Tarja Katajisto

Copepod eggs survive a decade in the sediments of the Baltic Sea

Many planktonic calanoid copepod species have been proved to spend a part of their life cycle as benthic resting eggs. In addition to avoiding seasonally occurring unfavourable conditions resting stages may also be used as a long-term survival strategy. The aim of this study was to find out for how long eggs of calanoid copepods retain their viability in the sediments of the Baltic Sea. The occurrence of viable copepod eggs in sea bottom sediment was studied in Pojovik Bay, SW coast of Finland. Eggs were found throughout a 25 cm deep core but deeper than 20 cm they were very scarce. Eggs were incubated at 12 °C in order to check the viability, and their age was estimated by determining the sediment accumulation rate with 137Cs-method. Viable eggs were estimated to be 10–13 yrs old, some possibly even 18–19 yrs. Most eggs in the top 8 cm were viable, their age being up to 7–8 yrs. Nauplii that hatched from the eggs belonged to Acartia bifilosa and Eurytemora affinis, A. bifilosa dominating the hatchers of the top sedment layers and E. affinis the deeper layers. Preliminary evidence is presented that E. affinis produces true diapause eggs in the Baltic Sea.

Seasonal dynamics of Acartia bifilosa and Eurytemora affinis (Copepoda: Calanoida) in relation to abiotic factors in the northern Baltic Sea

The seasonal response of the two most abundant calanoid copepods of the northern Baltic Sea, Acartia bifilosa Giesbrecht and Eurytemora affinis (Poppe), to abiotic factors was studied in Tvarminne, the SW coast of Finland, using long-term data from the years 1973 to 1984. The relationship between copepods and hydrography during the four seasons was evaluated by calculating correlations between deseasonalized and detrended time series of calanoid abundances, temperature and salinity. The dependence of the copepods on temperature and salinity was not the same during the different seasons, nor for copepods of different age. In general, A. bifilosa preferred waters with a salinity of between ca 4 and 7‰, and E. affinis avoided salinities above 6.5‰. The correlations between copepods and temperature were usually positive, but large season- and stage-specific variation occurred. We suggest that the different copepod generations were seasonally adapted to the prevailing hydrographical conditions and therefore responded to changes in a different way.