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  1. Outline • Loading –concentrations • littoral-open water interactions • cyanobacteria and nutrients , N:P • nitrogen-macrophytes • temperature cyanobacteria • diffuse loading problem • focus on Lake Taihu…

  2. Plake= Pin/(1+tw), tw = y-1

  3. After 10 years

  4. Chen et al, 2003

  5. P-lake/P-in*100 P-lake=P-in/(1+retention time**0.5) Lake Taihu retetention estimated: 39% - ”measured” 35% (Qin, 2007) Hydraulic retention time (years

  6. Bioavailable P Bioavailable P typically > 500-2000 in DK lakes Zhu et al, 2006

  7. Lake Arresø Area 40 km2 % of country area 0.09 Mean depth 3.1 m Max depth 5.9m Tw 3.5 y Lake Taihu Area 2338 km2 % of country area 0.02!!! Mean depth 1.9 m Max depth 4.5m tw 0.8 y

  8. Hydraulic retention time: 3.5 years Simple dilution Three retention times (10.5 years) reduced by 95% Jeppesen et al ,2007

  9. Jeppesen et al ,2007

  10. Resuspenion does not prevent improvements-even in relatively large shallow lakes Fraction of suspended solids made up by detritus and inorganic SS

  11. Seasonal changes in P concentration Seasonal phosphorus concentrations (relative to winter values) in 265 shallow Danish lakes with different P levels.

  12. Seasonal P concentration and retention in 14 shallow Danish lakes with reduced P

  13. Phosphorus release mechanisms/factors • Physical: temperature • Chemical • Biological • P release increases at rising temperatures due to increased mineralization and oxygen/nitrate consumption, thus diminishing the oxidised surface layer. Cumulative P release from four lakes at 7, 14 and 21 oC. Thickness of oxidised surface layer in four lakes at 7, 14 and 21 oC Based on Jensen et al. (1992)

  14. Temporal variation in P

  15. Model dPV/dt = (Q/V)*[Pin]-SED+REL water dPS/dt = SED-REL-IMM sediment SED = bS*PV/Z REL = bF*(tF^(TEMPV-20))*PS IMM = bI*PS parameters from the Danish model: bS=0.0695, bF=0.000468, tF=0.0867, bI=0.0000923. - 16 lakes-many years PV og PS [g/m^2], rates [g/m^2/d]. Time step 1 month

  16. Examples of the seasonal P-model Borup Useful for Lake Taihu? Maybe if including the hydrodynamics

  17. Lake Engelsholm Before fish removal: 1989-1993 After fish removal : 1994-1999 Søndergaard et al,2003

  18. P dynamics Simple models useful? Complex models taking hydrodynamics into account better? Sediment –water interactions – how to do realistic experiments Spatial and temporal variability Role of N for P release, role of macrophytes (bays), role of fish

  19. What happens when N is passing through a lake

  20. lake intercept inlet mean depth retention time Lake Taihu: 1.9 m mean depth, 5month retention time prediction 56% loss - ”measured” 49%

  21. Loading N-lake related to: -Depth -TW -N-in -Temp Retention Retention (%)

  22. N-dynamics (chemical perspective) Assimilation PON NO3- NO2- Excretion N2 NH4+ Fixation Sinking DON N2O Water column Sediment NO3- NO2- NH4+ N2 DON PON DNRA NO3- N2O DENITRIFICATION Transformations within the nitrogen cycle in shallow aquatic systems (Slide modified from version of Soonmo An) UTMSI

  23. Mark in Action at Lake Taihu China

  24. McCarthy et al,2007

  25. McCarthy et al,2007

  26. N dynamics Simple models useful? Complex models taking hydrodynamics into account better? Sediment –water interactions – how to do realistic experiments Spatial and temporal variability Role of macrophytes (bays), role of fish

  27. Physical: resuspension by wind/benthivorous fish • Resuspension increases turbidity and sediment-water interactions, particularly in very shallow lakes. • Submerged macrophytes and floating leaved plants may put a damper on the impact of resuspension. (Huang et al,2007) Lake water changes during 10 days at changing wind speed (from 0-2 to 5-7 m/s to 2-3 m/s) in Western Stadil Fjord, Denmark (450 ha, mean depth: 0.8 m).

  28. Pelagic- littoral zone interaction , Bay-main lake interactions Huang et al, 2007 - resuspension 50% of open water in Trapa beds. Wang ,2007 - Higher concentration of nitrate in the reed zone than 200 m away – indicating that trapped nutrients are released from in here and trabsported to the open water. Littoral-open water interactions (trap-release) an important research area . Redsistribution of sediment as well. Local removal of sediment a relastic symptom treatment?

  29. Diatoms Silicate Cyanobacteria green algae Lake Arresø Jeppesen et al ,2007

  30. Chen et al,2003

  31. Water clarity increases not least after zooplankton biomass and body size increases

  32. Shallow Danish lakes N-fixing Cyano then non-N fixing Cyano then Green algae with increasing TP Jensen et al, 1994

  33. Green algae and not N fixing cyanoes at the lowest inorganic N N fixers at intermediate TN:TP not low TN:TP Jensen et al, 1994

  34. Søndergaard et al, 2005

  35. Deep Shallow

  36. Deep

  37. High N input-- Enhances or reduces importance of cyanobacteria? or is P affinity the important factor ? is N:P at all of importane in shallow lakes? Should we focus on nutrient release /turnover instead and on benthic pelagic coupling

  38. Changes in water temperature in Danish survey lakes 1989-2005 simpel lineær regression

  39. Model: log(cyanobacteria biomass) = log(TP), log (mean depth), log(wat. temp.) Jeppesen et al, in prep.

  40. Model: log(cyanobacteria biomass) = log(TP), log (mean depth), log(wat. temp.) Jeppesen et al, in prep.

  41. Climate warming will likely enhance the risk of cyanobacteria dominance and the duration of blooming during the year