Susanne Worischka

Is the difference in population dynamics of Daphnia galeata in littoral and pelagic areas of a long-term biomanipulated reservoir affected by age-0 fish predation?

We analysed population dynamics of Daphnia galeatain littoral and pelagic areas of the long-term biomanipulated Bautzen reservoir during spring and early summer of two consecutive years. Simultaneously, consumption of the age-0 fish community was estimated by means of a bioenergetics model. As a consequence of biomanipulation, the abundance of adult planktivorous fish in Bautzen reservoir is low and the interactions between underyearling percids and Daphniaare a key process in the food web of this water. In both years, density and mean size of D. galeatawere markedly lower in littoral than in pelagic areas, whereas no differences were found in the size at maturity and clutch size. However, the lower proportion of adults in the littoral zone resulted in drastically reduced birth rates, which could not explain the strong fluctuations in Daphniaabundance. These findings were consistent during the whole investigation period. Daphniaconsumption by age-0 fish was markedly higher in the littoral than in the pelagic zone and accounted for up to 50% and even 100% of Daphniamortality in this habitat. However, due to gape-size limitation, age-0 fish did not feed on daphnids before June. We conclude that besides underyearling percids older planktivorous fish contribute to the suppression of D. galeatain the littoral zone of Bautzen reservoir and that recruitment of daphnids in this habitat mainly takes place via (active or passive) immigration.

Fish predation can induce mesohabitat-specific differences in food web structures in small stream ecosystems

Diverse benthic communities in streams include a wide variety of predators with different habitat preferences, e.g. for pools or riffles. We hypothesised that these preferences result in mesohabitat-specific predator community structures with quantitative differences concerning predation intensity by vertebrate and invertebrate predators, importance of intraguild predation, or top–down pressure. This hypothesis was evaluated for a small submontane stream by means of mesohabitat-specific quantification of prey consumption by two benthivorous fish species (Gobio gobio and Barbatula barbatula) and several invertebrate predators. The estimation was based on daily food rations and diet composition of predators and mesohabitat-specific predator biomass. We found clear differences between the two mesohabitat types. Predator food webs were less complex in pools than in riffles. Fish predation was more important than invertebrate predation in pools, and intraguild predation had a higher relative importance in these mesohabitats. These differences were probably caused by the mesohabitat use of G. gobio, the largest top predator, which preferred pools. Consequently, the predator food webs were more similar between the mesohabitats when fish were absent. Top–down pressure on primary consumers by all predators together was lowest in pools without fish, but the effect was not significant. Omnivory (including cannibalism) was intense, but its potentially destabilising effects were probably counterbalanced by mesohabitat connectivity. From the results of our experimental study, we conclude that even in small stream ecosystems, food web structures and predation pathways can differ between mesohabitats and that a mesohabitat-specific consideration will help to explain the variety of top–down effects on benthic communities.

River-specific effects of the invasive amphipod Dikerogammarus villosus (Crustacea: Amphipoda) on benthic communities

The invasive amphipod Dikerogammarus villosus is assumed to threaten native biodiversity in rivers. In spite of small-scale experiments and field observations, its impact on natural communities is largely unknown because it seems to be variable and long-term analyses are rare. We analysed long-term data from the Upper Elbe and Middle Rhine (Germany) for invasion patterns and changes in the community structure. In addition, mesocosm experiments were performed in both rivers to identify density effects of D. villosus on the communities. We assumed that D. villosus is a driver of changes in the macroinvertebrate community and that effects are river-specific due to differing benthic communities. We found two invasion patterns for D. villosus with fast invasion in the River Elbe and slower invasion in the River Rhine. The impact of D. villosus on the species composition was weak in both river communities. Invasion seems to have reduced taxa number and individuals and increased Shannon diversity in the River Rhine, but not in the River Elbe. The correlations between the densities of the invader and other taxa in the long-term data were mostly positive with the exception of two native taxa in the River Rhine, indicating a lack of strong negative species interactions. Also in the mesocosm experiments, the biomass gradient of D. villosus adults did not cause significant changes in the communities. The community in the River Rhine seemed to be more vulnerable to the D. villosus invasion than that in the River Elbe. This might be caused by a dominance of invasive species interacting positively with one another, as suggested by the ‘invasional meltdown’ theory. The study suggests that community-level effects of invasion may differ between rivers, probably due to differences in the community composition.