Arctic Lakes and Diatom Change - A Brief Summary

As promised, I'm going to cover the Smol et al (2005) article this time, as well as some associated references which will form part of the discussion. This post will focus on the Diatom changes seen in a wide range of Arctic lakes and their inferred causation. It's generally accepted that the Arctic regions will experience the effects of climate change disproportionately to other regions. This, coupled with the sensitivity of Arctic lakes, (potentially) makes this form of study an early indicator of the speed and severity of climate change in the future on a global scale. (Oops, I may have partly answered the final question). Oh well, on with the post...

A subarctic lake from Northern Quebec

Arctic Lake ecology is nice summarised by Smol and Douglas (2007) and shows that
diatom response to climate in the Arctic is indirect. That is, the climate affects an environmental condition, which in turn influences diatom community assemblages. In this case the 'middle-man' is snow and ice cover.

As seen in the diagram below, the extent of snow/ice determines the habitat availability for diatoms. As Diatoms are a group of algae, they require a certain light level to survive. Therefore, part a) shows a lake with a permanent cover of ice with only a small 'moat' developing during the summer, allowing littoral diatoms to grow. As warming occurs and ice melt is sustained (c) diatom ecology is shifted. Also, longer growing season is likely to result in a more diverse diatom community and potentially allow growth of moss (which increases epipyhtic diatom numbers) in the shallow areas. Of course, that's a hugely simplified explanation and ignores a whole load of other feedbacks and chemical processes (which are explained in Ch13 of this book). I'll post again when I've learnt a bit more about it!

Snow and Ice cover is a dominant climate-controlled influence
on Arctic Lake Ecology (direct from Smol and Douglas 2007)

Anyway, Douglas et al (1994) found significant diatom shifts at three sites in the Northern Canadian Arctic, including the Elison Lake record shown below. It's clear from even a quick glance that there's a huge change at about 3cm which continues to the surface. Here, there's a (relative) growth in the number of Pinnularia balfouriana at the point corresponding to the 19th century (determined by 210Pb dating). It's impossible to tell whether the Fragilaria pinnata and construens var venter populations crashed at this time because no diatom concentration data are shown in this paper. Pinnularia balfouriana is an epiphytic species (grows attached to plants) and is therefore supportive of the 'growth in aquatic mosses' hypothesis.

The authors argue that climate change is the only reasonable explanation as other changes to the environment began either too early (increasing UV levels) or too late (pollutants) to explain the shift. Also, there is little/no land use in these areas, discounting direct impacts from herding.

Elison Lake Diatom record from Douglas et al 1994

However, to be sure that this wasn't just a one off (or three?) and to further test the hypothesis that climate was causing this change, further studies were required. Smol et al (2005) is one of these follow up studies. The paper is a synthesis of 55 lakes in the Arctic and sub-Arctic regions, taken from across a broad chemistry and morphometry range, which have long-term palaeolimnological records with botanical and/or zoological proxies.

Results also showed a shift towards littoral diatom growth and epiphyton associated with moss, backing up the findings by Douglas et al (1994). For those lakes with longer-term records available, the recent change (~150yr) is unprecedented when compared to long term fluctuations. Also, they argue in support of their climate hypothesis, sites where temperature records show little warming do not exhibit the same shifts. However, correlation does not mean causality and temperature records are usually short.

In conclusion, it is clear that Diatom community structure is changing across the Northern Canadian Arctic, a pattern which is likely replicated around the high Arctic. It is likely that the dominant cause, as has been asserted by these authors, is changing climate, but this doesn't mean the other causes have not contributed to the changes. Perhaps a project directly relating recorded ice extent to live diatom communities from year to year would give even more weight to this argument? What we can be sure of, though, is that these communities have experienced a significant change in recent years, and judging by the long term records, this is down to us and our increased development. Therefore as the authors assert, "any hopes of cataloging natural assemblages may already be fruitless." (Douglas et al (1994))

Next time we're going to look up the trophic levels to chironomids...