Freshwater Quality Challenges in Southern Europe Under an Increasingly Warmer and Drier Climate Scenario

1. Scope

Climate change is driving major transformations across ecosystems [1,2,3]. Freshwater systems are particularly vulnerable to changes such as rising temperatures and declining precipitation levels (leading to decreased water levels). In surface freshwater bodies, warmer temperatures can lead to health-threatening events for local communities [4,5,6,7,8,9,10], with environmental, societal, political, and economic impacts. Groundwater quality is also significantly affected by climate change because of alterations in its hydrogeochemistry but also in the microbial communities [11,12], changing the succession of bacteria that play a major role in biogeochemical cycles for nutrient recycling or introducing and promoting contamination by pathogenic bacteria and viruses [13]. Global warming and droughts can also lead to overpumping and freshwater salinization in coastal regions [14,15].

It is now more important than ever to promote and aim to develop an integrated approach to preserve the quality of freshwater [12,16]. However, overgeneralization can be a delicate approach to employ [17], and more research information is needed to help us anticipate hydrogeochemical and biological impacts of climate change. In the era of a “knowledge-based” society, information is vital to increase understanding and awareness of the impacts of climate change and prepare society to find and implement ways to deal with these changes through collaborative solutions involving stakeholders, academia, and policymakers.

Over the past few decades, there has been a general trend of decreasing precipitation in Southern Europe, with more frequent heatwaves and prolonged droughts of increasing severity [18,19,20,21,22], seriously impacting freshwater bodies [23,24] and groundwater resources [25,26]. All these facts indicate that more attention should be paid to environmental research regarding freshwater ecosystems in this region in order to develop more integrated and effective mitigation policies.

Therefore, the aim of the present Special Issue, “Freshwater Quality Challenges in Southern Europe under an Increasingly Warmer and Drier Climate Scenario”, is to contribute to interdisciplinary and transdisciplinary research on freshwater quality under changing climate scenarios, considering surface freshwater and groundwater to be equally important components of the hydrological cycle. Within the context of climate change and with a geographical focus on Southern Europe, this Special Issue features a diverse array of contributions, including research papers and reviews on treatment strategies for the removal of metal contaminants from freshwater resources [contribution 1], groundwater spatiotemporal bacterial biodiversity [contribution 2], pathogenic bacteria in surface freshwaters [contribution 3], the impacts of water retention reservoirs [contribution 4], and the contamination of atmospheric water [contribution 5].

2. Contributions

Sočo et al. [contribution 1] presented a new approach to treating heavy-metal-contaminated freshwater involving the use of a low-cost and ecological methodology based on the biosorbent properties of microalgal powder (derived from dietary supplements made of Chlorella vulgaris), providing new insights for future applications for water treatment facilities and broader areas. Połomski & Wiatkowski [contribution 2] evaluated the impacts of a water retention reservoir in Poland to help predict the effects of the construction of a new reservoir to mitigate water scarcity problems. Their work included a multidisciplinary approach conducted from both ecological and social perspectives. The results indicate areas where the impacts could be minimized or even neutralized and how local communities are reacting (mostly positively) to the reservoir’s construction. Araújo et al. [contribution 3] provided new insight into the occurrence, antibiotic resistance, and virulence of pathogenic bacteria (from the genus Klebsiella, namely, the species K. pneumoniae) isolated from Portuguese surface freshwaters. The results revealed a high antibiotic resistance rate in about 30% of the isolated strains, suggesting potential public health risks via waterborne dissemination, which can also be enhanced by climate change. De Figueiredo et al. [contribution 4] conducted the first study on the overall bacterioplankton community of geographically dispersed groundwaters from the largest karst aquifer in Portugal during both the wet and dry seasons. The hydrogeochemical and molecular results (obtained using 16S rRNA gene barcoding) showed a bacterial community variation mainly linked to redox conditions, SO₄, PO₄, Fe, Mn, Sr and Cl but also some minor and trace elements such as Al, Cr, Cu and Pb. The dominant bacteria belonged to the phylum Proteobacteria, with most of the classes represented, but there was a strong dominance of OTUs related to nitrification/denitrification. These findings highlight the important role of these groundwater systems in nutrient cycling. Finally, Santos [contribution 5] reviewed the potential for the contamination of rainwater and its impacts, considering its role as a major source of freshwater for aquatic systems, particularly during drought periods. Several studies from Southern Europe were highlighted regarding potential toxic elements (PTEs) in rainwater, addressing environmental impacts as well as human health risks. These insights have implications for rainwater quality management improvement and policy development.

In conclusion, this Special Issue contributes to a multidisciplinary standpoint, offering new insights to address present and future freshwater quality issues, particularly within the scope of the challenges posed by extreme drought periods in Southern Europe.