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Our study focused on the seasonal dynamics of total Hg in the phytoplankton (living and dead) of two dystrophic shield lakes (Mouse, and Ranger). Phytoplankton samples were taken from metalimnetic and hypolinmetic depths in the euphotic zone and were collected and analyzed using ultraclean techniques. In both lakes, phytoplankton Hg (PHYTO-Hg) levels (pg/L) in the metalimnion did not significantly change amongdates over the season, although Ranger Lake exhibited significant differences between Hg values measured at the beginning and end of the season. In contrast, PHYTO-Hg significantly increased in the hypolimnia of both lakes by the end of the season. Combined influences of external Hg inputs, remineralization, phytoplankton sedimentation, and increased methylmercury production in the hypolimnia over the season may have contributed to these trends. A highly significant positive relationship existed between PHYTO-Hg levels and whole-water Hg-levels (r2 = 0.90), and the mean bioconcentration factor for Hg between the water column and phytoplankton was significantly higher inthe hypolimnion compared to the metalimnion for both lakes. In most cases, parameters associated with algal biomass had significant positive correlationss with PHYTO- Hg levels. Weight-specific PHYTO-Hg (pg/mg dry weight) varied significantly over the season, and there were interlake differences with respect to seasonal trends. On the basis of these results, we recommend that future sampling regimes include collection of phytoplankton at different limnetic depths throughout theseason to account for spatial and temporal variations. Weight-specific Hg levels in phytoplankton could not be explained well by the parameters tested, and the only significant regressions were with parameters reflecting algal biomass. This study provides in situ evidence ofHg accumulation in lake phytoplankton as a function of algal biomasson a seasonal basis (as opposed to biodilution) and stresses the need to [ABSTRACT FROM AUTHOR]