ALBERT

Description: This thesis shows different aspects of the distribution of zooplankton and the resulting influence on larval fish survival. The vertical distribution of the key calanoid copepod species in the Bornholm Basin (Baltic Sea), Pseudocalanus acuspes (Giesbrecht 1881), Temora longicornis (Müller 1792), Acartia bifilosa (Giesbrecht 1882), Acartia longiremis (Lilljeborg 1853) and the only cyclopoid copepod Oithona similis (Claus 1863), were investigated. Sampling was carried out with multiple opening/closing nets. The results showed distinct seasonal differences in the vertical distribution for all species. For P. acuspes an ontogenetic migration was detected, with the younger stage dwelling near the surface and the older stages moving successively deeper. The adults inhabited the water layers around the halocline. In November the vertical distribution disintegrated and all stages were distributed over the whole water column. However in this period the older stages (C4, C5) dominated the community. A diel vertical migration was observed for T. longicornis in the summer month (June 2001, July and August 2002). The migration was most distinc for the older stages, moving from 20 metre at night to almost 60 metre at day. No diel migration was observed in April and November. For Acartia species an extended daily migration was not observed, even though the weighted mean depth of Acartia longiremis in July shifted from 10 metre at night to 20 metre at day. Both Acartia species inhabited the water column above the thermocline in spring and summer. However, in November the distribution was broader for all stages in both species. All stages of O. similis inhabited the water layers around the halocline irrespective to the season, showing no vertical migration. The distribution of reproducing female P. acuspes was investigated with the Video Plankton Recorder, an in situ imaging device. The distribution was closely related to high salinity although low oxygen values were avoided. Based on images of female P. acuspes carrying egg sacs the number of eggs per clutch was calculated and using the temperature dependent development time an in situ egg production rate was estimated. Albeit the production was the same between years, the naupli to female ratio was higher in 2003 leading to the conclusion that the naupli mortality might have been reduced. A simple individual based model of P. acuspes connected to a circulation model of the Baltic Sea was used to reveal possible influence of drift to biomass changes. The results showed that in contrast to the 1980’s during the 1990’s the prevailing barotrophic circulation forced the majority of P. acuspes out of the Bornholm Basin into easterly Basins and shallow coastal regions. However, the transport of organisms produced in the eastern Basins (Gotland Basin and Gdansk Deep) back to the Bornholm Basin was limited. Lower salinity and higher temperature in the near shore water might influence the survival and reproduction. The same circulation model was used to investigate the drift of larval cod and possible consequences on the overlap with food organisms. Again, a shift in the baroclinic circulation between the 1980’s to the 1990’s led to an increased dispersion of larvae in the latter period. It was shown that this decreased the overlap with larval prey and thus decreased the survival probability of larval cod. Furthermore, the influence of variability in prey abundance and diversity on the results of an individual based model on larval cod growth and survival was investigated. It was shown, that the use of idealized mean prey fields led to an overestimation of larval survival. Using field data derived from net sampling the survival was partly reduced to 50%. Another investigated factor was the size of prey organisms. Again, the use of mean value led to an overestimation of survival. Using measurements from samples reduced the survival up to 75%.