Hidrológiai Közlöny, 2017 (97. évfolyam)

2017 / 3. szám - TRANSNATIONAL EFFORTS - Sandu, Cristina - Dumitrache, Alina - Radu, Emilia - Dobre, Doru - Tudorache, Mädälina - Mihăilescu, Simona: The importance of good ecological status for the successful revival of Danube sturgeon

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Cristina Sandu et al: The Importance of Good Ecological Status for the Successful Revival of the Danube Sturgeon

and the frequency of extreme meteorological events has increased significantly. At European level, an increase of precipitation level by Sup to 10-40% is expected in the Northern part of the continent, while in the Central and Southern part it will decrease by up to 20% (IPCC 2008). Widely accepted climate change scenarios suggest more frequent droughts in summer, as well as flash-flooding, leading to uncontrolled discharges from urban areas and land sources into receiving water courses and estuaries, resulting in possible microbiological (faecal coli, E. coli) and heavy metals contamination (former mining areas), increased loads of suspended solids (Lane et al. 2007) and soil erosion. Lower flows, reduced velocities and higher water residence times in rivers and lakes will enhance the potential for toxic algal blooms (Whitehead et al. 2009). Adaptation policies such as the culture of biofuels and increased water demand for irrigation, construction of new hydropower plants or higher dikes and embankments for flood protection will aggravate the impact on the freshwater ecological status. As the establishment of invasive alien species is more successful in disturbed ecosystems, with simplified trophic networks and reduced competition, any factor affecting ecosystem functionality may favour their further expansion: it has already been acknowledged that changes in forest species, the establishment of invasive alien species and disease outbreaks have been caused or enhanced by global climate change (EEA 2016).

The increase in water temperature leads to the decrease of dissolved oxygen content, affecting the selfpurification capacity of natural water bodies and increasing the occurrence of hypoxia in the water column. Such environmental changes lead to a decline of oxyphylous species, which tend to migrate towards upper reaches with lower temperatures, and a proliferation of thermophilic species (Sandu et al. 2009). In the case of habitat fragmentation and connectivity disruption, the lack of migration corridors jeopardizes the survival of species with low tolerance of environmental changes.

Additionally, water temperature increase accelerates chemical processes and the decomposing activity of microbial communities, controls the growth rate of algae and macrophytes (Wade et al. 2002), regulates the emergence and abundance of aquatic macro-invertebrates (.Durance and Ormerod 2007) and fish migration (eel, shad, sturgeons), rendering freshwater bodies sensitive to rising temperatures, with major implications for meeting WFD objectives and reference conditions for the restoration and improvement of the ecological status (Whitehead et al. 2009).

In the Danube river basin, according to the climate scenarios, the impact is expected to be particularly aggravated in the Southern part, as reduced precipitation ratios, increased temperatures and frequent droughts will occur, especially in summer (1CPDR 2013)

The water temperature of the River Danube increased by 1-3°C in the last century (EEA 2007), while the discharge recorded a decreasing trend after 1960 (Michaylov 2004). In fragile environments, such as coastal and deltaic areas, the climate change impact is even stronger; recent trend analyses emphasize that the area near the Black Sea will very likely become more arid in the next decades (Cheval et al. 2017). The climate change has already affected the Danube Delta, where the River Danube is the main water source. Some lake complexes already record decreasing depths and increasing temperatures (Dumitra- ehe el al. 2017) with subsequent changes in species dominance: higher temperatures and reduced depths favour mass development of cyanobacteria, outnumbering other algal or macrophyte species, which result in cascading changes in the food webs.

Due to modified precipitation ratios, discharge fluctuations, increased frequency of floods and droughts, increased temperatures and evapotranspiration, climate change may significantly affect the river basins. Improving the climate models and the predictions of climate change may significantly contribute to the adaptation of the management strategies and to increasing the resilience of aquatic ecosystems. Identifying vulnerable areas, creating natural water retention measures to mitigate the impact of floods/droughts, restoring riparian and flood- plain areas or creating buffer strips along the rivers to filter the pollutants brought by flash-floods, limiting water abstraction for human consumption during droughts, limiting deforestation and habitat fragmentation are just some adaptive measures that can be taken to mitigate the impact of climate change on aquatic ecosystems.

Invasive Alien Species

Invasive Alien Species are rated worldwide as the second major cause leading to biodiversity decline. Brought to Europe for aquaculture, farming, aquariums and the pet trade or accidentally (transport, e.g. ballast water, cargo ships), these species have escaped into the wild and have spread since, outcompeting the native species and establishing stable populations, while pushing indigenous species towards extinction.

Several of the 49 species included in the EU list of Invasive Alien Species of Union concern (COM 2016, 2017) are already present in countries of the Danube Basin, and may negatively impact the ecological status by modifying the characteristics of aquatic habitats and by eliminating native species, reducing aquatic biodiversity and inducing high economic damages on fishery, aquaculture, flood defence, agriculture and recreational activities. Due to their potential impact on the biological communities and reference species assessed according to the requirements of WFD (planktonic and periphytic algae, macrophytes, macroinvertebrates, fish), they should be taken into account in the evaluation of the ecological status of water bodies.

According to a recent assessment undertaken at EU level (EC 2017), some of the worst groups of “invaders” include: (1) aquatic plants, able to overgrow and create dense mats at the surface, impacting the whole ecosystem by preventing light penetration in the water column and inducing oxygen depletion, leading to the decline of native species; (2) crayfish, due to their predatory behaviour