Comparative salinity tolerance of three indigenous tropical freshwater cladoceran species; Moinodaphnia macleayi, Ceriodaphnia rigaudii and Diaphanosoma brachyurum.

The acute salinity tolerance of three tropical freshwater cladoceran species, M. macleayi, C. rigaudii and D. brachyurum was determined. The existence of these species represented new records for Trinidad and Tobago, a tropical Caribbean island. It has a large oil based industry, in which, the primary effluent (saline produced water) is discharged directly into many freshwater systems. Cladoceran species are used routinely in temperate regions, to assess the impacts of chemicals and effluents in freshwater systems. However, relatively few studies have assessed the salinity tolerance of these organisms. Test organisms were assayed using seven salinity treatments (0, 1, 3, 5, 10, 15, 20, 25 per thousand) prepared by volumetrically mixing natural filtered seawater with dilution water. C. rigaudii had a 48 h LC50 of 1.6 per thousand, M. macleayi 1.5 per thousand and D. brachyurum <1 per thousand. The CSmax (critical salinity maximum) for C. rigaudii and M. macleayi was 3 per thousand after 48 h, compared to 1 per thousand for D. brachyurum. There was no salinity value at which there was 100% survival. The resulting salinity response curve was a straight line which indicated that each species was intolerant of salt. Consequently, any of these can be considered as a representative freshwater organism for toxicity testing in Trinidad and Tobago.