Effect of the Gabcikovo Waterworks (Slovakia) on riparian floodplain forest ecosystems in the Danube inland delta: vegetation dynamics and trends

Human alteration of watercourses is global phenomenon that has had significant impacts on local ecosystems and the services they provide. Monitoring of abiotic and biotic changes is essential to mitigating long-lasting effects, and the 23-year dataset from the Gabcikovo Waterworks provided a rare opportunity to assess the impact of groundwater regimes on vegetation. The main aim of this study was to describe the effect of the Gabcikovo Waterworks on vegetation structure and species composition of the adjacent riparian floodplain forests over the past 23 years. The results are based on studies of three permanent monitoring plots (PMPs) located in the Danube inland delta - two outside (PMP 1 and 3) and one (PMP 2) fully under the influence of the artificial supply system. Our results demonstrate that the Danube inland delta was negatively affected by the Gabcikovo construction, particularly for sites outside of the artificial supply system. There was a significant decrease in soil moisture and increase in nitrogen at both external PMPs (1 and 3). Alter soil conditions were accompanied by negative changes in plant species composition demonstrated by decreases in the number of typical floodplain forest species that are characteristic for the alliance Salicion albae and either number or cover of neophytes. These changes resulted in slow degradation of the riparian floodplain forests. Vegetation in the artificially-fed PMP 2 was the most stable in terms of the evaluated parameters, however even this site did still experience negative trends in some site characteristics, including soil moisture fluctuations and the number and cover of neophytes. The artificial supply system is not sufficiently replacing previous natural floods, but in some aspects it helped to decrease the negative effect of the Gabcikovo Waterworks on softwood floodplain forests in the studied area.