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Field observations in an oxycline in relation to laboratory determinations of oxygen requirements in some species of marine zooplankton Fish, Arthur Geoffrey


Preliminary investigations of Saanich Inlet, Vancouver Island, British Columbia, indicated that there was a stratification in the depth of occurrence, during daytime, of the euphausiid Euphausia pacifica and the amphipods Cyphocaris challengeri and Orchomenella pinguis. The presence of an oxycline and a gradient in the carbon dioxide concentration suggested that the stratification was associated with these environmental factors. A programme was conducted between July 1962 and June 1963 to determine whether the stratification was a persistent phenomenon and whether physical and chemical characteristics of the water were associated with the distribution of the organisms. Clarke-Bumpus plankton samplers were towed above, within, and below the oxycline on seven cruises. The survival of the species was measured above, within, and below the oxycline, for exposures of 1 to 12 hours for Euphausia pacifica and Cyphocaris challengeri and of 9 to 24 hours for Orchomenella pinguis, by lowering specimens in cylinders which were in communication with the environment. Laboratory experiments measured the oxygen consumption of the species when carbon dioxide produced by the organisms in experimental chambers accumulated and when it was absorbed. Stratification was a persistent phenomenon in which Euphausia pacifica occupied the upper position, Orchomenella pinguis the lower position and Cyphocaris challengeri an inter-mediate or upper position in the daytime aggregation of organisms. The position of the aggregation was not influenced by temperature or salinity, but was related to the concentration of oxygen, and carbon dioxide. The field experiments showed that Orchomenella pinguis could tolerate lower oxygen and higher carbon dioxide concentrations for much longer durations of exposure than could the other two species. The laboratory experiments indicated Orchomenella pinguis and Cyphocaris challengeri consumed less oxygen than Euphausia pacifica, but Cyphocaris challengeri was more susceptible to high concentrations of carbon dioxide than the other species. These facts offer partial explanations for the stratification of the species.

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