Lakes And Global Change

1. Lakes And Global Change

2. Global climate change • Climate change is one of the most critical global challenges • Phenomenon known as global warming. • anthropogenic climate change. • Affecting agriculture • Sea-level rise • Accelerated erosion of coastal zones • Natural disasters • Species extinction

3. Climate changes on freshwater body • Climate change could bring to the dynamics of the lake's water system • rainfall and snowfall amounts. • Water temperature change. • Depth of mixing layer • Species composition …etc… • Effects of climate change can be unpredictable.

4. Climate changes on freshwater body • Freshwater reserves fluctuate widely due to natural variations in climate, and changes in climate caused by human activities could have dramatic and unpredictable effects. • Extreme case: Lake Tahoe Just in the last six or seven years alone, the surface has warmed by an average of 5 degrees • It is not known if a warmer Lake Tahoe would be bluer and clearer or greener and murkier.

5. Various effects of Climate changes on freshwater body • Lake Erie, (one of the Great Lakes in North America) the shallowest of the Great Lakes, is typically frozen by late December. • missing frozen mantle, water are evaporating. • Lower water level

6. Climate change have different effects on lakes from different ecosystems. • Climate change can have very different effects on lakes from different geological regions. • Lakes of different ecosystem form different region have various physical and chemical properties • Climate and environmental condition at a specific location can be very different • Climate change effects at a tropical lake and a boreal lake are compared.

7. Lake Tanganyika, Africa (Lake Tanganyika is a large, deep north-south trending valley lake. (~50km wide, ~650km long, ~570m deep, ~1470mMAX. depth) Second deepest lake. Important nutrition and revenue sources to the bordering countries. (provides 25~40% animal protein supply ) The world’s most productive pelagic fisheries. Oligotrophic, permanently thermally stratified, anoxic hypolimnion.

8. Bordering countries of Lake Tanganyika

9. Causes of decreased productivity • Primary productivity of Lake Tanganyika may have decreased by about 20%. • Fish yields decreased by 30% (suggested by carbon isotope records in the sediment cores) • Caused by global climate change (regional effects) on aquatic ecosystem functions and services OR local anthropogenic activity or overfishing? • Experiment provides evidence that global effects are the major cause.

10. Causes of decreased productivity • Local effects: • Increased population in border countries. • Overfishing. • large-scale deforestation and farming led to a dramatic increase in soil erosion rates. • Biologically sensitive: Large diversity since the lake is very old.

11. Mixing event in the lake • Cool and windy season: (May-Sept) strong southerly winds tilt the thermocline. • Causing up-welling of deeper nutrient-rich water at south end of lake. • Results in weaker thermocline. • Provide some dominant source of limiting nutrients (P, Si) to surface water. • Climate change causes decreasing in upwelling of nutrients.

12. Climate change • Local air temperature from the lake region show warming. (consistent with global patterns) • Historical records of lake region: increase 0.5-0.7 ºC average annual air temperatures. (global: 0.6±0.2 ºС)

13. Water temperature change • Climate warming also seen in water temperature. • Historical data: upper water column: significant warming trend increase 0.1±0.01ºС/decade since 1913. deep water column: increased from 23.10 to 23.41ºС since1938 to 2003. (homothermal between 400m to 1000m) • Other lakes (deep water temperature): Lake Victoria: increase 0.3 ºС from 1960s to 1991. Lake Malawi: increase 0.29 ºС since 1953. Lake Albert: warmed 0.5 ºС since 1963

14. Wind speed • Declined 30% since late 1970s. • Records show: North: Wind velocity (monthly average) in cool windy season remained constant until 1985, after which decreased significantly. south: also decreased significantly after 1977. • Winds in the south have higher velocity the in the north.

15. Climate changes • Combined effect of increasing temperature and decreasing wind speed is to increase the stability of the water column. • Mixing depth is reduced. • Decline in primary productivity.

16. Analysis of sediment cores: evidence of decreased primary productivity. • Atomic C/N ratios of bulk organic matter along sediment cores along eastern shoreline was analyzed. • Low C/N ratios in all four cores. • Carbon isotopes are indicators of phytoplankton productivity. • Carbon stable isotope records showed : trend more toward negative values. (less productive)

17. Phytoplankton carbon isotope composition • Evidence for rapid uptake of introduced nitrate from deep water after an upwelling event. • No significant uptake of deep water DIC. (Deepwater 13C isotope is more negative than that of surface DIC. Phytoplankton δ13C values do not change significantly after upwelling. ) • DIC not accompanied with upwelling of nitrate. • Deep-water DIC is limited. • Also: Records: decrease in carbon mass accumulation in sediments cores.

18. The timing of shift towards to more negative isotopes matches the climate changes in temperature records. • Declining isotope trend in early 1900s, right after initial warming.

19. fishery • Dominated by 2 clupeid species and 1 piscivore species. • High yields under similar fishing pressure in late 1970s • Closely associated with seasonal patterns of upwelling during cool windy season. • Recent unknown decline can be explained by decrease in primary productivity.

20. Historical and paleolimnological data provide evidence that climate change has contributed to the diminished productivity in Lake Tanganyika over the past 80 years.

21. Climatic warming effects on central boreal forest • Experimental Lakes Area (ELA) of northwestern Ontario. • Condition of the boreal ecosystems of northwestern Ontario: Warm and arid . Thin and sandy soil layers. Small water storage capacities. Forest fire common

22. Climate change prediction • Result of greenhouse effect in the next several decades: Increases in air temperatures—Summer increases up to 9ºС Decreases in soil moisture—greater than 50%

23. Data collected at ELA

24. DATA records collected at the ELA • Data collected throughout 20 years. • Continuous warming increase incidence of drought. Temperature increased about 2 degrees in the air. Increases in mean and MAX. water temperature. Increases heat content during ice-free season Increases duration of ice-free period. Increasing evapotranspiration

25. DATA records collected at the ELA • Data collected throughout 20 years. • Continuous warming increase incidence of drought. Temperature increased about 2 degrees in the air. Increases in mean and MAX. water temperature. Increases heat content during ice-free season Increases duration of ice-free period. Increasing evapotranspiration Decreased volume of runoff from terrestrial basins and Decreases water renewal rate for the lakes

26. DATA records collected at the ELA • Data collected throughout 20 years. Data from Lake 239 • Continuous warming increase incidence of drought. Temperature increased about 2 degrees in the air. (A) Increases in mean and MAX. water temperature. (B) Increases heat content during ice-free season Increases duration of ice-free period.(C) Increasing evapotranspiration Decreasing percipitation Decreased volume of runoff from terrestrial basins and rates of water renewal for the lakes

27. DATA records collected at the ELA • Data collected throughout 20 years. • Continuous warming increase incidence of drought. Temperature increased about 2 degrees in the air. (A) Increases in mean and MAX. water temperature. (B) Increases heat content during ice-free season Increases duration of ice-free period.(C) Increasing evapotranspiration Decreasing percipitation (D) Decreased volume of runoff watershed from terrestrial basins and rates of water renewal for the lakes (E)

28. Decreased water flow Concentrate chemical solutes in lakes: for both dissolved N (G) and more conservative ions (F); dramatic decline of P after 1985 N:P ratio doubles (from 25:1 to 50:1) P limitation (causes by both decreased water flow and forest fire) Note: A decreasing relative abundance on cyanobacteria might be expected .

29. Disappearance of forest Increase wind velocity (I) Decreased water flow and forest fire affects physical properties of lakes. denudation of large lake areas affects physical properties of lakes. Lakes becomes clearler Increase penetration of solar energy(H) Cause thermalcline in lakes to deepen (J)

30. Increase penetration of solar energy(H) deeper thermalcline (J) Increase in average phytoplankton biomass in the epilimnion and diversity(slightly) (K)

31. Additional information • Ice free season increased by about 20days • Mainly reflects early ice-out dates in spring. • Increases of air temperature more pronounced in Apr~May. • Below average snow covers and warm temperature in March causes snow to disappear from lake surface earlier in the later years (L). • Solar radiation absorbed by lake increases in spring.

32. Other effects of climate change • Filamentous green algae (order Zygnematales), which are similar to algae that present in early stages of lake acidification, is favored by warm temperature at littoral zone of lakes. • Increased water temperature observed would be sufficient to extirpate some temperature-intolerate species. • Small lakes where deepening of the thermocline would destroy cold, oxygen-rich hypolimnion. A number of cold stenothermic glacial relicts cannot survive there.