Table 1.

Results of the multiple regression models

Logistic GAMM	edf	Ref. df	χ2	Probability value	Significance
Random slope
s(urban area)	34.0	46.0	118.6	6.3 × 10−14	***
s(cultivated area)	1.9	2.3	3.5	0.24
s(pastured area)	3.3	4.1	5.2	0.27
Random smooth
s(logurban,lake)	65.4	204	200.2	<2 × 10−16	***

A logistic GAMM showed that the probability of hypoxia onset in lakes increased as the proportion of urban area increased over the past 300 years, but was unrelated to the changes in cultivated and pastured land area [adjusted R²_adj (R²_ad ) = 0.227]. By incorporating a term that accounted for differences in the trajectory of urban land use among lakes, our logistic GAMM was able to explain a much larger proportion of the variation in the timing of hypoxia onset (R²_adj = 0.396). ***p = 0.001. edf, effective degree of freedom; Ref., reference degree of freedom.