Limnology and diatom ecology of shallow lakes in a rapidly thawing discontinuous permafrost peatland

Lakes in discontinuous permafrost peatlands are on the front lines of climate change, sensitive to even modest increases in air temperature. The aim of this study was to provide the first limnological characterization of shallow (∼1-2 m depth) lakes in the Scotty Creek basin (Northwest Territories, Canada), a field site of circumpolar significance due to the existence of long-term ecohydrological monitoring going back decades. We use this as a foundation from which to advance our process-based understanding of the potential drivers of lake ecosystem change. Our results showed that dissolved organic carbon (DOC) and lake color were not correlated, a pattern that appears to be an important driver of diatom (siliceous single-celled algae) assemblages in these lakes. Diatoms in the study lakes tended to fall into one of two assemblage clusters. One cluster, comprised of small benthic Fragilariaceae and small Navicula species (sensu lato), was found associated with higher lake color. The second cluster, comprised of Encyonopsis and large Navicula species, was found associated with high DOC, lower color, and the presence of a benthic moss mat. From this, we suggest that DOC quality is a primary control on lake ecology in this region for its role in controlling light penetration to the lake bottom. We hypothesized that the prevalence of nearshore fens and collapse scar wetlands would be important drivers of DOC, but this was not supported in the 9 study lakes for which we had available data to map shoreline features.</p

Exploratory Hydrocarbon Drilling Impacts to Arctic Lake Ecosystems

<div><p>Recent attention regarding the impacts of oil and gas development and exploitation has focused on the unintentional release of hydrocarbons into the environment, whilst the potential negative effects of other possible avenues of environmental contamination are less well documented. In the hydrocarbon-rich and ecologically sensitive Mackenzie Delta region (NT, Canada), saline wastes associated with hydrocarbon exploration have typically been disposed of in drilling sumps (i.e., large pits excavated into the permafrost) that were believed to be a permanent containment solution. However, failure of permafrost as a waste containment medium may cause impacts to lakes in this sensitive environment. Here, we examine the effects of degrading drilling sumps on water quality by combining paleolimnological approaches with the analysis of an extensive present-day water chemistry dataset. This dataset includes lakes believed to have been impacted by saline drilling fluids leaching from drilling sumps, lakes with no visible disturbances, and lakes impacted by significant, naturally occurring permafrost thaw in the form of retrogressive thaw slumps. We show that lakes impacted by compromised drilling sumps have significantly elevated lakewater conductivity levels compared to control sites. Chloride levels are particularly elevated in sump-impacted lakes relative to all other lakes included in the survey. Paleolimnological analyses showed that invertebrate assemblages appear to have responded to the leaching of drilling wastes by a discernible increase in a taxon known to be tolerant of elevated conductivity coincident with the timing of sump construction. This suggests construction and abandonment techniques at, or soon after, sump establishment may result in impacts to downstream aquatic ecosystems. With hydrocarbon development in the north predicted to expand in the coming decades, the use of sumps must be examined in light of the threat of accelerated permafrost thaw, and the potential for these industrial wastes to impact sensitive Arctic ecosystems.</p></div

Map of study area, schematic of typical drilling sump and image of a degrading sump.

<p>A) The location of the 101 study lakes in the Mackenzie Delta uplands (Northwest Territories, Canada) (triangles – drilling-sump lakes; squares – thaw-slump lakes; circles – control lakes). Inset shows the region in the context of Canada. B) Image of a degrading drilling sump from the Mackenzie Delta uplands, near Parsons Lake, exhibiting significant surface and perimeter ponding. C) Generalized schematic of a drilling mud sump. A large pit is excavated into the permafrost and filled with the drilling wastes and fluids. These drilling fluids are then allowed to partially or completely freeze, and backfilled with the excavated material. The assumption is that the material will be permanently contained in the permafrost. Redrawn from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078875#pone.0078875-Dyke1" target="_blank">[8]</a>.</p

Spatial and Temporal Assessment of Mercury and Organic Matter in Thermokarst Affected Lakes of the Mackenzie Delta Uplands, NT, Canada

We examined dated sediment cores from 14 thermokarst affected lakes in the Mackenzie Delta uplands, NT, Arctic Canada, using a case-control analysis to determine how retrogressive thaw slump development from degrading permafrost affected the delivery of mercury (Hg) and organic carbon (OC) to lakes. We show that sediments from the lakes with retrogressive thaw slump development on their shorelines (slump-affected lakes) had higher sedimentation rates and lower total Hg (THg), methyl mercury (MeHg), and lower organic carbon concentrations compared to lakes where thaw slumps were absent (reference lakes). There was no difference in focus-corrected Hg flux to sediments between reference lakes and slump-affected lakes, indicating that the lower sediment Hg concentration in slump-affected lakes was due to dilution by rapid inorganic sedimentation in the slump-affected lakes. Sedimentation rates were inversely correlated with THg concentrations in sediments among the 14 lakes considered, and explained 68% of the variance in THg concentration in surface sediment, further supporting the dilution hypothesis. We observed higher S2 (algal-derived carbon) and particulate organic carbon (POC) concentrations in sediment profiles from reference lakes than in slump lakes, likely because of dilution by inorganic siliciclastic matter in cores from slump-affected lakes. We conclude that retrogressive thaw slump development increases inorganic sedimentation in lakes, and decreases concentrations of organic carbon and associated Hg and MeHg in sediments

Mean chloride concentration in the 101 lake dataset separated into three <i>a priori</i> defined groups.

<p>Vertical error bars representing the standard deviation are included. Letters A and B indicate drilling-sump impacted lakes have significantly higher chloride levels than either thaw-slump affected lakes or control lakes, which are not statistically different (Tukey HSD post-hoc test, following ANOVA run on normalized environmental data)</p

Stratigraphic profile of the most common cladoceran taxa from the three study lakes.

<p>Relative abundance diagrams from: lakes A) I20, impacted by drilling sump degradation; and control lakes B) C23; and C) C1A. Species assemblages (x axes) are scaled by relative abundance. Down-core sedimentary profiles (y axes) are scaled by date, based on ²¹⁰Pb radiometric dating techniques, with the depth in the sediment core included as a secondary axis. For all three lakes, two biostratigraphic zones were identified (constrained incremental sum of squares cluster analysis with the broken stick model) and are plotted with the background colour of one zone in grey the other white. The known timing of construction of the compromised drilling sump near Lake I20 (industry ID: Parsons F-09) is included as a horizontal line. The vertical red lines represent the pre- and post-sump construction mean of the relative abundance of the cladoceran <i>Alona circumfimbriata</i>.</p