Antarctic Science 23(3), 235–242 (2011) & Antarctic Science Ltd 2011 doi:10.1017/S0954102011000046 Summer–winter transitions in Antarctic ponds I: The physical environment IAN HAWES 1 , KARL SAFI 2 , BRIAN SORRELL 3 , JENNY WEBSTER-BROWN 4 and DAVID ARSCOTT 5 1 Gateway Antarctica, University of Canterbury, Private Bag 4800, Christchurch, New Zealand 2 NIWA Ltd, PO Box 11-115, Hamilton, New Zealand 3 Dept of Biological Sciences, Aarhus University, 8000 Aarhus 3, Denmark 4 Waterways, University of Canterbury, Private Bag 4800, Christchurch, New Zealand 5 Stroud Water Research Center, Avondale, PA 19311, USA ian.hawes@canterbury.ac.nz Abstract: Meltwater ponds are one of the most widespread aquatic habitats in ice-free areas of continental Antarctica. While most studies of such systems occur during the Antarctic summer, here we report on ice formation and water column attributes in four meltwater ponds on the McMurdo Ice Shelf during autumn, when they went from ice-free to . 80 cm thickness of ice. Ice thickness grew at an average rate of 1.5 cm d -1 in all ponds and as ice formed, salts and gases were excluded. This resulted in conductivity rising from 3–5 to . 60 mS cm -1 and contributed to the ebullition of gases. Incorporation of gas bubbles in the ice resulted in a high albedo and under-ice irradiance declined faster than incident, the former falling below 1Wm -2 (daily average) by early April. After two months of ice formation, only 0–15% of the volume of each pond was still liquid, although this represented 5–35% of the pond sediment area, where much of the biological activity was concentrated. We suggest that the stresses that the freezing process imposes may be as important to structuring the biotic communities as those during the more benign summer growth period. Received 18 May 2010, accepted 10 September 2010, first published online 4 February 2011 Key words: conductivity, ice formation, irradiance, McMurdo Ice Shelf Introduction During summer, meltwater ponds are common features in many ice-free parts of Antarctica, where they typically represent isolated foci of biological activity and diversity in otherwise barren, desert landscapes (Vincent & James 1996, Howard-Williams & Hawes 2007, Quesada et al. 2008). These are often closed-basin ponds, lying in shallow depressions and may be long-lived phenomena. Water lost by evaporation, ablation and sublimation is replenished by melting of winter snow and/or glacial ice. During summer, water is maintained in the liquid state by relatively mild air temperatures and the absorption of incident solar radiation within the pond/sediment system (Hawes et al. 1993, 1999). When melted, growth conditions in these ponds can be rather benign, with water temperature reaching up to 108C. The biotic assemblages in these ponds are dominated by species-rich, perennial mats of algae and cyanobacteria that can grow to centimetre thickness and cover the entire benthic area. Photosynthesis by these microbial mats can reduce nutrients and inorganic carbon to extremely low concentrations and drive pH to values of over 10 (see reviews by Howard-Williams & Hawes 2007, Hawes et al. 2008 and references therein). A paradox of our understanding of polar ponds is that most studies occur during this relatively benign period when researchers can most easily access them, while the most extreme stresses are likely to occur during and after the process of freezing. Previously (Hawes et al. 1999), based on over-winter data logging, experimental manipulations and limited direct observations (e.g. Schmidt et al. 1991), we have speculated on how the environmental changes that must accompany pond freezing may affect the microbial communities that dominate these ponds. We postulated that the extreme autumn and winter conditions were more likely to impose the uniquely ‘‘Antarctic’’ characteristics of these systems than the relatively mild summer conditions. In January 2008, a unique opportunity to observe the freezing process arose when the United States and New Zealand Antarctic Programmes combined to extend field science support in the McMurdo Sound region from January through until April. This allowed us to access ponds on the McMurdo Ice Shelf for much of the freezing period and to obtain detailed records of physical, chemical and biological change. This contribution sets the scene for a series of reports on the freezing process by describing physical conditions during freeze-up. Study area The McMurdo Ice Shelf (MIS) is a small part of the Ross Ice Shelf, located in the south-west corner of the Ross Sea (Fig. 1). It is bounded by Minna Bluff to the south and the coast of southern Victoria Land to the west. On the eastern side, the MIS grades into the Ross Ice Shelf proper, while 235 https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0954102011000046 Downloaded from https://www.cambridge.org/core. IP address: 207.241.231.81, on 27 Jul 2018 at 18:17:57, subject to the Cambridge Core terms of use, available at