Dr Stéphan JACQUET

Station INRA d’hydrobiologie Lacustre

Equipe «Microorganismes aquatiques»

75, Avenue de Corzent

74 203 Thonon-les-Bains cedex, France

Email:

tel (fax): + 33 (0)4 50 26 78 12 (07 60)22/05/02

Lake Bourget

 NOTE: All the data presented in this document have to be mentioned as Jacquet & co-authors (unpublished)

Lake Bourget is the largest natural French lake. It is localized in the Alps (in Savoie) at an elevation of 231 m. Its mains characteristics are as follows:

Surface area = 42 x 106 m2; Volume = 3.5 x 109 m3; max depth = 145 m; mean depth = 80 m; total length = 18 Km; max width = 3 Km; Catchment area = 560 Km2; residence time of water = 7 years in theory, probably more; Principal axe = North-south orientation; Principal alimentation = 2 rivers : Sierroz and Leysse

The nutrient concentration inflows from rivers to the lake (fluxes in tons) from the 70’s are summarized in the following table. It resumes efforts made to reduce nutrient pollution in the lake in the late 70’s - early 80’s

Mineral nitrogen (tons) / Total phosphorus (tons)
rivers / 1974 / 1982 / 1987 / 1996 / 1974 / 1982 / 1987 / 1996
Sierroz
Leysse
Others / 100
1300
400 / 110
330
140 / 150
200 / 136
314 / 20
230
50 / 40
90
20 / 60
75 / 16
78
total / 1800 / 580 / 350 / 450 / 300 / 150 / 135 / 94

In 1996, 18 tons of orthophosphates are brought to the lake. Concentrations of orthophosphates and total P in the Leysse river are 30 µgP.l-1 and 150 µgP.l-1, respectively.

The following figures give the evolution of nutrients and transparency (that can be related to chlorophyll a) until now. The data presented below correspond to all measurements made throughout the water column in the middle of the lake and available to date.

Take home messages:

The re-oligotrophisation process took place in the early 80’s in response to lake restoration. By reducing nutrient loading to the lake, phosphorus concentrations have been considerably reduced (75% in 25 years). Below, the figure summarizes the evolution of total phosphorus (mean value normalized to depth) for the 3 main lakes in French Alps (Léman, Bourget and Annecy).

By comparison, nitrogen concentrations for lake Bourget have only been reduced modestly (i.e. 25% in 25 years).

The N/P ratio in early spring has evolved as follows :

-6 in 1978-1981

-15 in 1988-1991

-28 in 1998-2001

In 1996, phosphorus fluxes (in tons) can be summarized as follows:

Stock / P input / P export / P sedimentation / P release
144 / 94 / 15 / 80 / 2

There is a potential of P release around 60 tons.year-1

Transparency data suggest that phytoplanktonic production reduced but the quantity of phytoplankton is still high. On average, throughout the year, concentrations of chlorophyll a evolved from 9.2 µg.l-1 in 1988 to 6.2 µg.l-1 in 1996 and to 7.2µg.l-1 in 2001. In addition, its composition still traduces a meso- to eutrophic system (criteria of OECD). A shift occurred in the planktonic community structure with a perennial bloom of the toxic cyanobacterium Planktothrix rubescens recorded since 1996. This is shown by the two figures below that show the temporal dynamics and the relative contribution of the cyanobacterium to total
chlorophyll a as measured indirectly using a submersible spectrofulorometer .


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