Monthly MLT

The monthly means of HSP MLT in winter were the lowest, reaching around −10 ◦C in January (Figure 5b). In turn, because of the shallow depth, the MLT of HSP and D1F were the highest in summer, reaching about 14 ◦C in July. Hence, lake depth was the main factor controlling the magnitude of the annual and summer monthly MLT in the three simulations. In contrast, salinity determined the winter MLT minima by preventing ice cover formation in saline lakes.

In NL, the long-term trends of monthly MLT were less than 0.6 ◦C/decade, and variations therein were much smaller than those of monthly mean LSWT (0.2–1.6 ◦C/decade) (Figure 5). In NL, the monthly MLT showed a strong positive trend from April to November and stayed almost stable during the ice period, while the monthly LSWT became higher throughout the whole year, especially in freezing and breakup months.

In shallow lakes (HSP and D1F), the trends of monthly MLT in summer coincided with monthly LSWT trends (0.4–0.8 ◦C/decade); the difference (<0.3 ◦C/decade) between MLT and LSWT in shallow lakes was only seen in winter and at times of ice-formation and melting. Still, their differences were much smaller than in NL.

Akin to the surface temperatures, the MLT of HSP in winter (February to April) increased at the highest rate among the three simulations, i.e., at about 0.6 ◦C/decade, compared to around 0.2 ◦C/decade in D1F and <0.1 ◦C/decade in NL.

### 3.2.3. Bottom Temperature and the Variation Trends Annual BLT

An annual BLT of 3.63 ◦C in HSP was the lowest among the three experiments, with 4.36 ◦C and 5.33 ◦C in NL and D1F, respectively (Figure 4). The simulated BLT of the two shallow lake experiments, i.e., in S-HSP and D1F, were warming by 0.52 and 0.44 ◦C/decade, while that of NL tended to become slightly cooler ( −0.03 ◦C/decade) without passing the significance test. This is consistent with results from other deep dimictic lakes [16,50,51].
