Ecology of zooplankton communities: climate, dormancy, and benthic-pelagic coupling

Sammanfattning: This thesis describes how influences, such as top down and bottom up forces, shape zooplankton communities in shallow lakes. I have also extended the traditional food-web theory by investigating the effects of climate on total biomass, taxonomic composition, and temporal properties of zooplankton communities. A field experiment showed that the total biomass of both phytoplankton and zooplankton increased with nutrient enrichment. This increase was, however, lower for zooplankton and higher for phytoplankton when planktivorous fish was present, indicating cascading effects of top down forces from fish to phytoplankton. A study of 81 lakes, covering a climate gradient from Southern Spain to Northern Sweden, confirmed the role of lake productivity (total phosphorus concentration) as the most important predictor of total phytoplankton and zooplankton biomass. Moreover, macrophyte cover and climate also significantly affected total zooplankton biomass. Increased macrophyte cover led to an increase in total zooplankton biomass through its positive effect on macrophyte associated and benthic taxa. Enrichment mainly affected pelagic taxa and the main effect of enrichment was an increase in cyclopoid copepods relative to calanoid copepods and an increased proportion of Daphnia of the total cladoceran biomass. Warmer climate was associated with lower biomass of zooplankton, mainly through its negative impact on pelagic species. Enrichment and increased temperature (especially the combination of these two) also reduced the temporal stability of zooplankton communities. Zooplankters are usually considered to be short-lived, transient creatures, but their ability to produce resistant dormant stages can prolong their life span considerably and carry populations through periods during which active stages are unable to survive. Dormancy has implications for zooplankton ecology, genetic diversity and evolution of species. In a field study hatching from diapausing eggs was shown to affect seasonal succession in a cladoceran community only during spring. An analysis of published data together with my own data suggest that cues, used to break dormancy, are exclusively of an abiotic type, signalling seasonal change. The type of cues used to induce diapause, on the other hand, appears related to the generation length or life span with long-lived taxa relying more on seasonal cues, whereas short-lived taxa rely on cues signalling environmental change on a shorter time scale. The presence of large pools of dormant eggs or individuals in lake sediments also suggest that in studies of zooplankton populations, the dynamics of both active and dormant stages should be considered. Spatial distribution of dormant stages in lake sediments seems to be the result of passive processes for diapausing eggs but possibly an active process for copepodites. Reanalysis of earlier studies regarding diapausing copepodites revealed a size-dependent distribution both horizontally in the lake and vertically in the sediment. Predation, or risk of predation, from benthivorous fish is proposed as one potential factor creating these patterns. This exemplifies the importance of viewing zooplankton life histories as the product of both benthic and pelagic selection pressures.