Evolving Ecosystems

Despite the climatic fluctuations characterising past Arctic climate, recent warming over the last 150 years has been unprecedented. It has exceeded warming in other regions and even that experienced during the Pleistocene-Holocene transition (which concurred with large-scale vegetational shifts and faunal extinctions in the Arctic). However, the impacts of climate change on Arctic ecosystems has received relatively little attention compared with tropical, temperate and montane biomes (Post et al., 2009). In looking at the impacts of climate change in the Arctic, I will begin at the local and regional scale, investigating how climate change is affecting lake ecosystem structure and functioning.

Forcings, thresholds and regime shifts

Natural ecosystems respond to a number of external forcings and are sensitive to any changes. However, this response is not necessarily linear and instead alternative stable states may exist, which can be reached when changing conditions are sufficient to force the system past the threshold. What's also interesting is that reversal back to prior conditions may not return the ecosystem to its previous state, instead, the ecosystem must be pushed back beyond another switch point. In addition, gradual changes may not directly cause change but reduce the resilience of the system to deal with perturbation and shifts can then be dramatic and unexpected (Scheffer et al., 2001).

This leads to the question, how have arctic lake ecosystems responded to climatic changes? - Has warming reduced system resilience and have any irreversible shifts occurred?

Ecosystem state at five different conditions showing different degrees of resilience to perturbation (Scheffer et al., 2001)

'Lakes as sentinels of climate change'

The remoteness of Arctic lakes, as well as the sensitivity of the region to warming, makes their ecological components particularly useful indicators of climatic warming. Both directly and indirectly, lake ecosystems respond to climate warming and integrate terrestrial catchment responses, thus acting as 'sentinels' of change (Adrian et al., 2009).

Declines in ice cover have increased light penetration and lengthened the overturn period in spring, increasing turbulence and nutrient cycling and the availability of pelagic habitats (van Donk and Kilham, 1990). As a result, diatom community changes in response to warming have been recorded across Russia, Greenland and Finland, with a distinct shift from benthic taxa (e.g. Achnanthes and Fragilaria) and tychoplanktonic species (e.g. Aulacoseira) towards planktonic genera (e.g. Asterionella and Cyclotella) (Rühland et al., 2008). In addition, small-celled diatoms (e.g. Cyclotella) with high surface area to volume ratios are increasingly favoured as warming strengthens thermal stratification leading to declines in turbulence and upward nutrient transportation during the summer months (Winder et al., 2009).

Impact of climatic warming on stratification, nutrient flux and diatom cell size (Winder and Sommer, 2012)

Hobbs et al. (2010) investigated diatom compositional changes across 52 lakes in Greenland and North America from the Little Ice Age (~1550-1800) to the present day. Diatom β-diversity was implemented to capture the community turnover and the results indicated high rates in the 20^th century that has not previously been seen. This indicates that while natural climatic changes have previously not induced regime shifts, recent environmental factors (climatic warming and nitrogen deposition) have driven dramatic alterations in community dynamics. Lower β-diversity values in regions that have experienced lower rates of warming suggest climatic warming is the primary driver. It is evident that thresholds have been exceeded with regime shifts toward new ecological states. 

Cascading effects

Intensifying thermal stratification and subsiding turbulence will continue to disfavour diatoms, as their cellular requirements and silica frustules mean that their cell size is restricted to 2-4μm. Thus, phytoplankton species adapted to such conditions will become increasingly favoured (Winder et al., 2009). As diatoms contribute 20-25% of primary production global, such a significant decline in abundance could have dramatic ecological implications (Rühland et al., 2015).