Karst is a widely spread natural phenomenon which provides essential benefits to human society, such as drinking water. The water cycle in the karst geosystems is the main factor for their formation and at the same time one of the main drivers for ecosystem services (ES) provision. The monitoring of the water cycle can provide valuable information regarding its functioning and ensure data for ES assessment. This paper aims to present an overview of the monitoring of the water cycle in the karst geosystems and the opportunities to integrate the monitoring data into the water regulation ES assessment. The monitoring of the water cycle is based on the methodological framework ProKARSTerra. It is applied in model karst geosystems, which are representative of the main karst types in Bulgaria. One of them is the Brestnitsa karst geosystem, which is the case study of this work. The monitoring ensures data for analyses of the water cycle which can be used in the assessment of water-related ecosystem services. The results from the analyses of the data requirements and availability show that some services such as water flow regulation and regulation of chemical condition of freshwaters can be easily provided through data for quantification, while for others further studies are needed. The results of the long-term integrated monitoring in Brestnitsa karst geosystem provide the foundation for important conclusions and models for the karst genesis and function under global changes and active anthropogenic pressure. Their integration into the assessment framework and mapping of ecosystem services is an essential step towards the development of models for sustainable use of natural resources in the karst areas.

This study examines climate change, anthropogenic impacts, and their relationship with the sustainable development of the Brestnitsa karst geosystem. It is representative of the karst in Bulgaria, which is developed on a quarter of its territory. The geosystem approach was used to reveal the interrelationships in a typical karst territory. The data were collected from more than 10 years of integrated monitoring of the geosystem, including continuous instrumental monitoring in the show cave Saeva Dupka, which is part of the geosystem. Various data from other sources were also used, such as satellite observations (a digital terrain model, land cover, and satellite images and products), ground data, and climate reanalyses. A spatial analysis of the karst geosystem in the context of climate change and sustainable development was conducted using a complex of remote sensing methods, geographic information systems, and statistical methods. The main results include the state and trends in the climate of the area, changes and trends in the speleoclimate and gas composition of the air in the Saeva Dupka cave, and changes in land use in the territory of the geosystem. Conclusions about the connections between climate change, current karstogenesis, and the carbon cycle in the geosystem, as well as the response of the geosystem to the consequences of the combined impact of climate change and human activities, have been made. All these impacts on the karst geosystem have been assessed in order to make adequate management decisions to guarantee its sustainable development in the future.