A comparison of the response of Simocephalus mixtus (Cladocera) and Daphnia magna to contaminated freshwater sediments

The southeast region of Mexico is characterized by intensive oil industry activities carried out by the national public enterprise Petróleos Mexicanos (PEMEX). The freshwater lagoon "El Limón", located in the municipality of Macuspana, state of Tabasco, Mexico, has received over 40 years discharges of untreated waste waters from the Petrochemical Complex "Ciudad PEMEX", located on the border of the lagoon. To assess the toxicity of the sediments and, hence, to obtain information on the biological effects of these contaminating discharges, the cladoceran Simocephalus mixtus was used as a test organism in acute (48 h) and chronic (12 d) toxicity assays. For comparison purposes, bioassays were also conducted with the reference cladoceran Daphnia magna. The sediments of this lagoon contain important amounts of metals and hydrocarbons that have been accumulated over time; however, the acute tests only registered reduced lethal effects on the test organisms (maxima of 10% and 17% mortality for D. magna and S. mixtus, respectively). This may be due to low bioavailability of the pollutants present in the sediments. On the other hand, partial or total inhibition and delay in the start of reproduction, reduction in clutch sizes, reduced survival, as well as reduction in the size of adults and offspring were recorded in the chronic assays. The most evident chronic effects were found in S. mixtus; in this species, reproduction was inhibited up to 72%, whereas D. magna was only affected by 24%. We determined that S. mixtus is a more sensitive test organism than D. magna to assess whole-sediment toxicity in tropical environments, and that chronic exposure bioassays are required for an integrated sediment evaluation. The sediments from "El Limón" lagoon induced chronic intoxication responses and, therefore, remediation measures are urgently needed to recover environmental conditions suitable for the development of its aquatic biota.