Long-term changes in boreal lake and stream chemistry: Interaction of climate and other stressors.
Dillon, Peter *,1, Watmough, Shaun1, Molot, Lewis2, Eimers, M.1, Futter, Martyn1, 1 Trent University, Peterborough, Ontario, Canada2 York University, Toronto, Ontario, Canada
ABSTRACT- Many lakes and streams in the boreal ecozone have been adversely affected by multiple stressors, including acid deposition, nutrient enrichment, etc., and are now impacted by changing climate. We have evaluated the interaction between climate and these stressors based on measured long-term trends in elemental concentrations and budgets of 8 lakes and 20 sub-catchments over periods ranging from 16 to 28 years. Over the duration of these studies, sulphate deposition has decreased by 40-45%; however, N deposition has remained unchanged, and as a result total N deposition now significantly exceeds S deposition. Lake recovery from acid deposition, however, has been modest, as catchment-related processes have led to net output of sulphate from almost all of the catchments. Lake sulphate has decreased, but only about half as much as expected based on the deposition change, and lake alkalinity and pH have improved only marginally. Base cations have also declined in conjunction with the sulphate, while inorganic nitrogen has remained consistently low in both lakes and streams. Because acid deposition is still greater than the critical load at many of these sites, the possibility of recovery of base saturation and improvements in soil water chemistry are limited. In fact, soil pH and base saturation have continued to decline. It is noteworthy, however, that the chemical changes are synchronous; that is, the temporal pattern of change is identical in all lakes, as is the pattern in all streams. These patterns have a strong climate signal, notably a drought-related signal that is influenced strongly by the Southern Oscillation. During the study period, there have been other significant changes in chemical parameters. Dissolved organic carbon (DOC) changed in a non-linear, but again synchronous, fashion. The lake DOC changes were correlated with incident solar radiation and precipitation. Total phosphorus, the critical nutrient in these boreal systems, has also changed very substantially (figure below); the declines of up to 65% in the study lakes over 2 decades indicate that lake productivity has dropped substantially. This decline can be only partly attributed to decreasing runoff, and may also be related to changes in the iron and aluminum cycle in the soil.
Key words: climate, acidification, stressor, biogeochemistry
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