During the recent decades, alterations in the northern Atlantic climate conditions have resulted in major modifications in the environmental conditions of the Baltic Sea that allows for a unique opportunity to assess species responses to climate-induced changes in the ecosystem.

In this thesis, I examined long-term changes in the northern Baltic zooplankton community in comparison to changes in seawater salinity and temperature. In addition, I estimated how the bottom-up changes in the Bothnian Sea have manifested themselves at higher trophic levels by using the Baltic herring (Clupea harengus membras) and its preferred prey species, a freshwater copepod Limnocalanus macrurus, as case examples.

I found that in the northern Baltic Sea major modifications in the salinity and temperature have resulted in concomitant changes in zooplankton species dynamics and community structure. In the Archipelago Sea, the freshening and warming of water have been favorable to most small-bodied brackish-water taxa and unfavorable to marine taxa. Moreover, the hydrography changes have also resulted in earlier peaks in the timing of phytoplankton and brackish-water taxa. In the Bothnian Sea, the results showed that L. macrurus has both large lipid reserves and occurs in high abundance during spring-early summer, when the energy requirements of herring are high. Later during summer, the descent of adult L. macrurus to deeper water levels coincided with a decrease in its abundance and lipid store size, indicating that at least part of the population suffered from starvation. The results also implied that while the warming and freshening of water have indirectly improved herring condition and thereby enabled the positive development of the herring stock, the changes have also increased the energy consumption of fish and caused a decline in the availability of prey resulting in a reduction of its lipid reserves.

In conclusion, during the past decades, the climate-induced changes in seawater salinity and temperature seem to have resulted in various changes in the northern Baltic ecosystem, which in the zooplankton data are manifested as a decrease of marine species and shifts in brackish-water species phenology. In the Bothnian Sea, the changes in the physical environment seem to have caused both indirect and direct effects on the energy cycles, which in the monitoring data appear as a decline in the lipid content as well as a decline in herring body size. The results of my thesis provide new information on the food web dynamics that can be used for further research and highlight the importance of long-term environmental monitoring to foresee climate-induced changes in the ecosystem and to provide an accurate picture of the trophodynamics.