Animal behaviour and water movement as co-determinants of plankton distribution in a tidal system

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Abstract
In tidal sounds and rivers along the Atlantic coast of the southern U.S.A., pelagic calanoid copepods exhibit striking patterns of horizontal distribution which oscillate according to the rhythm of the tides. Acartia tonsa has a marked density maximum in the middle of the tidal Duplin River, Georgia, at low water, although there are no appreciable gradients of temperature, salinity, or food. During flood tide, the maximum is shifted to the upper end of the river. The distribution is shown to be the result of the combined action of a) the vertical migration of the copepods, b) the asymmetrical tidal flooding and draining of the marsh, and c) the upstream increase of the ratio of water in the marsh to water in the river at high water.