SAME 8 Applications of Aquatic Microbial Ecology Section 8: water quality

Toxic cyanobacterial proliferations in response to re-oligotrophication: The paradox of large lake restoration ? S Jacquet, JF Humbert, C Leboulanger, JF Briand O Anneville, B Leberre, G Paolini, JP Bosse

Congress hall, Taormina, Italy, 29 October 2002

Location of interest

Geneva lake The largest natural O European lake km2;

Lmax = 72 km; S = 582 V = 89 Zmax = 310 m ; SCA = 7974 km2

km3

Lake Bourget The largest natural French lake Lmax = 18 km; S = 42 km2; V = 3.5 km3 Zmax = 145 m ; SCA = 560 km2

Nutrient pollution: total phosphorus Bourget

120

P. tot. (µg.l-1)

100

Geneva 80

60

40

20 0 1960

1965

1970

1975

1980

1985

Time (years)

1990 1

1995

2000

Water quality restoration process The case of Lake Bourget 180

1969 - 2002

-1

P-PO4 (µg.l )

150 120 90 60 30 0

Year 2001 PO4 ~ 18 µg.l-1 Ptot ~ 26 µg.l-1

Water quality restoration process The case of Lake Bourget 7

Chl a (µg.l-1.year-1)

Transp. (m)

1984 - 2001

6

5

4

Spring months

10 9 8 7 6 5 4

Year 2001 Transp mean ~ 6.2 m Transp min ~ 3.3 m Chloro ~ 7.7 mg.m-3

1988 - 2001

However, since 1996 Depth (m)

0

20

40

Fluoroprobe (µg / L chlorophylle a)

Fluoroprobe

Depth (m)

0

Cell counting (Number of cells / mL)

20

40

Planktothrix rubescens

Cell counts Summer 2000

Autumn 2000

Winter 2000 / 2001

Spring 2001

03 -A ug 31 -99 -A u 13 g-9 -S 9 e 29 p-9 -S 9 ep 14 -99 -O c 03 t-9 -N 9 o 16 v-9 -N 9 o 29 v-9 -N 9 o 07 v-9 -D 9 e 22 c-9 -D 9 e 05 c-9 -J 9 an 18 -0 -J 0 an 31 -00 -J an 15 -00 -F eb -0 0

Conc. (µg.l-1)

Bloom and toxin production

0m

50 m July 99

3

April 00 July 00 April 01

Eq. MCYS LR (µg.l-1)

6

WHO drinking water 5 guideline conc. of 1µg.l-1 10 m

4 15 m

20 m

2

1

July 01 April 02

Lab experiments OD (570nm)

Population growth

2

1

0 0

20

40

60

80

100

Eq MC-LR (µg.l-1)

Time (d)

extra intra

600 400 200 0 2

Toxin production

6

20

64

Time (d)

87

112

Why ? Cyanobacterial blooms result from competitive situations between phytoplanktonic species

Environmental factors favoring these situations : ! Nutrient pollution (54 % of eutrophic lakes in Europe) ! Stability of the water column (blooms occur principally in summer)

Why cyanobacteria are often the winner in competitive situations ? - Control of their buoyancy - Heterocysts - Storage capacity

nutrient/light uptake

- Accessory pigments (phycoerythrin…) - Multicellular organization (filament, colony) - Poor food quality - Synthesis of toxins

defense against predation

The particular case of P. rubescens (lab exp., in situ dynamics, literature) # low light, low temperature, low nutrient adapted # enhanced by P pulses # late summer – early autum species # requires stable water column # able to regulate its buoyancy # filamentous and toxic and hence little grazed # probably little affected by viral control #… Years 0 P +++

24 °C

P +++

P-

10

P+

20 30

P +++

7 °C

Eutrophic conditions

P +++ Mesotrophic conditions

84

Lower limit of P