Ice Phenology and Thickness in Small Arctic Lakes: Field Observations and Mechanistic Controls

Abstract The Arctic is the fastest‐warming region globally. Lake ice is a sentinel indicator of Arctic change, with wide impacts on hydrological regimes, biodiversity, and ecosystem services. While small lakes are ubiquitous across northern boreal and tundra zones, ice observations remain biased toward large lakes with distinct freezing and melting dynamics. We present high‐resolution, field camera‐based ice phenology records of 10 small lakes (2–20 ha) in northwest Finnish Lapland spanning low (LE, ∼300 m) and high (HE, 770–1,010 m) elevations over two consecutive seasons. Ice‐on timing was uniform across elevations, occurring at cumulative degree‐day sums of −10°C·d on shallow LE lakes, and −30°C·d on deeper HE lakes. Thawing from groundwater upwelling was observed across many LE lakes. While the recorded melting process at LE occurred over several days during late May, at HE it extended over weeks into late June to early July, due to long‐lasting continuous snow cover, cold meltwater inflow, and increased sublimation. Our thermodynamic lake ice model accurately predicted total ice thickness ( R = 0.99, RMSE ≤ 5.8 cm), reaching ∼80 cm at LE and ∼100 cm at HE, but the snow‐ice fraction was underpredicted. Freezing and melting were strongly modulated by snow, highlighting the impact of future precipitation changes on ice thickness, quality, and ice‐off timing. The rapid and spatially uniform freezing suggests a direct response of small lake phenology to Arctic warming—unlike large lakes, where the summer heat storage, depth, and turbulent mixing are important modulators.