A number of expeditions to the area of Mar de Atacarna, Chile, 68" 15'W, 20" 30'S, have involved studies of the biological and chemical features of Lake Tebenquiche, situated in the interior of the salar. Chemically, Tebenquiche is hypersaline, with practically anoxic waters dominated by sodium and chloride ions but with high concentrations of sulphate also. The lake is surrounded and invaded by macrophytes, dominated by Scirpus olmeyi and Juncus, which provide organic material for the formation of bacterial mats. The fauna of limnetic crustaceans is almost exclusively ofArtemia salina. The most important genera of bacteria are: Marinomonas, Halobacterium, Acinetobacter and the sulphur reductors Vibrio and Bacillus. The Cyanobacteria are represented exclusively by Oscillatoria.
Abstract. In the present study, the sensitivity of four coexisting cladoceran species to ultraviolet (UV)-B radiation was evaluated. First, the LD 50 under conditions inhibiting the action of photoenzymatic repair was calculated by exposing cladocerans to different doses of UV-B light only. Animals in another treatment group were exposed to both UV-B and visible light, which allows for photoenzymatic repair. Photoenzymatic repair contributed significantly towards the degree of total tolerance to UV-B light in all groups of cladocerans, although the magnitude of the contribution varied among species. When no photoenzymatic repair was allowed, the most tolerant species was Moina micrura, followed by Daphnia ambigua and Ceriodaphnia dubia, with Diaphanosoma chilense being the most sensitive species. Under conditions permitting the action of photorepair, M. micrura was again the most tolerant species to UV-B light. Ceriodaphnia dubia showed an intermediate tolerance value, whereas D. chilense and D. ambigua were the least tolerant species, with no significant differences between them. Adults and juveniles of C. dubia differed in their sensitivity to UV-B light under conditions with and without photorepair.
Eutrophic temperate and subtropical lakes often exhibit a marked vertical structure during the warm season that involves important spatial differences of physical, chemical and biological variables. Therefore, zooplankton is exposed to a highly heterogeneous environment in the vertical dimension. In this work, We analyze the depthdistribution of the cladoceran Daphnia ambigua in the eutrophic, monomictic lake El Plateado at midday and midnight, along with its relationship with the vertical distribution of water temperature and dissolved oxygen concentration. We also attempt to define whether or not this population exhibits a diel vertical migration. The results show significant changes in the day/night vertical distribution of D. ambigua during its growing season, with the exception of the last date. Also, the data revealed that average depth selected by D. ambigua becomes shallower with time, and the amplitude of the vertical migration decreases throughout the season. During the period of lake stratification, temperature appears positively correlated, and oxygen negatively correlated to the frequency of D. ambigua. It is suggested that oxygen concentration plays a crucial role in modulating the vertical migration behavior of D. ambigua in lake El Plateado, which has important consequences for understanding the atypical pattern of population dynamics exhibited by this species.
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