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The Arctic tadpole shrimp Lepidurus arcticus has a circumpolar distribution where the Hardangervidda mountain plateau in Norway marks its southernmost limit. Within this area, we searched for L. arcticus in 238 lakes in 27 catchments. On Hardangervidda, the distribution pattern of L. arcticus is highly skewed. In the 16 catchments located in the central and eastern parts, L. arcticus was recorded in 70% of all the lakes studied (n=191). The remaining 11 catchments located in western areas, are almost free of lakes with L. arcticus (n=47). The most striking difference between these two areas is the significantly higher level of snow deposition in the western areas. This delays the ice break-up, which results in lower water temperatures and a shorter growing season. The water of lakes in western areas (N=36) is also more dilute than those in the central and eastern areas (N=201), with mean calcium concentrations of 0.81±0.48 and 1.62±1.12 mg L-1, respectively. In the lakes in the central and eastern areas hosting L. arcticus (N=95), the mean value was slightly higher (1.67±1.14 mg L-1). The combination of low water temperature, a short growing season and dilute water low in calcium may explain the near total absence of L. arcticus in the western part of Hardangervidda. All lakes contain brown trout Salmo trutta, and as L. arcticus is heavily sought for as food, the analyses of fish stomachs are the most reliable method of detecting the species. However, this prey-predator relationship may severely reduce the population of L. arcticus, and their presence may also be a function of the proximity of species refugia. This is evident in the context of fish predation, but also of water quality. Hence, in the central and eastern parts of the plateau, where L. arcticus is common, their occurrence increased significantly with lake size, being found in 54% of the lakes <1.0 km2, as opposed to 97% in the bigger lakes. Furthermore, L. arcticus is most frequently found in lakes at altitudes between 1100 and 1299 m a.s.l. We conclude that environmental constraints limit the distribution of L. arcticus on Hardangervidda. The projected increase in temperature towards the end of this century may exterminate L. arcticus from the lower parts of Hardangervidda, especially in the most shallow lakes. Many of the lakes have water quality with pH <6.0 and calcium concentration <1.0 mg L-1. In such lakes L. arcticus is living on the edge of its survival, and the projected increase in precipitation may dilute the waters even further, pushing L. arcticus nearer to its extinction threshold.
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