Coronavirus pandemic: An opportunity to study the anthropogenic impact on micro-climate conditions and CaCO3 crystal morphology in the Nerja Cave (SE Spain).

Following the declaration of the COVID-19 pandemic, the Spanish Government restricted non-essential movements of all citizens and closed all public spaces, such as the Nerja Cave, until May 31, 2020. This particular condition of the closure of the cave provided a unique opportunity to study the micro-climate conditions and carbonate precipitation in this tourist cave without the presence of visitors. Our results show the significant effect of visitors on the air isotopic signature of the cave and on the genesis of the extensive dissolution features affecting the carbonate crystals formed in the tourist sector of the cave, alerting us to the possible corrosion of the speleothems located there. The movement of visitors within the cave also favours the mobilisation of aerial fungi and bacterial spores and their subsequent sedimentation simultaneously with the abiotic precipitation of carbonates from the drip water. The traces of these biotic elements could be the origin of the micro-perforations previously described in the carbonate crystals formed in the tourist galleries of the cave, but they are subsequently enlarged due to abiotic dissolution of the carbonates through these weaker zones.

Selection of LED lighting systems for the reduction of the biodeterioration of speleothems induced by photosynthetic biofilms in the Nerja Cave (Malaga, Spain).

Electrical lighting favours the development of photosynthetic biofilms in caves which can induce biodeterioration in the colonized substrates. The use of specific lights as a limiting factor for biofilm growth could be effective in their control and represents an alternative to chemical methods since they can damage the substrate. However, studies about lighting and the photosynthetic activity of organisms in caves are scarce. In order to select the most effective LED light source in reducing photosynthesis and therefore, in reducing the growth rates of microalgae and cyanobacteria, four biofilms in the Nerja Cave were illuminated by several light emitted diodes (LEDs) with different spectral compositions and the photobiological responses were measured both by empirical and theoretical methodologies. The empirical approach was based on the photosynthetic efficiency, by measuring the in vivo chlorophyll a (Chl a) fluorescence and the theoretical approach was based on the photonic assimilation performance related to the proportion of the light quality used for photosynthesis, according to the action spectra for photosynthesis available in the literature. The photobiological responses showed differences between the empirical and theoretical approach mainly in biofilms dominated by cyanobacteria and red algae, probably because the available action spectra were not useful for monitoring these Nerja Cave biofilms. However, the expected spectral responses of photosynthesis were observed in green microalgal biofilms with maximum photosynthetic efficiency in red and blue light although the green light was also unexpectedly high. The high photosynthetic efficiency in green light could be explained by the predictable high chlorophyll content due to a very dark environment. The results were not conclusive enough for all the biofilm types to be able to recommend a specific lighting system for the photocontrol of biofilm expansion. Therefore, new action spectra for photosynthesis of the extremophile organisms of the Nerja Cave are required. This approach, based on theoretical and empirical methodologies, is a useful tool to obtain information to allow the design of the most adequate lighting systems to reduce photosynthetic activity and favour the conservation of the caves.

Coupling air temperature records and gravimetric data to interpret ventilation patterns in a Mediterranean karstic system (Nerja-Pintada caves, southern Spain).

Microclimate and geophysical studies are commonly applied to the characterization of karst systems although they are usually used separately. The main purpose of this manuscript is to show how the analysis of the data from both these research methods is a useful tool in the characterization of karst systems and we present the analysis of a specific case study: the Nerja- Pintada caves system. The joint analysis of the Nerja Cave and external air data (mainly temperatures) and the pre-existing gravimetric data of its surroundings (residual gravity anomaly map) have allowed us: 1) to postulate the existence of an unknown great cavity located near to the Nerja Cave and with direct influence in its ventilation and 2) to propose a new model of the Nerja-Pintada caves ventilation based on the changing connection between a "main cavity" system (Nerja Cave), with basically a transmissive function of airflows and an "annex cavities" sub-system, with different functioning as far as the airflow is concerned: transmissive in the case of Pintada Cave and capacitive in the case of the geophysically-located cave.

Highlighting the importance of transitional ventilation regimes in the management of Mediterranean show caves (Nerja-Pintada system, southern Spain).

This study shows the utilization of the air CO2 exhaled by a very high number of visitors in the Nerja Cave as both a tracer and an additional tool to precisely evaluate the air circulation through the entire karst system, which includes non-touristic passages, originally free of anthropogenic CO2. The analysis of the temporal - spatial evolution of the CO2 content and other monitoring data measured from January 2015 to December 2016 in the Nerja-Pintada system, including air microbiological controls, has allowed us to define a new general ventilation model, of great interest for the conservation of the subterranean environment. During the annual cycle four different ventilation regimes and two ventilation modes (UAF-mode and DAF-mode) exist which determine the significance of the anthropogenic impact within the caves. During the winter regime, the strong ventilation regime and the airflow directions from the lowest to the highest entrance (UAF-mode) contribute to the rapid elimination of anthropogenic CO2, and this affects the whole karstic system. During the summer regime the DAF-mode ventilation (with airflows from the highest to the lowest entrances) is activated. Although the number of visitors is maximum and the natural ventilation of the karstic system is the lowest of the annual cycle, the anthropogenic impact only affects the Tourist Galleries. The transitional ventilation regimes -spring and autumn- are the most complex of the annual cycle, with changing air-flow directions (from UAF-mode to DAF-mode and vice versa) at diurnal and poly diurnal scale, which conditions the range of the anthropogenic impact in each sector of the karst system. The activation of the DAF-mode has been observed when the temperature difference between the external and air cave is higher than 5°C.