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Rotifers are important components of freshwater ecosystems and sensitive indicators of environmental changes. This study was carried out to test the hypothesis that, among environmental variables, salinity and aquatic vegetation have significant effects on rotifer diversity and abundance. We analyzed rotifer assemblages in the littoral zone of 22 hydromorphologically different shallow waterbodies in West Azarbaijan, Iran. Rotifer diversity and abundance were not significantly associated with basin morphology, but were positively correlated with the percentage of vegetation cover. Salinity and electroconductivity positively influenced rotifer abundance, while they had significantly negative effects on rotifer diversity. Halobiont species from the genera Brachionus, Hexarthra, Synchaeta, and Notholca reached their highest abundances in the waterbodies with pronouncedly higher salinities. Our findings are in agreement with recent records showing that distinct rotifer assemblages occur in saline and non-saline waterbodies. The role of salinity and aquatic vegetation as the most important environmental drivers in shaping rotifer communities is confirmed. The results of this study suggested that environmental changes could be significant on the micro-biogeographical level, and that the interaction of salinity and observed human impact, i.e., trophic level, promote rotifer abundance as sensitive indicators of environmental changes.

Intrinsic rates of population increase (r) were evaluated as a measure of population dynamics of four strains of Brachionus plicatilis and two strains of B. urceolaris from Iran in response to different salinities and feeding algae. Each rotifer strain was cultured at four salinities: 5, 20, 25 and 30‰ and fed with two microalgal species: Chlorella vulgaris and Nannochloropsis oculata. Salinity of 5‰ was critical for all the examined strains, at which r was at minimum and was different from the other salinities (P < 0.05). For B. plicatilis strains, the maximum r was observed in those fed on Chlorella at salinities of 10 and 30‰ (64 ± 0.01 day −1). While, in B. urceolaris, maximum r was for Nannochloropsis fed rotifers at salinity of 20‰ (0.69 ± 0.01 day −1). Maximum final population density (FD) was obtained for a strain of B. urceolaris fed on Nannochloropsis at 20‰ (329.3 ± 10.9 ind.mL −1). FD was relatively lower in B. plicatilis strains among all examined salinities. ANOVA showed the significant effect of salinity and rotifer strain, and algae × rotifer strain on both r and FD, and salinity × rotifer × algae on FD (P < 0.05). IntroductionRotifers are considered as ideal model organisms for various kinds of biological laboratory studies including population dynamics (YOSHINAGA et al., 1999;SARMA, 2006;SNELL and DESROSIERS, 2008). They have also become a valuable and, in many cases, indispensable food organism for first feeding of a large variety of cultured marine finfish and crustacean larvae (DOUILLET, 2000).Success in rotifer culture seems, to a great extent, to be dependent on providing adequate information on the life history characteristics and environmental preferences of each biotype. An increasing number of studies have been carried out on the effects of salinity, temperature and the type and concentration of food on the demographic traits and population growth of rotifers (CABRERA et al., 1993;OLTRA and TODOLI, 1997;FIELDER et al., 2000;SARMA and NANDINI, 2002;ATHIBAI and SANOAMUANG, 2008). These studies were accomplished using several rotifer species (XI et al., 2001;SUCHAR and CHIGBU, 2006;WULLUR et al., 2009

Effects of feeding on four algal species (Chlorella vulgaris, Nannochloropsis oculata, Isochrysis galbana andScenedesmus obliques) at salinities of 5, 15 and 25 on the population growth parameters of two Iranian strains of the widespread rotifer species Brachionus plicatilis were examined. Maximum specific growth rate of 0.55 ± 0.02 day -1 and final population density of 7 961 ± 3 267 ind. ml -1 were observed in the Ba strain fed with Chlorella at a salinity of 15. The minimum population density of 10 ± 0 ind. ml -1 occurred in the Ba strain fed with Scenedesmus at a salinity of 5. There were significant differences in the egg ratio (number of eggs ⁄ number of females) between the two rotifer strains when fed with Scenedesmus at a salinity of 15. Multivariate post hoc comparisons demonstrated the significant effects of algal species on all estimated parameters in both rotifer strains and the significant effects of salinity and salinity × algae on specific growth rate and population density of the Ba strain.

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