Divergent Thermal Habitat Responses to Climate Forcing in Shallow and Deep Lakes: The Role of Solar Radiation

Climate warming is expected to reduce thermal habitat for cold‐water fishes, yet the magnitude and direction of habitat change may depend on interactions among multiple climatic drivers. We analyzed two contrasting lakes in the same region—Lake Kasumigaura (a shallow, eutrophic lake) and Lake Inawashiro (a deep, oligotrophic lake)—to clarify whether increases in air temperature or solar radiation drive thermal habitat loss or expansion of pond smelt ( Hypomesus nipponensis ) over the last 30 years. One‐dimensional numerical simulations revealed that their thermal habitat changes proceeded in opposite directions in the two lakes. In Lake Kasumigaura, recent increases in air temperature warmed the water column to levels exceeding historical observations. Summer water temperatures frequently exceeded the critical thermal maximum of pond smelt (CT max = 29.1°C), resulting in a loss of optimal thermal habitat (15.0 < T opt < 29.1°C). In contrast, in Lake Inawashiro, exceedance of CT max was rare, and the duration and vertical extent of the optimal thermal habitat increased. This downward expansion of the thermal habitat was driven by increased surface solar radiation during the springtime. These results demonstrate that combined effects of warming and increased solar radiation do not uniformly reduce thermal habitat for cold‐water fishes; instead, biological responses can diverge depending on interactions among multiple climatic drivers and intrinsic lake properties. These contrasting patterns highlight the importance of lake morphology and water clarity in shaping the thermal habitat of cold‐water fish under continued climate change.