Diurnal vertical migration of an estuarine cladoceran,Podon polyphemoides,in the Chesapeake Bay

Populations of the marine cladoceran Podon polyphemoides (Leuckart) in the Chesapeake Bay (USA) zooplankton typically occur in large patches, extending the length of the estuary. These patches are characterized by well defined maxima, which do not move seaward during a given season. During the day, the bulk of the population is situated in the upper part of the two-layered estuarine circulation, where it is subjected to transport by net non-tidal seaward current. One mechanism for maintaining the population within the estuary is provided by the diurnal behavior of the podonids, which migrate to deeper water during the night. The landward advection of the deeper currents, thus, helps to counterbalance seaward population loss during the day. The extent of the vertical movement of the population can be modified by various environmental factors, but a suggestive correlation with light was found. It is thought that the downward component of the migration is due to passive sinking.