for other lakes in northeastern Ontario. The Turkey lakes had probably been relatively acidic, therefore, since pre-industrial times. Down-core pH tended to be lower in upstream lakes. While a pH minimum was typically observed at the sediment–water interface, the surficial sediments were not acidified relative to down-core sediments.