Environmental Management of the Waitomo Glowworm Cave, New Zealand: Effects of Visitors and Ventilation on Carbon Dioxide Concentrations and Air Moisture Content

Abstract

Environmental management of the Waitomo Glowworm Cave aims to mitigate the potential conflict between presenting the cave to visitors and protecting it. In the case of the cave’s microclimatic environment, it aims, firstly, to control rise in carbon dioxide (CO2) levels above a certain threshold, and secondly, to keep evaporation rates low so that cave drying is minimised. Reduced ventilation combined with CO2 from visitor respiration can lead to elevated CO2 concentrations in the cave. High ventilation rates during cool or cold conditions outside can lead to cave drying. Both of these can be regulated by managing visitor numbers and airflow through the cave. Earlier work has shown that three ventilation regimes exist, namely, downflow, upflow, and neutral, which are determined by cave-to-outside air temperature gradient (Td). Cave ventilation can be regulated by a door at the cave’s upper entrance which, when closed, seals the entrance and restricts air flow though the cave. Ineffective management of cave ventilation and visitor numbers can affect cave microclimate thereby compromising the glowworms’ habitat. In light of this, the work here examines the extent to which cave management effectively minimises both cave drying and rising cave air CO2 concentrations in the cave. To assess this, a three-part null hypothesis is tested, namely: cave air CO2 levels are not related to visitor numbers; cave air CO2 levels are not related to Td; and cave drying is not related to Td. Four detailed monitoring experiments are carried out inside the cave over a 10-month period. The results of these experiments together with a statistical analysis of a five-year data archive show the extent to which visitor numbers and cave ventilation affect CO2 concentration in the cave. Spatial and temporal distributions of cave air temperature and humidity, and thus drying of the cave, are determined by: the external temperature and humidity; moisture state of the cave environment; advection of heat and moisture from outside the cave; the sensible and latent heat flux to and from the air moving through the cave and cave surfaces; and the vapour flux between the air and cave surfaces. Overall, the results show: cave air CO2 levels are moderately related to visitor numbers; cave air CO2 levels are not related to Td; and cave drying is not related to Td. From this it may be concluded that cave management practices through ventilation and visitor control is effective.