THE ECOLOGICAL MONITORING AND ASSESSMENT NETWORK
SECOND NATIONAL SCIENCE MEETING
January 17-20, 1996
Halifax, Nova Scotia
SUMMARY OF PLENARY PRESENTATIONS
Declining Productivity of Lakes and Streams Due to Climate Change and Acidification
D.R.S. Lean
National Water Research Institute, Burlington, ON.
A new problem has emerged in some lakes which was totally unexpected. In some remote lakes, far from any direct human influence, lakes which had relatively low levels of phosphorus, now have even less. Consistent data for 15 years or more is necessary to reliably show the roughly 2 percent per year change. Now, phosphorus levels are about 30 percent lower than the values in the 1970s in lakes in the Dorset area and in some of the lakes contained in the Experimental Lakes Area (ELA) northwest of Kenora, Ontario.
This has overall effect of reducing energy flow through the food chain and yield of sportfish.
Tentatively, it seems that the problem stems from the effects of the recent warmer, drier climate conditions and acid deposition. Phosphorus export from the drainage basins is less, partly because there is less rain and partly because the catchments have been acidified making more aluminum and iron available to complex with phosphorus. When this happens, phosphorus is retained in the soil and lake productivity declines.
In addition, less dissolved organic carbon (DOC) is now being exported from the drainage basins. DOC is the principal attenuator of UV-B radiation and the reduced DOC in some lakes will cause a greater impact from UV-B radiation than increased UV-B resulting from stratospheric ozone depletion. With less rain, water is retained in lakes for longer periods and increased phosphorus sedimentation and increased photodegradation of dissolved organic carbon occurs. Together, these factors will also make the lakes clearer and less productive.
In contrast to the above data sets, similar work at Turkey Lakes, north of Sault St. Marie, do not show any decline in either total phosphorus or DOC. Nevertheless, these results are also consistent with the above hypothesis. Perhaps because the Turkey Lakes are near Lakes Huron and Superior, no decline in precipitation has occurred. Values remain near 1250 mm per year compared with a decline from 800 to 500 mm at ELA and 1200 to 900 mm at Dorset.
These observations illustrate the vital need for reliable long-term data sets for streams and lakes in drainage basins with different soil types if we are to be able to predict the impact of changing land use with nutrient biogeochemistry and changes in precipitation, acidification and UV-B radiation.
