Understanding how thermokarst lakes on arctic river deltas will respond to rapid warming is critical for projecting how carbon storage and fluxes will change in those vulnerable environments. Yet, this understanding is currently limited partly due to the complexity of disentangling significant interannual variability from the longer-term surface water signatures on the landscape, using the short summertime window of optical spaceborne observations.
Here, we rigorously separate perennial lakes from ephemeral wetlands on 12 arctic deltas and report distinct size distributions and climate trends for the two waterbodies. Namely, we find a lognormal distribution for lakes and a power-law distribution for wetlands, consistent with a simple proportionate growth model and inundated topography, respectively.
Furthermore, while no trend with temperature is found for wetlands, a statistically significant decreasing trend of mean lake size with warmer temperatures is found, attributed to colder deltas having deeper and thicker permafrost preserving larger lakes.
Plain Language Summary
Arctic river deltas are landscapes facing significant risk from climate change, in part due to their unique permafrost features. In particular, thermokarst lakes in ice-rich permafrost are expected to both expand and drain under warming-induced permafrost thaw, reconfiguring deltaic hydrology and impacting the arctic carbon cycle.
A limitation in understanding how thermokarst lake cover might be changing is the significant interannual variability in water cover in flat regions such as deltas, which makes it difficult to distinguish between perennially inundated, thermally relevant waterbodies, and ephemerally inundated waterbodies. Here, we present a pan-Arctic study of 12 arctic deltas wherein we classify observed waterbodies into perennial lakes and ephemeral wetlands capitalizing on the historical record of remote sensing data. We provide evidence that thermokarst lake sizes are universally lognormally distributed and that historical temperature trends are encoded in lake sizes, while wetland sizes are power law distributed and have no temperature trend.
These findings pave the way for quantitative insight into lake cover changes on arctic deltas and associated carbon and hydrologic cycle impacts under future climate change.
Coastal river deltas are landscapes at significant risk from sea level rise and sediment deprivation (Nienhuis et al., 2020; Syvitski et al., 2009). Arctic deltas are likely more vulnerable than their temperate counterparts due to the presence of thermokarst lakes in permafrost, which are sensitive to rapid Arctic warming (Emmerton et al., 2007; Piliouras & Rowland, 2020; Walker, 1998). Pan-arctic thermokarst lake coverage is responding to warmer temperatures in complex ways, as temperature-driven ground ice loss drives lake growth through retrogressive thaw slumping along lake shorelines (Grosse et al., 2013) but also generates surface and subsurface hydrologic connectivity that can cause lake drainage (Grosse et al., 2013; Jones et al., 2020; Rowland et al., 2011; Yoshikawa & Hinzman, 2003).
Observed changes in lake area over the last 50 years have shown both positive and negative trends depending on local hydrology, climate, permafrost zonation, ice content, landscape age, and geomorphic setting (Arp et al., 2011; Chen et al., 2012; Jones et al., 2011; Nitze et al., 2018; Plug et al., 2008; Smith et al., 2005). Irrespective of whether lake coverage is expanding or decreasing, the reorganization of thermokarst lake cover will have significant implications for polar atmospheric carbon fluxes (Engram et al., 2020; Grosse et al., 2013; Petrescu et al., 2010; Rowland et al., 2010; van Huissteden et al., 2011; Walter Anthony et al., 2018).
Moreover, thermokarst lakes in deltas modulate the transport of riverine freshwater, sediment, and nutrient fluxes to the Arctic ocean, by trapping and holding sediment (Marsh et al., 1999; Piliouras & Rowland, 2020) and modifying the residence times and pathways of nutrient transport through the delta (Cunada et al., 2021; Emmerton et al., 2007; Lesack & Marsh, 2010; Squires et al., 2009; Tank et al., 2009). Therefore, changing deltaic lake coverage and its spatial distribution will also alter the timing and magnitudes of riverine fluxes to the Arctic Ocean, which has broader implications for near-shore circulation and ecosystem productivity (Lique et al., 2016).