Unusual Tarn on Rock and Pillar Range, Central Otago, New Zealand

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• published: 15 Jul 2014
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Request to alpine wetland ecologists: Can you assist with an explanation for this lone alpine pond (tarn) at 1400 m on the crest of the Rock and Pillar Range (1450 m), Central Otago, south-central New Zealand? The video and 7 images describe a single lone alpine pond (tarn) in the upper reaches of the bed of a small catchment (watershed) with only an intermittent stream on a gentle north-aspect (sunny) slope. The climate is periglacial and other periglacial features (terraces, stripes, hummocks, ploughing boulders) occur on the range and have been published. The tarn has a distinctive bow-shaped front composed of very dark, organic-rich, peaty (non-stoney) material with a permanent pond that varies only a few cm in relation to rainfall but has no obvious overflow point. It is seasonally frozen (see Fig. 5), suggesting the concave front may be due to periodic lateral ice expansion or ice-thrusting: we have been calling it an "ice-wedge tarn" which, however, may confuse it with the ice-wedge polygons so common in high-arctic regions that are clearly different in both origin and maintenance. Significant wave action seems unlikely given the small size and relatively sheltered location of the pond. Apart from the several slabs of flat schist rock (see figs), which are not unusual in the area, the bed of the pond is soft and silty, with some filamentous algae. The pale surrounding vegetation is a late snowbank community dominated by the NZ endemic Celmisia haastii and this is surrounded by an alpine herbfield of C. viscosa. Neither of these could be described as wetland communities (which also occur locally on the mountain), apart from its narrow rim. Other studies on the mountain suggest the snowbank community would be snow-covered for almost 200 days a year and the herbfield for about 120 days. Other types of small alpine ponds occur on the New Zealand mountains but all of these are associated with wetland plant communities where the water flow is obviously more continuous. One example are alpine patterned mires about 1 m deep, with the pools generally elongated across the usually broad gentle slopes, held in by vegetation moats and with the pools often contain circular pedestalled islands (see. Figs 7 & 8). These also occur in southern Tierra del Fuego (see Mark et al. 1995. "Southern hemisphere patterned mires, with emphasis on southern New Zealand. J. Roy. Soc. NZ 25: 23-54.) and generally common across the arctic. A second type is alpine stepped ponds (tarns), being a series of ponds of varying sizes and shapes, down some alpine streambeds. These are reasonably common in New Zealand and elsewhere. A subalpine small lake, I saw on Niwot Ridge of the Colorado Front Range when visiting back in 1966 has some similar features but unfortunately is not known to my contact at the University of Colorado. Perhaps someone else knows it close to the main access. Local geologists consider this feature 'quite unusual' among other ponds in the Central Otago region and suggest 'periglacial tarn' might be an appropriate name. Any information on alpine ponds similar to ours on the Rock & Pillar Range (its name comes from the shaft tors of outcropping schist rock on this and other Central Otago mountains) would be greatly appreciated, including ideas as to its likely origin and maintenance, and any other similar cases you know of. We will be pleased to acknowledge your assistance when we publish on this interesting feature. Alan F. Mark, Botany Department University of Otago Dunedin, New Zealand. Email: alan.mark@otago.ac.nz and Ulf Molau, Department of Biological and Environmental Sciences University of Gothenburg Gothenburg, Sweden. Email: ulf.molau@bioenv.gu.se