Diversity of lakes and ponds in the forest-tundra ecozone: from limnicity to limnodiversity

Arctic and subarctic landscapes have unique hydrological and limnological features and are now experiencing rapid change due to climate warming and permafrost thaw. The highly abundant lakes, ponds, and rivers across these landscapes play an increasingly important role in global biogeochemical cycles and are sentinels of environmental changes. However, studying these remote waters poses challenges for both in situ sampling and remote-sensing analysis. Here we developed a synergistic remote-sensing strategy that combined PlanetScope and Sentinel-2 satellite data to estimate limnicity (water fraction per land surface), limnodensity (density of water bodies), and limnodiversity (optical diversity of water bodies) along a boreal forest-tundra transect, from the non-permafrost to the continuous permafrost zones of western Nunavik (Subarctic Canada). Our analyses show that this region hosts 335,281 water bodies, around 90% in the 0.0001 to 0.01 km2 size range. In bedrock outcrops, large water bodies were mostly associated with glacially carved depressions (higher limnicity). In contrast, small water bodies were predominately found in sedimentary infills along valleys (higher limnodensity). The discontinuous permafrost zone had the highest limnodensity and limnodiversity. This was likely due to permafrost thaw (thermokarst), particularly the collapse, subsidence, and erosion of palsas (organic permafrost mounds), resulting in ponds with black- and brown-colored waters, and lithalsas (mineral permafrost mounds), resulting in ponds with brown, light-brown, and sometimes white-colored waters. Some of these limnodense and limnodiverse landscapes, although covering only 2 to 7% of the total area of the study region, contained over one-third (34%) of the total number of water bodies, 97% of which were <0.01 km2; they accounted for a small proportion of the total black-colored water bodies (23%), but a high proportion of the total brown- (60%) and light brown-colored water bodies (92%) throughout the region. This research underscores the utility of optical satellite remote sensing for assessing water body types and for evaluating their individual and distinct aquatic responses to climate change. The dataset may be used to improve the modeling of carbon fluxes by better categorizing small water bodies affected by organic or mineral soil type settings. This is an important factor dictating biogeochemical responses, with effects on albedo, climate feedbacks, and ecosystem dynamics in the boreal forest-tundra region. The framework developed here may be applied to landscapes elsewhere in the world that have high densities of water bodies of variable size and optical properties.