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 were studied at two sites with different hydrographical and sediment properties off the SW coast of Finland, the northern Baltic Sea between 1991 and 1993. Vertical distribution of the resting eggs in the sediment was studied down to 10 cm depth, and hatching of the eggs extracted from different 1-cm thick sediment layers was experimentally tested. The larvae of eight mesozooplankton taxa emerged from the eggs: Calanoida (mostlyAcartia bifilosa), cladoceransBosmina spp.,Daphnia spp.,Evadne nordmanni, andPodon spp. (mostlyP. polyphemoides), and rotifersSynchaeta spp.,Keratella spp. andAsplanchna priodonta. Some calanoid and cladoceran eggs hatched after 7 mo storage of the sediment at 3°C. A few calanoid and cladoceran eggs hatched from the 8 to 10 cm deep sediment layer. Their age, estimated by means of¹³⁷Cs analysis, was ca. 8 to 10 yr. Vertical distribution of the benthic eggs reflected the different sediment structures of the study sites. The eggs of all zooplankton taxa were more stratified in the laminated sediment than in the disturbed sediment. We suggest that the intensity of deep water mixing is the key factor for regulating the vertical distribution and hatching probability of mesozooplankton 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.