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Absence of visitors during lockdown reveals natural variation in carbon dioxide level in the Glowworm Cave, Waitomo, New Zealand

Waitomo Glowworm Cave is a highly visited cave where the highlight is viewing the bioluminescence display of a large colony of glowworms. Anthropogenic carbon dioxide build-up in the cave is prevented by management of chimney-effect ventilation aided by a network of microclimate sensors. A cave door prevents ventilationunder drying conditions and promotes it when necessary to clear CO2 and when inflowing air has high relative humidity. A COVID-19-related nationwide “lockdown” in New Zealand from March 2020 resulted in neither staff nor visitors being present in the cave for 60 days, and provided an opportunity to assess the natural microclimate of the cave, especially the natural variation in partial pressure of carbon dioxide (pCO2). In addition, comparison to the previous year showed that the presence of people in the cave increased the cave temperatures but the effect was short-lived due to cave ventilation. During the period of lockdown, the daily increase of carbon dioxide partial pressure (pCO2) due to visitors was absent. When the cave door remained sealed, pCO2 varied and tended to lie at levels above that of the external atmosphere (410 ppm). Notably, rain events raised pCO2 by up to 200 ppm (v/v), which appeared to be sourced from both stream water and drip water. These natural CO2 sources rarely reached the levels associated with cave visitation. The results support current management practices that use door control to enhance cave ventilation when people are in the cave or when natural conditions (high stream levels and high drip-water levels) promote CO2 outgassing into the cave. Suppressing ventilation outside of those times reduces the risk of introducing dry air that could desiccate the glowworms.