| Summary: | This study was carried out to assess the chronic effects of both an insecticide (spirotetramat) and a herbicide (2,4-Dichlorophenoxyacetic acid) on two phytoplankton species: Monoraphidium griffithii and Coscinodiscus sp. The effects of the chemicals on an exponentially-growing population of the two species were evaluated using single compounds and a compound mixture. The experiments lasted for 27 days. The results show a decrease in the populations’ growth for both species after the application of single pesticides and the mixture. 2,4-D applied alone and the mixture of both chemicals have affected the organisms the most. Both pesticides have induced an increase in chlorophyll-a content in M. griffithii while no effect was observed on chlorophyll-a in Coscinodiscus sp. A significant effect was recorded in the cell size of both species with both pesticides. The two spirotetramat tested concentrations have induced an increase in cell size, while the opposite effect was obtained for the (2,4-D) herbicide concentrations. The mixture of compounds induced an additive effect on the cell size of both species. These results indicate the risk which the two chemicals represent in a natural environment. They are agents that can have chronic impairments at the physiology or behavior level of such phytoplankton species as M. griffithii or Coscinodiscus sp. Such impairments might consequently affect the health of the ecosystem.
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