Seasonal occurrence and hatching of calanoid eggs in sediments of the northern Baltic Sea

Abstract

The seasonal occurrence and hatching of benthic eggs of calanoid copepods 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 106 eggs m-2). At the archipelago site, most eggs belonged to Acartia bifilosa and A. tonsa; at the bay site, eggs of Eurytemora affinis and Acartia spp. occurred. At the archipelago site, the egg numbers in the surface sediment followed closely the seasonal abundance of the planktonic Acartia spp. females. The eggs collected from the sediment were Incubated at temperatures corresponding to the in situ bottom temperatures. Hatching of the A. bifilosa eggs occurred throughout the year, but it was most intensive in autumn when water stratification broke and the deep water warmed up to 13°C. It is suggested that a large number of the A. bifilosa eggs sink to the bottom prior to hatching m 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 benthc condtions and processes (e.g. temperature, sediment resuspension and bioturbation). At the bay site, where the surface water layer was hydrographically separated from the deep water, the coupling between the benthic egg abundance and the planktonic populations was not so obvious. The dominant species E. affinis carries its eggs in an egg sac unhl hatching, and probably only the diapause eggs, which are produced in autumn, fall to the bottom. A. tonsa was abundant in the water column at both study sites in autumn, even outnumbering other Acartia spp. in some samples. The eggs of the species only hatched in autumn, when the incubations were conducted at 10 to 13°C. It is probable that A. tonsa spends most of the year as benthic resting eggs In the northern Baltic Sea. In contrast, A bifilosa and E. affinis occurred in the plankton in winter at both study sites, though in low concentrations. They thus have 2 possible sources of recruitment when conditions are again favourable for population growth in spring: hatching of benthc eggs and reproduction by the overwintering population.