The ecology of Planktothrix rubescens in Lake Bourget and Lake Geneva: in situ dynamics, laboratory experiments, and modeling Laura Oberhaus, C.Avois-Jacquet, J-F.Briand, J-F.Humbert, S.Jacquet, B.Leberre, C.Leboulanger, G.Paolini, B.Tassin, B.Vinçon-Leite, N.Zervos Thesis directors: Jean-François Humbert, Daniel Thévenot UMR CARRTEL – CEREVE – LSEE – CTLB (France)

Presentation outline 1. Planktothrix rubescens 2. Lake Bourget and Lake Geneva 3. Questions and hypotheses 4. Light quantity and quality experiments 5. Community structure, competition, predation 6. Data analysis and modeling

Planktothrix rubescens

(Photo M. Roux CSP)

Autumn 2001, Lake Bourget (south side)

Two large subalpine lakes

Lake Geneva Lake Bourget Volume = 89 km3 Maximum depth = 310 m

Volume = 3.5 km3 Maximum depth = 145 m

Occasional P. rubescens blooms

Regular P. rubescens blooms since 1998

Restoration: long-term evolution of total phosphorus Bourget

120

100

P. tot. (µg.l-1)

Geneva 80 60 40 20 0 1960

1965

1970

1975

1980 1985

Time (years)

1990

1995

2000

Questions and hypotheses Why are P. rubescens blooms observed in L. Bourget and not in L. Geneva?

Why the occurrence of these blooms in L. Bourget only recently, after many years of restoration?

Questions and hypotheses Improvement of water clarity in L.Bourget

P +++

24 °C

P +++

P +++

PP+

7 °C

Eutrophic conditions

P +++ Meso-trophic conditions

Questions and hypotheses Role of phytoplankton communities, competition, predation

Questions and hypotheses Differences in lake physical characteristics

Thermocline

Thermic gradient Stratification

Mixing depth

Light climate

Mixing

Methods

Field Survey

Hypotheses

Lab Data

1D Vertical Model

Questions and hypotheses Improvement of water clarity in L.Bourget

P +++

24 °C

P +++

P +++

PP+

7 °C

Eutrophic conditions

P +++ Meso-trophic conditions

Light quantity and quality Growth of Planktothrix rubescens in a range of intensities of: • white light • red light • green light (that found near the metalimnic layer) • blue light • blue-green light And in competition with Planktothrix agardhii

Light quantity and quality • Range of light intensities Light spectra

P. rubescens

P. agardhii

White (Imax =483) Blue (Imax =225) Blue-Green (Imax =150) Green (Imax =255) Red (Imax =280) White (Imax =420) Green (Imax =285)

µ max (day-1) 0,16 0,11 0,13 0,11 0,12 0,11 0,11

µmax = maximum growth rate (calculated) Imax = maximum light intensity reached in each experiment

Light quantity and quality • Competition with P. agardhii • Lumière VERTE Pr

0

15 32 Time (days)

100 80 60 40 20 0

% Biovolume

100 80 60 40 20 0

120 µEm-2s-1

50 µEm-2s-1

Pa

% Biovolume

% Biovolume

10 µEm-2s-1

0

15

Time (days)

32

100 80 60 40 20 0 0

15

Time (days)

32

Light quantity and quality • Competition with P. agardhii • Lumière VERTE Pr

0

100 80 60 40 20 0

% Biovolume

100 80 60 40 20 0

120 µEm-2s-1

50 µEm-2s-1

Pa

% Biovolume

% Biovolume

10 µEm-2s-1

0

15 32 Time (days)

15

100 80 60 40 20 0

32

0

Time (days)

15

32

Time (days)

• Lumière ROUGE 10µEm-2s-1

100

80 60 40 20 0

% Biovolume

% Biovolume

100

% Biovolume

120µEm-2s-1

50µEm-2s-1 100 80 60 40 20 0

1

15

Time (days)

29

80 60 40 20 0

1

15

Time (days)

29

1

15

Time (days)

29

Questions and hypotheses Role of phytoplankton communities, competition, predation

Community structure, competition, and predation • Exchange of L. Geneva and L. Bourget phytoplankton communities • Competition between P. rubescens and Mougeotia gracillima • Zooplankton predation using the competition precedent: is grazing more limited by filamentous structure or by toxins?

Community structure, competition, and predation • Exchange of L. Geneva and L. Bourget phytoplankton communities • Competition between P. rubescens and Mougeotia gracillima • Zooplankton predation using the competition precedent: is grazing more limited by filamentous structure or by toxins?

Community structure, competition, and predation • Exchange of L. Geneva and L. Bourget phytoplankton communities • Competition between P. rubescens and Mougeotia gracillima • Zooplankton predation using the competition precedent: is grazing more limited by filamentous structure or by toxins?

Questions and hypotheses Differences in lake physical characteristics

Thermocline

Thermic gradient Stratification

Mixing depth

Light climate

Mixing

Data analysis Field Survey • Data from Lake Bourget: – physico-chemical – light climate – phytoplankton

• Meteorological data

Data analysis Dynamics of P. rubescens in Lake Bourget What is the impact of climatic factors on the seasonal and annual dynamics of P. rubescens ? 0m

50 m Summer 1999

Summer 2000

Summer 2001

Summer 2002

Data analysis Modeling • Model-calculated indicators on lake physical characteristics: test hypotheses on physical differences in L. Bourget and L. Geneva – mixing depth – stratification intensity

• Validation, interpretation of lab results • Long-term aspects

Conclusions • Many hypotheses still to test, data to analyze and interpret

• Lots of work to do!