Contrasting responses of dimictic and polymictic lakes to environmental change: a spatial and temporal study

Although comparative analyses between dimictic and polymictic lakes have noted the key role of mixing regime in governing water quality, limnologists have historically focused on dimictic lakes, leaving polymictic lakes relatively understudied. In this study, we investigated whether the effects of agricultural development on water quality differed between dimictic and polymictic lakes in a naturally nutrient-rich region of Alberta, Canada. Through a spatial limnological analysis of 36 sites, we found that the relationship between surface water total phosphorus concentration and the percent of agriculture (% Agr) in the catchments differed between dimictic and polymictic lakes, where the proportion of variance explained was much more pronounced in the dimictic (79% explained) than in the polymictic systems (7% explained). Paleolimnological analyses of subfossil chironomids in surface sediment samples (0–1 cm) from 18 of the 36 study lakes, and in sediment core profiles from the dimictic and polymictic basins of a eutrophic lake, showed that water quality differed between mixis groups. We found that the surface sediment chironomid assemblages differed significantly between polymictic and dimictic lakes. Detailed analyses of the sediment cores demonstrated that the two basin types differed in their responses to land-use change through time, as only the dimictic basin showed a parallel increase in anoxia with increasing agricultural development. We suggest that in naturally-fertile landscapes, external nutrient loading exerts a more notable effect on dimictic lakes, whereas internal nutrient loading is more important in polymictic systems.