2). Although small- and large-sized cladocerans had relatively similar responses (survival rates

were 81%, and 79% respectively), medium-sized D. magna were significantly more vulnerable to the crude oil (survival rate 70%) (ANOVA post hoc Bonferroni pmedium sized vs. other size groups < 0.05). The median lethal concentrations (LC50) at 24 h for small, medium and large size classes were Pexidartinib manufacturer 1025, 610 and 900 mg L−1, respectively. At 96 h, however, the values were much lower at 210, 213 and 216 mg L−1. Furthermore, there was a significant interaction between cladoceran size and crude oil, i.e. different sized cladocerans responded differently on increasing crude oil concentration. The post hoc Bonferroni test indicated that most of the treatment levels above 100 mg L−1 were statistically different after 24 h but not after 96 h (Figure 3 and Figure 4). Specifically, none of the cladocerans, being in contact with oil concentrations above 100 mg L−1, showed recovering signs and died after 96 h even Ipilimumab chemical structure if placed back

into their normal oil-free environment. In the control flasks all animals survived. Above 100 mg L−1 the survival rates of small- and large-sized D. magna decreased almost linearly with increasing oil concentrations: the large-sized specimens were more tolerant to the lowest dilution but their survival rate was decreasing more steeply with the raising oil concentration. However, medium size-class had lowest survival rates at all studied concentrations and declined nearly exponentially

with increasing oil concentration. Our experiments supported the hypotheses that an increasing crude oil concentration decreases the survival of D. magna and the crude oil having different effect on each of the cladocerans’ size-class was supported by current study. In contrast, the hypothesis that the interactive effect of crude oil concentration and the cladocerans’ life stage may dominate over the separate effect of crude oil concentration was not supported. We were also able to establish a threshold value of 100 mg L−1 below which the effects of crude oil on the cladocerans was negligible. In our study the very overall LC50 values were considerably higher as compared to, e.g. Bobra et al. (1983). Such variation in LC50 values may be attributed to differences in, e.g. test methodology, test duration and crude oil type. The effects of oil pollution to plankton are complex involving many indirect and direct mechanisms. However, most effects are due to the increasing oil concentration. The indirect impact of oil pollution to plankton may result in the decrease of dissolved oxygen concentration and related degradation in water quality parameters (Harrel, 1985, Li and Boufadel, 2010 and Neff and Stubblefield, 1995). Very high concentrations of crude oil may eliminate primary producers from the area, thus decreasing the food resource for heterotrophs (Chao et al., 2012 and Karydis, 1982).