A simple model for predicting the date of spring stratification in temperate and subtropical lakes

The date of onset of spring stratification in northern hemisphere lakes is predictable from mean annual air temperature. The prediction isfurther improved by also considering lake surface area (surrogate of wind exposure) and by the ratio (R) of lake surface area to mean depth, with R reflecting aspects of lake morphometry and heat storage capacity. Mean annual air temperature and R together explain 68% of the variance in the date of onset of stratification in a combined 70-lake data set from North America, Europe, and Asia. Nearly identical fractions were explained when the lakes were separated into dimictic and warm monomictic data sets. The remaining unexplained variation is probably largely attributable tointeryear variation in weather, but also to unexamined ifferences in regional wind strength, local protection from winds, lake shape, and lake turbidity, as well as differences inthe intervals at which temperature profiles were taken. The stratification flakes and its onset has preoccupied limnologists ince the late 19th century, which reflects not only an interest in being able to categorize lake types by their stratification regimes, but also stems from a recognition of the importance of temperature and season in determining the structure and productivity ofaquatic communities. The onset of spring stratification marks amajor change in the light climate for phytoplankton i lakes that are sufficiently deep to stratify. Stratification allows a spring bloom to develop in many lakes (Reynolds 1984; Marshall and Peters 1989). The date of stratification has, furthermore, a major bearing on the length of the stratification period and thus on the probability of an anoxic hypolimnion developing in eutrophic lakes. Niimberg (1988) recently developed simple empirical models that allow estimation of the date by which north temperate lakes can be expected to have completed their autumnal Acknowledgments This manuscript isbased on an undergraduate limnology course project by E.D. D. J. McQueen and K. L. Barry are gratefully acknowledged for comments. Contribution 322 of the McGill Limnology Research Centre. destratification. The goal of the present study is to produce a complementary model able to predict he spring date by which temperate and subtropical northern hemisphere lakes can be expected to stratify. Lakes in cold regions lose a portion of the annual heat income to melting of ice (Welch et al. 1987). Because this heat is not available to warm water, cold lakes stratify later than they otherwise would. For this reason, we examined the utility of mean annual air temperature in predicting the onset of stratification. When not available directly, the mean annual air temperature was estimated from published climatic maps (Wernstedt 1972). Temperature was used rather than latitude because the latter does not account for temperature differences associated with altitude and longitude (Kalff 1991). Large lakes can be expected to stratify later than small ones in the same region because of greater wind exposure. We therefore explored the importance of lake surface area in determining date of stratification. We also reasoned that the volume of water to be heated should affect he date of stratification; deep lakes should stratify later than shallow ones of the same surface area (Gorham 1964). Lastly, we examined the utility of the ratio of surface area to mean depth (R), which includes both a volumetric and a morphometric component. R proved to be superior to mean depth as a predictor of stratification. To examine whether there were major systematic differences between dimictic and warm monomictic lakes in the timing of their stratification, wealso separated the lakes according to whether they developed an ice-cover. Data on 70 north-temperate lakes were assembled from six sources (Janus and Vollenweider 1981; Davis et al 1978; Seyb and Randolph 1977; Milway 1970; ILEC/UNEP 1988, 1989). The data set includes 34 lakes from North America, 22 from Europe, and 14 from Asia, with the majority of the latter from Japan. With water temperature profiles determined at intervals, the date of stratification