MESOZOOPLANKTON RESTING EGGS IN THE BALTIC SEA - IDENTIFICATION AND VERTICAL-DISTRIBUTION IN LAMINATED AND MIXED SEDIMENTS

Benthic resting eggs of rotifers, cladocerans and calanoid copepods we re studied at two sites with different hydrographical and sediment pro perties off the SW coast of Finland, the northern Baltic Sea between 1 991 and 1993. Vertical distribution of the resting eggs in the sedimen t was studied down to 10 cm depth, and hatching of the eggs extracted from different 1-cm thick sediment layers was experimentally tested. T he larvae of eight mesozooplankton taxa emerged from the eggs: Calanoi da (mostly Acartia bifilosa), cladocerans Bosmina spp., Daphnia spp., Evadne nordmanni, and Podon spp. (mostly P. polyphemoides), and rotife rs Synchaeta spp., Keratella spp. and Asplanchna priodonta. Some calan oid and cladoceran eggs hatched after 7 mo storage of the sediment at 3 degrees C. A few calanoid and cladoceran eggs hatched from the 8 to 10 cm deep sediment layer. Their age, estimated by means of Cs-137 ana lysis, was ca. 8 to 10 yr. Vertical distribution of the benthic eggs r eflected the different sediment structures of the study sites. The egg s of all zooplank- ton taxa were more stratified in the laminated sedi ment than in the disturbed sediment. We suggest that the intensity of deep water mixing is the key factor for regulating the vertical distri bution and hatching probability of mesezooplankton resting eggs, since it largely determines the stratification of the sedimenting material, distribution of oxygen within the sediment, and the living conditions of benthic animals causing bioturbation.