Swapping Space and Time

A transect is a straight path between two points along which a number of sampling sites are established. The same procedures are generally carried out at each sampling point so that changes in a certain variable can be related to distance along the transect. Transect studies have been used to examine the spatial variability of various glaciological and microbiological phenomena on the Greenland ice sheet since the early expeditions in the late 1800s.

The major transects undertaken during the late 19th and early 20th centuries (from Hodson et al, 2008)
The major transects undertaken during the late 19th and early 20th centuries (from Hodson et al, 2008)

A recent Greenland transect was undertaken in 2010 in order to examine microbial processes on the ice sheet, described in detail in Yallop et al (2012), Stibal et al (2012) and Cook et al (2012). Measurements made along this transect revealed the importance of a surface dwelling algal community on the Greenland ice sheet that was estimated to fix significantly more C than cryoconite hole organisms (Cook et al, 2012) and also have an important albedo lowering effect (Yallop et al, 2012) while morphological and microbiological variations along the transect were linked to abiotic processes by Stibal et al (2012). Variations in all these phenomena with distance inland on the ice sheet were observed.

Traversing the "K-transect" near kangerlussuaq, Greenland in 2010
Traversing the “K-transect” near kangerlussuaq, Greenland in 2010

Transects can also allow temporal changes to be studied spatially – substituting “space for time” facilitates study of long term temporal changes without actually having to monitor over long time periods. This is a commonly used technique for ecologists wishing to infer past or future trajectories for biotic systems. For example, a glacier in retreat exposes newly deglaciated land, and it is usually fair to assume that land closer to the glacier snout was more recently deglaciated than land further away. From an ecological perspective, this means there is a gradient of older to younger biological systems with proximity to the glacier.

Another example is the seasonal recession of the snowline on a glacier. In winter, snow accumulates on glacier surfaces and then in spring and summer it recedes up-glacier. The bare ice nearer to the snowline can be assumed to have been more recently exposed than bare ice near the glacier snout. There is therefore an upglacier gradient of more recently exposed ice.

A newly deglaciated forefield is evident in shown here - prime land for a space-for-time study of ecological succession
A newly deglaciated forefield is evident in shown here – prime land for a space-for-time study of ecological succession

It is possible that  microbial communities at the snowline represent primary successions (initial colonisation of sterile ice); however supraglacial habitats such as cryoconite holes persist for multiple seasons, which might mean exposure initiates “reawakening” of an already complex community rather than development of a new one. Furthermore, ancient microbes stored within the body of the glacier might be released and there is evidence to suggest that microbial communities are shaped to some extent before they are deposited on ice by wind, due to microbes being subjected to specific environmental stresses while suspended in the atmosphere. Complex assemblages may also be delivered directly to the glacier forefield by glacier melt, and deglaciation may leave behind it microbial communities that survived in the subglacial and supraglacial zones. These factors complicate the relationship between space and time, and bring the notion of truly primary successions on ice and glacier forefields into question.

Cryoconite holes - active and diverse microbial habitats that might freeze under seasonal snow in winter, then thaw out during spring and summer
Cryoconite holes – active and diverse microbial habitats that might freeze under seasonal snow in winter, then thaw out during spring and summer

Despite these issues, space for time substitutions have frequently been shown to be robust and extremely useful for making long term ecological analyses that would otherwise be impossible. A famous example is the space-for-time substitution established at Glacier Bay by William Cooper in 1915. Continued glacier retreat in this location has since facilitated intensive study of ecological succession on newly deglaciated land. Space-for-time has also recently been employed to great effect by Wilhelm et al (2013) studying the effects of climate change on Arctic microbial ecology.

 

REFS:

Cook, J., Hodson, A., Telling, J., Anesio, A., Irvine-Fynn, T, Bellas, C. 2010. The mass-area relationship within cryoconite holes and its implications for primary production. Annals of Glaciology, 51 (56): 106-110

Hodson, A., Anesio, A.M., Tranter, M., Fountain, A., Osborn, M., Priscu, J., Laybourn-Parry, J., Sattler, B. 2008. Glacial Ecosystems. Ecological monographs, 78 (1): 41-67

Stibal, M., Telling, J., Cook, J., Mak, K.M., Hodson, A., Anesio, A.M. 2012a. Environmental controls on microbial abundance on the Greenland ice sheet: a multivariate analysis approach. Microbial Ecology, 63: 74-84

Wilhelm, L., Singer, G.A., Fasching, C., Battin, T.J., Besemer, K. 2013. Microbial biodiversity in glacier-fed streams. ISME Journal, 7: 1651 – 1660

Yallop, M.L., Anesio, A.J., Perkins, R.G., Cook, J., Telling, J., Fagan, D., MacFarlane, J., Stibal, M., Barker, G., Bellas, C., Hodson, A., Tranter, M., Wadham, J., Roberts, N.W. 2012. Photophysiology and albedo-changing potential of the ice-algal community on the surface of the Greenland ice sheet. ISME Journal, 6: 2302 – 2313