SEASONAL OCCURRENCE AND HATCHING OF CALANOID EGGS IN SEDIMENTS OF THENORTHERN BALTIC-SEA

The seasonal occurrence and hatching of benthic eggs of calanoid copep ods were studied for 1 yr in the surface sediments at 2 sites (a 33 m deep archipelago area and a 42 m deep site in an enclosed bay) off the SW coast of Finland, northern Baltic Sea. Eggs were abundant at both sites (up to 4 and 6 x 10(6) eggs m(-2)). At the archipelago site, mos t eggs belonged to Acartia bifilosa and A. tonsa; at the bay site, egg s of Eurytemora affinis and Acartia spp, occurred. At the archipelago site, the egg numbers in the surface sediment followed closely the sea sonal abundance of the planktonic Acartia spp. females. The eggs colle cted from the sediment were incubated al temperatures corresponding to the in situ bottom temperatures. Hatching of the A. bifilosa eggs occ urred throughout the year, but it was most intensive in autumn when wa ter stratification broke and the deep water warmed up to 13 degrees C. It is suggested that a large number of the A. bifilosa eggs sink to t he bottom prior to hatching in shallow coastal areas of the Baltic Sea . Hatching of the benthic eggs occurs throughout the year and the rate of naupliar emergence from the sediments depends on benthic condition s and processes (e.g. temperature, sediment resuspension and bioturbat ion). At the bay site, where the surface water layer was hydrographica lly separated from the deep water, the coupling between the benthic eg g abundance and the planktonic populations was not so obvious. The dom inant species E. affinis carries its eggs in an egg sac until hatching , and probably only the diapause eggs, which are produced in autumn, f all to the bottom. A. tonsa was abundant in the water column at both s tudy sites in autumn, even outnumbering other Acartia spp, in some sam ples. The eggs of the species only hatched in autumn, when the incubat ions were conducted at 10 to 13 degrees C. It is probable that A. tons a spends most of the year as benthic resting eggs in the northern Balt ic Sea. In contrast, A. bifilosa and E. affinis occurred in the plankt on in winter at both study sites, though in low concentrations. They t hus have 2 possible sources of recruitment when conditions are again f avourable for population growth in spring: hatching of benthic eggs an d reproduction by the overwintering population.