Arctic Lakes and Zoological Indicators

As we saw in the last post, it is highly likely that climate is indirectly altering Diatom communities in high Arctic lakes through changing levels and patterns of ice cover. Quinlan et al. (2005) used the same cores as the (previously reviewed) Douglas et al. (1994) study and counted the fossil chironomid (non-biting midge) larvae remains down the cores to investigate whether invertebrate communities have also 'shifted', and if they have, how likely it is that the changes are linked to the results from the previous diatom investigations. It's important to note, however, that these results are only taken from three archives (ie lakes and ponds) from Ellesmere island so further study is required to confirm that these findings are widespread.

Chironomidae are a family of flies which are aquatic organisms until their adult stages (see above) and
are found from the Tropics right through to the High Arctic and Antarctic regions. While at the larval stage, they normally preserve well in lake sediments, making them useful in Palaeoecological studies. On average 1cm³ of sediment will contain around 50 fossil head capsules, used for identification and counting of assemblages. Some species are predaceous but most feed on algae and other organic matter within the lake system, meaning that various species of chironomid are influenced (in at least a small way) by algal community presence.

So, it's likely that chironomid communities have been affected by algal community shifts as well as the direct ecological effects of rising temperature in these sensitive regions. Quinlan et al. (2005) found changes at all three sites studied from around 1850, determined using ²¹⁰Pb dating. As can be seen in the plot below, firstly the number of head capsules has increased significantly as compared to the last c.4Kyr determined from ¹⁴C dating (the rising numbers are plotted as absolutes, not percentages as with most diatom data, meaning that the rise also indicates an increase in chironomid head capsule 'concentration' and therefore productivity).

Change in Chironomid ecology through last c4Kyr from Quinlan et al. (2005).
Data are plotted as absolute head capsules per gram of dry weight sediment HC/g DW.

The findings back up the 'algae influencing chironomid' hypothesis because of a rise in 'scrapers', adapted to consuming periphyton. As seen with the diatom studies, periphytic diatom numbers have increased due to the indirect impact of lowering ice levels and subsequent growth of aquatic moss. The Taxon Corynoneura/Thienemanniella is a 'scraper' and it's clear dominance in recent sediments therefore strongly suggests a food-web interaction. If this is correct, this is a clear knock-on effect of rising temperatures and changing seasonality. Firstly, climate affects lake ice extent, influencing aquatic moss growth, then diatom communities and subsequently chironomidae. This is not the end of the food chain however, meaning this knock-on effect is likely to continue up the trophic levels to fish and birds. Making this all the more likely is the fact that Chironomidae are the dominant macro-invertibrate in these Arctic waters.

Another finding was that species diversity has increased. It is very easy to link diversity to desirability, and in many instances this is understandable as biodiversity conservation is a dominant climate change topic. However, it is not sensible or reasonable to comment on the positivity of such findings, despite such claims being very easy to make. What is clear, is that climate change is having a clear and visible impact on the ecology of aquatic Arctic ecosystems, meaning that the region is far from 'pristine', unaffected by human activity despite their remoteness.

Of more concern, in my opinion, is the timing of these changes. Assuming accurate dating, major shifts began in the 19th century when industrial activity was limited. Add to this the slow build up sediment (40cm took just under 4000 years!) means that not only were these botanical and zoological communities responding to warming far lower than current levels, but that these changes are not responses to the most recent warming patterns seen in recent decades. Therefore, it is likely that the true impact of climate change on aquatic ecosystems in the Arctic is yet to be seen.

Next time we're back on dry land to consider climate change impacts on terrestrial ecosystems...