Low summer temperatures: a potential mortality factor for high arctic soil microarthropods?
Throughout the summers of 1992–1994 the low temperature performance of soil microarthropods at Ny Ålesund, Spitsbergen (78 °56′N 10 °53′E), was investigated. Species studied were the Collembola Hypogastrura tullbergi (Schäffer), Onychiurus arcticus (Tullberg) and Onychiurus groenlandicus (Tullberg) and the mites Diapterobates notatus (Thorell), Hermannia reticulata (Thorell), Camisia anomia Colloff and Ceratoppia hoeli (Thor). The results show that: (i) The supercooling ability of these animals decreased rapidly on regaining activity in spring. For example, the supercooling point (scp) of H. tullbergi when heat extracted from frozen ground, decreased from −20 to −8 °C within 4 h. Population scp profiles of all species determined throughout the summer showed distinct bimodal distribution; (ii) starvation for 14 days, desiccation or a combination of both, resulted in little change in the mean scp of the collembolan O. arcticus; (iii) survival of the animals after a brief exposure to a sub-zero temperature was poor, in either humid or dry atmospheres. For example, 77% of H. tullbergi died after cooling to −5 °C at 1 °C min−1. Comparison with scp data indicates that animals died before they froze; (iv) all species examined showed some locomotory ability at temperatures approaching −3 °C; (v) polyols occurred in low concentrations, although elevated levels of glucose were observed in early spring and late autumn in O. arcticus; and (vi) soil temperature declined to −29.6 °C in the winter of 1992/93 and remained below zero for up to 289 days and the animals can be encased in ice for 75% of the year. Average daily soil temperatures for July and August rarely exceed 8 °C and were typically in the range 3–6 °C. Estimation of previous years soil temperatures from screen temperature records indicate that July /August ground surface temperatures < 0 °C occurred on 25 and 28 occasions between 1969–1993 at the polar semi-desert and tundra heath sites respectively; but, that soil temperatures at a depth of 3 cm are buffered against temperature extremes and temperatures below 0 °C are rarely encountered. The consequences for the soil microarthropod fauna of such extended periods of low temperature and the effects of climate change on these species are discussed.