New results on the biological state of the oxbow lake



New results on the biological state of the oxbow lake The biological survey of the Old-Dráva continued in 2016. Since both species and habitats of European significance are present in the oxbow lake and on its shores, every survey provides valuable results.

Among the habitats of the oxbow lake, the most important ones are the pondweed associations of the natural lakes and oxbow lakes, the soft wood forest groves as well as the alder and ash groves. These habitats are spread out over approximately one hundred hectares, comprising one third of the oxbow lake’s area. In those sections of the oxbow lake which are expected to be affected more by the increasing water level, additional sampling has been conducted in order to detect the scope of effects in the most accurate way possible.

The most common natural habitats of the Old-Dráva oxbow lake are the willow-poplar alluvial forest, spreading over 72 hectares (which is 22% of the area) and the pondweed vegetation, the area of which is 32 hectares (which is 10% of the whole area). The natural composition of the pondweed associations is an important asset.  In addition, the majority of the vegetation sample comprised of protected species. However, the percentage proportion of weeds signaling degradation proved to be 0.

The water level increase planned in the Old-Dráva oxbow lake will likely have a positive effect on the pondweed vegetation, and the populations and association stocks of the individual species will reorganize based on the water level. Our forest monitoring records have shown naturalness states similar to those of the pondweed associations.

One of the points of interest of the survey was the examination of the dragonfly groups. The 2016 survey showed a low species-count, but this does not mean that the dragonfly-fauna of the Old-Dráva is poor. In order to keep track of the changes, a more extended, multi-year monitoring is recommended. A monitoring carried out more often and in multiple seasons would provide a more precise picture of the dragonfly-group’s state.

Aquatic bird monitoring has been concluded twice on the old-Dráva in May of 2016. However, due to the late date, the population is certainly underestimated with respect to several species. In our view, the population of other bird species can be determined very accurately. During the survey in early May, data was recorded at 46 locations, when 231 individuals of 45 bird species have been detected. During the May-end field visit, 250 individuals of 39 bird species have been detected at 39 points.

The full documentation of the monitoring report, along with the Croatian and English language summary can be downloaded from our website.