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Threshold Values for Trophic Recovery - Analysis of System Components and Causalities

Threshold Values for Trophic Recovery - Analysis of System Components and Causalities. Dr. Ingrid Chorus and Dr. Inke Schauser, German Environmental Agency. Lake Tegel 1980, near shore. P. agardhii , typical microscopical image of Schlachtensee in summer. 1 / 40 P load.

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Threshold Values for Trophic Recovery - Analysis of System Components and Causalities

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  1. Threshold Values for Trophic Recovery - Analysis of System Components and Causalities Dr. Ingrid Chorus and Dr. Inke Schauser, German Environmental Agency

  2. Lake Tegel 1980, near shore

  3. P. agardhii, typical microscopical image of Schlachtensee in summer

  4. 1 / 40 P load Berlin (West): Lake Tegel and Schlachtensee Lake Tegel (Main basin) Area 3.060.379 m2 Volume 23.148.783 m3 Max. depth 16 m Water retention time ~ 75 d OWA since 1985 P-concentration inflow 400-800  20 µg/L Schlachtensee Area 420100 m2 Volume 1971000 m3 Max. depth ~ 9 m Water retention time ~ 210 d OWA since 1981 P-concentration inflow 400-800  8 µg/L 1 / 100 P load

  5. Importance and Examples of Threshold Values Understanding reaction thresholds is important for determining (i) management targets and (ii) system sensitivity • Threshold determining phytoplankton biomass response to TP reduction • Threshold water transparencies determining phytoplankton species composition • Threshold temperatures and/or nitrate concentrations determining internal phosphorus loading

  6. Schlachtensee: TP and Phytoplankton Biomass (Chl.a) restoration ~ 50 µg/L TP • Exponential decline of TP, • 4 years delay for reaction of Chl.-a, only beneath TP threshold

  7. Lake Tegel: TP and Phytoplankton Biomass (Chl.a) P-Chl a Tegeler See restoration ~ 50 µg/L P • Exponential decline of TP, • 9 years delay for reaction of Chl.-a, only beneath TP threshold

  8. TP-treshold for Chl.-a response

  9. Total Phosphorus Thresholds for Phytoplankton Biomass W H Y ???

  10. 1982 1984 1986 1988 1990 1992 1994 1996 1998 0 1,0 2,0 3,0 depth in m 4,0 5,0 6,0 7,0 Schlachtensee 8,0 Schlachtensee: Transparency • increase 1985, at <10-20 µg/L Chl.a

  11. J F M A M J J A S O N D 40 35 1982 at TP ~ 300 µg/L Chl.-a 60-100 µg/L Zs = 0.3 – 1 m 30 25 cm³/m³ 20 15 10 5 0 Transparency thresholds switch phytoplankton species composition

  12. J F M A M J J A S O N D ? ? 40 35 Transparency threshold Transparency threshold 30 for species response for species response 25 cm³/m³ 20 15 10 J F M A M J J A S O N D 5 20 0 15 1989 cm³/m³ 10 5 0 J F M A M J J A S O N D e.g. 1989, 1998 at 20 TP ~ 20 µ µ g/L 15 Chl .a 10 - - 20 µ g/L and 1998 cm³/m³ 10 Z Z = 1.5 = 1.5 – – 4.0 m 4.0 m s s 5 0 Transparency thresholds switch phytoplankton species composition 1982 at TP ~ 300 µg/L Chl.-a 60-100 µg/L Zs = 0.3 – 1 m

  13. J F M A M J J A S O N D 40 35 30 25 cm³/m³ 20 15 10 J F M A M J J A S O N D 5 20 0 15 1989 cm³/m³ 10 5 0 J F M A M J J A S O N D e.g. 1989, 1998 at 20 TP ~ 20 µ µ g/L 15 Chl .a 10 - - 20 µ g/L and 1998 cm³/m³ 10 Z Z = 1.5 = 1.5 – – 4.0 m 4.0 m s s 5 0 Transparency thresholds switch phytoplankton species composition 1982 at TP ~ 300 µg/L Chl.-a 60-100 µg/L Zs = 0.3 – 1 m Species shift towards those with larger loss rates through sedimentation and or grazing by zooplankton enhances further phytoplankton biomass reduction  Positive feedback mechanism kicks in above transparency threshold

  14. 45 40 35 30 ³ 25 m / ³ 20 m c 15 No data No data 10 5 0 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 Schlachtensee: Cyanobacterial biomass • Reliably low at TP < 20 µg/L; Chl.a < 10 µg/L; Zs > 2 m

  15. Lake Tegel: Cyanobacterial biomass 45 40 35 30 ³ 25 m / ³ 20 m c 15 10 No data 5 0 1999 1987 1989 1991 1993 1995 1997 • Reliably lowat TP < 20 µg/L; Chl.a < 10 µg/L; Zs > 2 m • Why hasn‘t the situation been reliably stable?

  16. Questions arising from this long-term data set on trophic recovery: • What processes govern the phosphorus budgets and phytoplankton responses in these two lakes ? • What can we generalise from this experience for trophic recovery ?

  17. Targets of current research • Water mass balance (mixing calculation, numerical model) • Phosphorus mass balance (numerical model, optimization, sensitivity analysis) • Specific management models for Lake Tegel and Schlachtensee • Transferable P process model • Causalities of P release from sediment (time series, sediment investigations) • Threshold values as restoration targets (comparison of different lakes) • Management guidance for restoration measures

  18. Precipitation Evaporation (?) OWA Bankinfiltration Tegeler See Groundwater enrichment ? QOUT ? QIN Process water water works Tegel Havel 120 Validation (r²=0.63) Calibration (r²=0.79) 110 ] -3 100 90 80 Cl in Lake Tegel [g m 70 60 50 Measured values Best Model (r²=0.76) 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 40 Water Balance:Lake Tegel • QIN = f (QHAVEL, QOWA, QWW) • Water retention time: ~70 d • Fraction of Qin of total Q: ~40 %

  19. Precipitation Evaporation OWA Bankinfiltration Epilimnion Groundwater enrichment ? ? FBs FMIX Havel Hypolimnion Process water water works Tegel ? ? FBS FRL Sediment Measured Values 0.30 Model C (GS + RL, ‚Epilimnion' + ‚Hypolimnion'), r²=0.72) 0.25 0.20 TP in Lake Tegel [g m³] 0.15 0.10 0.05 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0.00 P Balance: Lake Tegel • P-sink (1993-1999), else P-source • FGS = f (QIN, QOWA,QWW) • FRL = (Temp, NO3)

  20. Lake Tegel [kg P yr-1] 10000 source 5000 0 -5000 sink Havel inflow (Fin) OWA discharge (Fowa) Process water of WW (Fcw) Precipitations (Frain) -10000 Runoff into Havel (Fout) Extraction by water works (Fww) Net sedimentation (Fns) -15000 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 P Sinks and Sources Lake Tegel (Annual Means 1990-2002)

  21. Niederschlag Verdunstung Regenüberlauf OWA Waldsee Schlachtensee Tiefenwasser-ableitung Rehwiese ? Grundwasser Andere ? Fns Krumme Lanke Sediment 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 P-BudgetSchlachtensee ? • P-Sink (1985-2002) • „final“ concentration: 0,02 g P m-³ • Other sources (leaves, dog crap, GW ?? • FNS = f (QREGEN, QOWA,QREHWIESE, Temp, Redox) 0.050 ] -3 0.045 0.040 0.035 TP in Schlachtensee [g m 0.030 0.025 0.020 Messwerte 0.015 Model B (Fns abh. v. Seekomponenten, r²=0.55) 0.010

  22. Temperature Oxygen + + + - - Effect of Temperature and Oxygen (Nitrate) on the P Release from the Sediment P-Release Mineralization Desorption P sorbed to Fe organic bound P

  23. 2.5 2 1.5 1 0.5 Temperatur 0 20 °C 15 10 5 0 0.25 0.2 0.15 0.1 0.05 0 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 Phosphorus, Nitrate and Temperature in 7,5 m in Schlachtensee (monthly means) Schlachtensee, 7,5 m NO3-N mg L-1 N TP mg P L-1 • Deterministic (?) Nitrate threshold for P release ? • Nitrate (desorption) & Temperature (mineralisation) are both important !

  24. 25 Temperature 20 15 °C 10 5 0 Phosphorus, Nitrate, and Temperature in 15 m Depth in Lake Tegel (monthly means) Lake Tegel (15 m) 10 NO3-N 8 6 mg L-1 N 4 2 0 TP 3 2.5 2 mg P L-1 1.5 1 0.5 0 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 • No deterministic nitrate threshold in Lake Tegel • Temperature (mineralisation) > nitrate (desorption)

  25. NH4Cl-P gelöstes und leicht sorbierbarer P BD-P redox sensitiv gebundener P NaOH-SRP P austauschbar gegen OH- NaOH-NRP P in Detritus und Mikroorganismen HCl-P säurelöslicher P RLP RLP Rest-P TP Konzentration TP 0 2 4 6 8 0 2 4 6 8 mg P g-1 TM mg P g-1 TM Total P depth profiles [mg g-1] and fractions [%] in sediment cores (Juni 1996) Tegeler See Schlachtensee 0% 20% 40% 60% 80% 100% 0% 20% 40% 60% 80% 100% 0 0 1 1 2 2 3 3 Sedimenttiefe [cm] 4 4 5 5 • Release potential : • Only in top few cm • Only a fraction of the redox sensititve P • In total a small fraction of TP 6 6 7 7 8 8 9 9 10 10

  26. 45 40 35 30 25 [mg P m-2 d-1] 20 15 10 5 0 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 P-release (RL) vs. Release Potential (RLP) P accumulation rate in the hypolimnion Lake Tegel RL (1995-2002) = 6,6 mg m-2 d-1 RLP (1996, 2002) = 2,2 – 3,6 g m-2  2-3 Jahre • Assumptions: • No further external load • Disruption of internal P cycle • P release rate unchanged • Release potential unchanged • Conclusion: • External load is still recharging sediment • Fast improvement after sufficient reduction of external load (< 5 yrs)

  27. Research targets for the next 2 years • Improve P models, e.g. to better depict release from sediments • Develop biological model components and interface them with the P models • Assess relative effects of external and internal measures • Continuous vs. threshold responses and levels of thresholds ? • Optimised management scenarios for the 2 Berlin lakes • Develop generalised guidance on managing restoration and predicti responses (include data from other lakes) • Provide sound long-term basis for assessing impacts of global change in future projects

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