VARIATIONS OF PLANKTOTHRIX RUBESCENS SPATIAL

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VARIATIONS OF PLANKTOTHRIX RUBESCENS SPATIAL DISTRIBUTION IN LAC DU BOURGET (FRANCE) S. Jacquet1*, B. Le Berre1, G. Paolini2, J.F. Humbert1* 1. INRA, Laboratory of Aquatic Microbiology, BP 511, 74 203 Thonon Cedex, France 2. Cellule Technique de l’Aquarium du Lac du Bourget, 73 100 Aix-les-Bains, France

Context and question: Lac du Bourget located in the Alps, is the largest natural French lake. Despite a strong decrease of phosphorus concentration during the last 20 years, proliferations of the toxic filamentous cyanobacterium Planktothrix rubescens regularly occurred since 1996 (Humbert et al., 2001; Jacquet et al. 2004; Briand et al., submitted).

N C 45° 46, 500 N 5° 51, 000 E

In the global framework of our studies dealing with the ecology of P. rubescens, we interested in the spatial (both horizontal and vertical) distribution of the cyanobacterium and asked the following question:

B : 45° 44, 750 N 5° 51, 650 E B1: 5° 52 , 500 B2: 5° 50 , 800

What is the heterogeneity in the vertical and the horizontal distributions of this cyanobacterium in this great sub-alpine lake (Lac du Bourget) during an annual cycle ?

M : 45° 42, 700 N 5° 53, 150 E M1: 5° 52, 500 M2: 5° 51, 900

2.5 km A : 45° 41, 000 N 5° 52, 150 E 45° 40, 600 5° 53, 410 BL: 45° 40, 500 5° 51, 700 P: 45° 39, 600 5° 52, 500 T:

Material and methods: Distribution and quantification of P. rubescens biomass were obtained using the BBE Moldaenke fluoroprobe (Leboulanger et al. 2002). 11 stations (Figure 1) were sampled twice a month during a two years survey (2002 and 2003).

Figure 1. Localization of the 11 sampling stations in the Lac du Bourget

Main results: Vertical distribution

Horizontal distribution

Depending on hydrodynamical processes (internal waves) on a daily scale basis, large differences were found in the vertical distribution of the P. rubescens biomass in the water column. This is symbolized in Figure 2 where a difference of up to 8 m could be found between the North and the South of the lake (data unpublished) for the same day of sampling.

Clear differences could also be recorded for the P. rubescens biomass on the horizontal scale. This can be observed when comparing the biomass distribution between the North and the South of the lake between September and December 2002 for instance (Table 1, Fig. 3).

C

Conc. Eq. Chl a (µg/L)

Depth (m)

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6m

Temperatures

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34 36 0

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P. rubescens

9m

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29 August 2002

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32 34

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27 November 2002

36 30

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30

Temperature (°C)

Figure 2. Distribution of the P. rubescens biomass at the eleven sampling stations of Lac du Bourget for three different dates in 2002/2003. Lines in yellow: correspond to depths where pumpings are carried out by the water treatment units

B

B1 B2 M M1 M2 P

BL A

T

04-Jun-02 19-Jun-02 03-Jul-02 16-Jul-02 30-Jul-02 13-Aug-02 29-Aug-02 11-Sept-02 26-Sept-02 24-Oct-02 20-Nov-02 27-Nov-02 23-Dec-02 14-Jan-03 23-Apr-03 22-Jul-03 05-Aug-03 19-Aug-03 02-Sept-03

Table 1. Classification of P. rubescens biomass values at the different sampling station and for all sampling dates. High values are symbolized in red. Low values are symbolized in blue. Intermediate values are symbolized in white. The grey color expresses missing data

Conclusions:

M2

A

B2 T M

S

M

Axis 1 (38.4 %)

M1

N

B

C

BL P

B1 Axis 2 (19.9 %)

Figure 3. Correspondance analysis performed on the results of the classification of P. rubescens biomass (Table 1). The two first axes account for 58..3 % of the total variance. The three ellipses regroup the sampling points in regard to their location in the lake (North, Middle, South)

This study showed that due to hydrodynamical processes, the vertical and the horizontal distributions of the P. rubescens biomass can vary largely throughout Lac du Bourget,. From a practical point of view, its appears that the use of an unique sampling station in the framework of the monitoring of this cyanobacterium was not able to track this important variability recorded in the distribution of P. rubescens biomass. This may have important consequences for the management of the water abstraction especially when the offtake structure does not have the provision for multiple offtake depths.

Acknowledgments: This program was funded by the Ministère de l’Ecologie et du Développement Durable (SACYTOX Project : www.sacytox.org) References: Briand et al. Submitted to Microbial Ecology Humbert et al. (2001) In Harmful Algal Bloom 2000, Hallegraeff, G., et al. (Eds), Intergovernmental Oceanographic Commission of UNESCO 2001, 496-499 Jacquet et al. (2004) In press in Harmful Algae Leboulanger et al. (2002) Aquat. Microbial. Ecol.30, 83-8 * Email: [email protected], [email protected] SIXTH INTERNATIONAL CONFERENCE ON TOXIC CYANOBACTERIA, BERGEN 2004