Alterations of phytoplankton assemblages treated with chlorinated hydrocarbons: effects of dominant species sensitivity and initial diversity.

Changes in composition of phytoplankton assemblages due to short-chained chlorinated hydrocarbons (tetrachloroethane, tetrachloroethylene and trichloroethylene) were studied in microcosm experiments with different initial diversities. Diversity decreased further during treatments in the less diverse 2011 summer assemblages, dominated by the euglenid Trachelomonas volvocinopsis (its relative abundance was nearly 70 %). Diversity did not change significantly during treatments in the more diverse 2012 summer assemblages, dominated by cryptomonads (their relative abundance was 40 %). The dominant Trachelomonas volvocinopsis in 2011, due to its insensitivity to the treatment and presumably high competition skills, filled released habitats occurring when sensitive species were not detectable any more. In contrast, cryptomonads were extremely sensitive to the treatments, their abundance decreased under detection limit in the treated assemblages, regardless of diversity conditions. Our results showed that population dynamics of dominant species determine the response to the contamination of the entire community, if these species display high resistance or resilience. If the dominant species was highly sensitive and recovered slowly, compensatory growth of rare species maintained high levels of ecosystem performance.