Christian Fesl

Dragonfly associations (Insecta: Odonata) in relation to habitat variables: a multivariate approach

In a dragonfly survey, carried out in a lowland wetland area in eastern Austria, a total of 19 resident species was recorded. Multivariate statistical procedures were used to analyse the relationship between dragonfly assemblage patterns and environmental variables. Besides widespread and euryoecious species with unspecific habitat requirements two dragonfly associations were identified: on the one hand species mainly occurring at temporary natural and near-natural ponds characterised by rush and reed vegetation, on the other hand species preferring permanent waters such as the artificial waterbodies in the investigation area characterised by floating macrophytes. Water persistence and the existence of floating macrophytes determined the formation of species assemblages.

Intercalibration of hydroacoustic and mark-recapture methods for assessing the spawning population size of a threatened fish species.

Hydroacoustic counting and a three-year mark-recapture study with passive integrated transponders (PIT tags) were used to estimate the size of a spawning population of nase Chondrostoma nasus, a threatened potamodromous cyprinid that undertakes annual spawning migrations into a tributary of the Danube River. In 2005, the estimates of the size of the spawning population from the hydroacoustic counts (N = 2234, 95% CL 1929-2538) and from the Jolly-Seber model (N = 1198, 95% CL 461-5842) corresponded well. Estimates from the jackknife-estimator based on the hydroacoustic counts yielded slightly higher values (N = 2783, 95% CL 2529-3037), but were still in the same order of magnitude as those from the hydroacoustic and mark-recapture approach. At low run-size, hydroacoustic counting was more time consuming and technically demanding than mark-recapture studies. At the same time, it was non-invasive, provided real-time data on a fine temporal scale, and estimates showed less variability than the Jolly-Seber model. Mark-recapture of fish in spawning streams involved substantial disturbance at a sensitive stage of the life cycle. Hence, hydroacoustics is highly suited for population estimates of threatened potamodromous fishes, where interference needs to be minimized.

Phytoplankton community structure in a Danubian backwater system: Response to environmental factors

Abiotic variables and phytoplankton composition were studied at an impounded part of the River Danube and at three backwaters upstream of Vienna. One of the backwaters was frequently connected to the Danube, whereas the other two sites were isolated with the exception of two flood events. Analysis of community stability revealed a pronounced change in the community structure at the flood pulses. The strongest impact was detected at the backwater that usually showed the highest differences to the main channel. Cluster analysis based on species abundance estimations resulted in a separation of the sites and detected similarities between the Danube and the permanently connected backwater. An Indicator Species Analysis was used to identify species indicative of the groups defined by the cluster analysis. A Detrended Canonical Correspondence Analysis was applied for examination of the relationship between the indicator species and the environmental factors. Connection and isolation, respectively, seemed to be important factors for the separation of sites and for the distribution of species. In times of isolation, seasonal variations of the environmental factors and, at one site, enhanced nutrient supply due to shallowness became determining factors.