Modulating lighting regime favours a sustainable use of show caves: A case study in NW-Italy

Abstract When artificial lighting systems are installed in show caves, alien photosynthetic epilithic prokaryotic and eukaryotic microorganisms —the so-called ‘lampenflora’— form extended biofilms on cave speleothems, causing structural and aesthetic damage. Among the environmental drivers influencing their proliferation, light intensity and duration of illumination are expected to play a key role. To unravel which environmental variables determine the proliferation of lampenflora, we designed a specific study in two Italian show caves with similar characteristics and management practices, but differing in terms of tourist use and thus in terms of duration of illumination. In each cave, we selected multiple illuminated speleothems at progressive distance from the cave entrance where we measured the chlorophyll-a concentration of the three main microorganism groups constituting phototrophic biofilms, namely cyanobacteria, diatoms and green algae, by means of the portable fluorimeter BenthoTorch®. In each sampling plot we also recorded the intensity of artificial lights and we visually evaluated the presence of seeping water on speleothems. By adopting advanced regression techniques, we revealed that the total chlorophyll-a and the concentration of the three groups differ between caves, increasing in the cave with higher number of visitors. Regarding the other environmental factors, light intensity positively influenced the total chlorophyll-a and the concentration of cyanobacteria, while humidity increased diatom concentration. We then provided suggestions to implement appropriate management actions addressed to the modulation of the lighting regime in show caves, especially in the areas most exposed to the proliferation of lampenflora.