Lake restoration and ecological effects: results from 80 Danish lakes

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1 Lake restoration and ecological effects: results from 80 Danish lakes Martin Søndergaard, National Environmental Research Institute, Department of Freshwater Ecology, Aarhus University

2 Overall effects/conclusions Type Number of lakes Ecological effect Remarks/buts Pike stocking 50 Few effects, timing very important Fish removal 40 Clear effects, but repeated removal after 5-10 years often needed Al treatment 6 Very clear short term effects, but long term effects? Oxygenation 6 Some effect, but long term treatment needed, permanent effects? Sediment removal 1 Depends on loading

3 Size and depth of Danish lakes

4 Landuse in Denmark Area: km² Max. altitude: 171 m (above sea level) Population: 5.4 mio.

5 Lake water quality of Danish lakes Some improvements during the past years, but most lakes still eutrophic and with unsatisfactory water quality

6 Lake restorations: effects Et af billederne fra Furesøopfiskningen Fish trawling

7 J. Appl. Ecology 2007 Ecosystems 2008 Hydrobiologia 2009

8 Two basic types of restoration methods relative to eutrophication (used in Denmark) Measures, which decrease phosphorus availability (increased bottom up control) sediment removal hypolimnetic oxygenation alum treatment Measures, which increase the zooplankton grazing on phytoplankton (increased topdown control) pike stocking removal zooplankti-benthivorous fish

9 Lake restorations in Denmark Total number = 80

10 Lake restorations in Denmark the past 30 years

11 Alum addition Lake Year Lyngby Sø 1974 Sønderby Sø 2001 Kollelev Mose 2003 Frederiksborg Slotssø Nordborg Sø Principle: Al is added to increase the sorption of phosphorus in the sediment and decrease the internal loading of phosphorus Fra Hedeselskabet

12 TP and PO4-P in Lake Sønderby before and after Al addition in Oct Fra: Jonas Hansen (Fyns amt)

13 0 3,2 2,4 Total-fosfor, mg/l TP Surface water of Lake Sønderby during summer ,6 0,8 0,0 3,2 2,4 1,6 0,8 Total-kvælstof, mg/l TN 4,5 3,6 2,7 1,8 0, Sigtdybde, m Klorofyl-a, µg/l Secchi depth Chl a 0,0 4 Alkalinitet, mekv/l Alkalinity FØR EFTER 0 Total-fosfor, mg/l FØR middel median og min/max-værdier EFTER Fra: Jonas Hansen (Fyns amt)

14 Alum treatment: results Immediate effects on lake water quality. No negative effects on fish and benthic fauna. Not to be used in low-alkaline lakes (Al toxic at low ph). Long term effects? Tendency of return to turbid conditions, but this may depend on external nutrient loading. Aging of Al?

15 Hypolimnetic oxygenation 2,4 0,7 0,5 10,3 6,2 50 0,6 2,2 5,8 16,5 17 3,6 4,5 5,3 4,3 5,6 5,8 5,6 6,6 6,6 6,3 7,5 6,7 6,6 7,6 8,6 9 1,3 0,8 4,5 5,1 25,4 13,7 21,3 21,8 2,8 21,5 20,1 18,7 16,5 16,5 15,8 15,4 11,8 11,3 6,8 19,1 18,7 19,7 19,4 10,6 9,9 15,5 19,5 19,5 18,5 18,7 18,5 18,5 18,7 18,8 25,8 25, , ,8 30,2 1,9 1,7 0,5 Lake Hald, oxygenated since 1985

16 Oxygenation of hypolimnion Principle: Oxygen is added to increase the redox sensitive sorption of phosphorus to iron and decrease the internal loading

17 Oxygen in Lake Furesøen before and after oxygenation 0 Furesø 2000 Furesø 2005 Iltindhold 0 (mg/l) Iltindhold (mg/l) Depth Dybde (m) Depth Dybde (m) J F M A M J J A S O N J F M A M J J A S O N Måned Month Måned Month

18 TP in Lake Furesøen before and after oxygenation 0 Furesø (dybeste sted) 2001 Furesø (dybeste sted) 2005 Totalfosfor 0 (µg/l) Totalfosfor (µg/l) Dybde (m) Dybde (m) J F M A M J J A S O N D Måned J F M A M J J A S O N D Måned

19 Seasonal oxygen concentration in four lakes before and after oxygenation

20 Seasonal hypolimnion temperature in four lakes before and after oxygenation

21 Seasonal hypolimnion TP in four lakes before and after oxygenation

22 Seasonal hypolimnion TP in two lakes when oxygenation was stopped for one year

23 Oxygenation: results Clear effects on reduced TP and TN (mainly reduced NH4) in hypolimnion, but more uncertain about epilimnetic effects. Risk of destratification (particularly in less deep lakes. Long term treatment necessary. Risk of increased mineralization and increased mobile phosphorus pool.

24 Pike stocking

25 Overall results from 34 lakes, does however not show promissing results (report from DTU-DFU) Only in very few lakes results can be seen. As a consequence it is now longer recommended as a separate restoration method in Denmark Yes Maybe Unknown Likely not No Effects of pike stocking on cyprinids

26 Removing the fish

27 Fish removal: methods

28 Fish removal: years and biomass Opfisket (kg ha -1 ) År Typically 10-80% of the fish stock are removed within 2-4 years.

29 What has been achieved?

30 Analyses of Danish examples of fish removal Data from the 40 lakes where fish were removed. Before values are mean of 1-3 years before the restoration. Variable Area (ha) Mean depth (m) Max-depth (m) TP-before (mg l -1 ) TN before (mg l -1 ) Chl a before (mg l -1 ) Mean Min Median Max

31 Reduced loading and concentration of TP in Danish lakes 1,4 1,2 Totalfosfor (mg P l -1 ) 1,0 0,8 0,6 0,4 0, Makes it more complicated to extract the effects of fish removal contra the effect of reduced loading and internal P loading

32 Presentation of data 90 % Relative units (relative to before restoration) Significant difference marked with blue color log scale 75 % Median 25 % 10 %

33 Secchi depth, Suspended solids and chlorophyll a

34 TP and TN

35 Seasonality of nutrients (absolut units) Before After

36 Seasonality of nutrients (absolut units)

37 Seasonality of TP when going from clear to turbid conditions (absolut units) Turbid? Clear Turbid

38 Phytoplankton biovolume

39 Biovolume of main phytoplankton classes

40 Seasonality of phytoplankton biovolume Absolute units

41 Zooplankton biomass

42 Fish (bream CPUE; weight and number)

43 Fish (roach CPUE; weight and number)

44 Biomanipulation: results If sufficent number of fish removed large effects on most trophic levels. Clear effects in most lakes for 6-10 years. Thereafter less clear effects (but only few long term data available). Repeated fish removal needed in most lakes to maintain effects (at least in nutrient rich lakes in this study).

45 Reasons for failures (fish removal) Insufficient loading reduction Rapid return of zooplanktivorous fish, particularly roach, insufficient removal Invertebrate predators Neomysis/Leptodora) High resuspension of loose sediment Internal P loading from a sediment pool Instability due to low macrophyte coverage

46 Prerequisites for a successful restoration External loading reduced sufficiently, lake TP reduced to below ug P/l in shallow lakes. Establishment of a high coverage of submerged macrophytes in shallow lakes. Internal P loading not controling lake water TP Long term stability? Sufficient action in restoration, i.e. high number of fish removal.

47 Main conclusions Low external nutrient loading essential for achieving and maintaining a good lake water quality. Internal measures (lake restoration) may be a method to speed up the recovery after reduced loading, but repeated manipulations often needed. Future challenges: agriculture, climate changes, changed legislation, improved restoration techniques.