To date, some studies have focused on the impacts of crude oil to plankton communities (e.g. Jung et al., 2012 and Varela et al., 2006). Nevertheless, most of these studies have correlative nature and the reported oil spill effects are likely confounded by other environmental variables that are not covered by sampling design. As a consequence, the adverse effect of crude oil cannot often be distinguished (Batten et al., 1998 and Hu et al., 2011). Moreover, most Ganetespib datasheet of the studies have not investigated the oil pollution induced responses of different

life stages of planktonic organisms although the size of organisms is expected to modulate the responses to the intoxication of biota (Arzate-Cárdenas et al., 2011, Brooks et al., 2003 and Kostial et al., 1978). Cladocerans within the genus Daphnia are one of the key organisms in aquatic ecosystems being an essential link between primary production and many important fish species and at the same time exerting a strong control over phytoplankton abundance ( Lampert, 1987). Daphnia magna is commonly found in brackish water ( Arner and Koivisto, 1993) but also inhabits freshwater environments. Therefore, D. magna is acknowledged as an

important test-organism in ecotoxicological DAPT nmr studies both in fresh and brackish waters. Our experiment focused on short-term effects of crude oil on the cladoceran Daphnia magna (Straus 1820) in order to assess the acute effects of crude oil on their survival rate. Furthermore, we explored a potential of different life stages of cladocerans to modulate the effect of intoxication. Previous studies quantified the crude oil effects mainly on the first developmental stages of D. magna (<24 h old in Martinez-Jeronimo

et al., 2005; <48 h old in Ullrich and Millemann, 1983; and <10 days in Ratushnyak et al., 2009) and in one case also mature adults ( Dowden, 1962). The hypotheses of this study are: (1) As an opportunistic species D. magna is not influenced by very low concentrations of crude oil; (2) An increased crude oil concentration decreases the survival rate of D. magna; (3) Different developmental Montelukast Sodium stages of D. magna have different sensitivity to crude oil, whereat the interactive effect of crude oil concentration and cladocerans’ life stage may dominate over the separate effect of crude oil concentration. D. magna specimens were obtained from continuous cultures maintained for several years at the Estonian Marine Institute of the University of Tartu. The experiments manipulating crude oil concentration and size-classes of the cladocerans were performed at the Estonian Marine Institute. The stock culture was maintained in 20 L aquarium and fed an ad libitum diet of Scenedesmus obliquus. The culture was kept in natural light conditions at room temperature (20 ± 2°C). The cladocerans were separated into three size classes: small (1.4 mm, SE = 0.013; 3 days old), medium (2.5 mm, SE = 0.026; 6 days old), and large (3.1 mm, SE = 0.022; 9 days old).