The thermocline as an ecological boundary

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Abstract
Plankton distribution in relation to the seasonal thermocline was examined off the Moroccan coast, by six night hauls with a flight of three 1 m2 nets. Catch dividing devices and a depth of net telemeter were used. Bathythermograph, chlorophyll, and hydrographic observations supplemented the sampling. Euphausiids, decapods, and ostracods were all caught in lower numbers in the discontinuity layer, compared with 20 m either side. Fish, however, showed some aggregation in the thermocline. Specific analysis of the 22 ostracod species showed that there was a general avoidance of the discontinuity. Maxima of chlorophyll and oxygen associated with the thermocline suggested that food was not the limiting factor. The physical gradients were too small to account for the avoidance behaviour. It is tentatively suggested that the current shears often found to occur at discontinuities may be avoided by many plankton species.

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