Reviews and Syntheses: Effects of permafrost thaw on arctic aquatic ecosystems

Abstract

Abstract. The Arctic is a water-rich region, with freshwater systems covering 16 % of the northern permafrost landscape. The thawing of this permafrost creates new freshwater ecosystems, while at the same time modifying the existing lakes, streams, and rivers that are impacted by thaw. Here, we describe the current state of knowledge regarding how permafrost thaw affects lentic and lotic systems, exploring the effects of both thermokarst (thawing and collapse of ice-rich permafrost) and deepening of the active layer (the surface soil layer that thaws and refreezes each year). Within thermokarst, we further differentiate between the effects of thermokarst in lowland areas, vs. that on hillslopes. For almost all of the processes that we explore, the effects of thaw vary regionally, and between lake and stream systems. Much of this regional variation is caused by differences in ground ice content, topography, soil type, and permafrost coverage. Together, these modifying variables determine the degree to which permafrost thaw manifests as thermokarst, whether thermokarst leads to slumping or the formation of thermokarst lakes, and the manner in which constituent delivery to freshwater systems is altered by thaw. Differences in thaw-enabled constituent delivery can be considerable, with these modifying variables determining, for example, the balance between delivery of particulate vs. dissolved constituents, and inorganic vs. organic materials. Changes in the composition of thaw-impacted waters, coupled with changes in lake morphology, can strongly affect the physical and optical properties of thermokarst lakes. The ecology of thaw-impacted systems is also likely to change, with thaw-impacted lakes and streams having unique microbiological communities, and showing differences in respiration, primary production, and food web structure that are largely driven by differences in sediment, dissolved organic matter and nutrient delivery. The degree to which thaw enables the delivery of dissolved vs. particulate organic matter, coupled with the composition of that organic matter and the morphology and stratification characteristics of recipient systems will play an important role in determining the balance between the release of organic matter as greenhouse gases (CO2 and CH4), its burial in sediments, and its loss downstream. The magnitude of thaw impacts on northern aquatic ecosystems is increasing, as is the prevalence of thaw-impacted lakes and streams. There is therefore an urgent need to address the key gaps in understanding in order to predict the full effects of permafrost thaw on aquatic ecosystems throughout the Arctic, and their consequential feedbacks to climate.

Carbon dynamics in highly heterotrophic subarctic thaw ponds

“…Increasing attention has been given to thaw ponds and lakes after recognizing the cumulative effect of their high abundance and greenhouse gas (GHG) emissions on global warming (Kling et al, 1992;Walter et al, 2006;Laurion et al, 2010), especially when they emit old carbon thus having the potential to act as a positive feedback mechanism on climate (Walter Anthony et al, 2014). Thaw ponds and lakes are dominant in continuous and discontinuous permafrost areas, for example in permafrost regions of Siberia where they represent 90 % of all lakes (Walter et al, 2006), but until recently little has been known on the limnological properties and microbial communities of this important freshwater ecosystem (Vonk et al, 2015).…”

Modern to millennium-old greenhouse gases emitted from ponds and lakes of the Eastern Canadian Arctic (Bylot Island, Nunavut)

“…Overall, thermokarst (thaw) ponds and lakes represent a major landscape feature in permafrost-affected regions (Grosse et al, 2013), and there is a growing interest in defining the specific role of various types of freshwater ecosystems in global carbon dynamics associated to permafrost degradation, and how they may rapidly respond to environmental changes (see Vonk et al, 2015, and other articles in this special issue).…”

Oxygen depletion in subarctic peatland thaw lakes

“…Temperature plays a key role in affecting oxygen dynamics in lakes and is especially important in cold environments such as Arctic and subarctic permafrost waters (Vonk et al 2015). On a seasonal timescale, temperature influences the duration and thickness of ice cover, which limits the flux of oxygen from the atmosphere to the surface waters of the lake.…”