Methods
Lake Sampling Design
In the late summer of 2023, 68 lakes were sampled across the Northwest Territories, Canada, spanning 10 degrees of latitude and targeting lakes of various regions and landforms (figure 4). Lakes were sampled at the end of the summer when the permafrost thaw is at its greatest extent to capture the peak influence of permafrost thaw on Northern lakes.
Water samples were collected from the center of the lake via helicopter to minimize the impacts of bank effects. Several physico-chemical properties were measured in situ (see table 3). A sub-sample of water was filtered to 0.45um (to capture the dissolved fraction of matter) for dissolved organic matter composition analysis and dissolved trace metal concentration.
Samples for dissolved organic carbon, major cations, and trace metal concentrations were analyzed at Taiga Environmental Laboratory, Yellowknife, NT. Samples for dissolved organic matter composition were analyzed at the University of Alberta, Edmonton, AB.
Figure 4. Map of lakes (n=68) sampled for water biogeochemistry in the summer of 2023 in the Northwest Territories, Canada.
Table 1. Landscape variables used to describe sampled lakes.
To understand the impact of landscape factors on lake biogeochemistry, 5 categorical landscape properties were selected (table 1). Ecodistricts are subdivisions of an ecoregion that are characterized by vegetation, landforms, relief, geology, and fauna. Thermokarst Lake Type refers to the lake feature resulted from underlying permafrost conditions and thaw conditions (figure 5). Permafrost zone describes the extent of permafrost, ranging from no permafrost to continuous. Ground ice describes the volume of ice contained in permafrost, ranging from low to high. Dominant vegetation cover describes the dominant vegetation type along the perimeter of the lake.
Figure 5. Images of a variety of thermokarst lake types sampled across the Northwest Territories, Canada with their identifying code associated with this study in brackets.
Lake Biogeochemistry Measurements
Water biogeochemistry was evaluated with 58 response variables, falling into 3 subcategories: dissolved organic matter, physico-chemical properties, and trace metal concentrations.
Organic Matter Analysis
Table 2. Overview of dissolved organic matter (DOM) composition indices used to evaluate the organic matter pool and their associated characteristics.
Dissolved organic matter composition was evaluated using the HORIBA Aqualog spectrophotometer. Absorbance data and excitation-emission matrices (EEMs) were produced for each lake sample. A Parallel Factor (PARAFAC) Analysis was completed using the EEMs and five unique component were identified to describe dissolved organic matter composition (see C1-C5 in table 2).
Organic matter can be described based on its lability, or how easily a compound can be broken down by microbes. Lability is a continuum from more humic-like compounds to more protein-like compounds. Humic-like compounds are typically more complex and harder for microbes to break down. They are often of terrestrial origin and have high molecular weights and complex bonds (e.g., aromatic bonds). In contrast, protein-like compounds are much simpler and smaller and are easier for microbes to break down.
Physico-Chemical Properties
Physico-chemical descriptions (table 3) provide information on the nutrient content and bioavailability and the conditions of the water that impact biological communities.
The physico-chemical properties of a lake can influence the biological organisms, including microbes, that can survive. For example, some microbes thrive under more acidic conditions and may not survive at a pH greater than 7. Water colour and total dissolved solids can influence the amount of light able to penetrate the water column, which can impact the photosynthetic life the lake is able to support.
Table 3. Overview of physico-chemical water measurements recorded for sampled lakes.
Table 4. Description of trace metal concentration measurement for sampled lakes.
Trace Metal Concentrations
The presence and concentrations of trace metals in lake biogeochemistry (table 4) impact the organisms that are able to survive in a lake and can pose issues for drinking water safety.
Higher trace metal concentrations can be associated with deeper flow pathways due to thawing permafrost.
