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Freshwater cyanobacterial bloomsand toxin production
S. Jacquet & J.-F. Humbert
UMR CARRTEL Thonon
EC, Brussels, 29 May 2002
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
- Accessory pigments (phycoerythrin…)
- Multicellular organization(filament, colony)
- Synthesis of toxinsdefense against predation
nutrient/light uptake
… and the winner is:
Predicting cyanobacteria dominance in lakes ?
Low N/P is not a key parameterThe risk is more associated to total P or total N
Enhancing factors:Shallow watersLong RT
CausesInsufficientlytreated sewage
Runoff from fertilizedagricultural areas
Manure, effluentfrom livestock industries
Runofffrom roads
EffectsFertilization of water, chiefly with P
Consequences
Mass developmentsof potentially toxic
cyanobacteria
Most common cyanobacterial toxins
!!!! Cyclic peptides
- Microcystins- Nodularin
!!!! Alkaloids
- Anatoxin –a, -a(S)- Saxitoxins- Cylindrospermopsins Hepatotoxicity
!!!! Lipopolysaccharides Potential irritant for any exposed tissue
Hepatotoxicity
Neurotoxicity
Impacts of cyanobacteria
! Ecological impact
- Perturbations of the ecosystem functioning- Shade- Trophic chains
- Anoxia at the end of the bloom
! Sanitary impacts
- Mortality and morbidity in aquatic and terrestrial invertebrates and vertebratesExample: In Switzerland, more than 100 cattle deaths were attributed during the last two
decades to cyanotoxin poisoning
- Human contamination
Human poisoning by cyanotoxins
! Short term effects- Gastrointestinal and hepatic illness- Death of kidney dialysis patients in Brazil
! Chronic term effects- Hepatic carcinoma
Principal routes of exposure! Oral exposure through drinking water, ! Oral and dermal exposure trough recreational water use! Oral exposure through consommation of contamined
products ?! Haemo-dialysis
Nutrient control of toxin production
Microcystis aeruginosa, microcystins LR (MC-LR)Several lakes investigated in US, Canada
↑Ptot ⇒⇒⇒⇒ ↑↑↑↑MC-LR production↑ N (N03, NO2, NH4) ⇒⇒⇒⇒ ↓↓↓↓ MC-LR production↑ light ⇒⇒⇒⇒ ↓↓↓↓ MC-LR production
High MC content at the later exponential and stationary phase of growth
MC production = f(growth rate, cell division)
Caution : N2 fixing vs. not fixing cyanobacteriaSpecies dependence ⇒⇒⇒⇒ case studies
Environmental control is little known
Biological significance, functional role of toxins :- ‘ fine-tuning’ metabolism and balancing uptake- assimilation and incorporation of nutrients for growth- beneficial associations with other microbes- protective role from zooplankton, bacteria, viruses, fungi- reserve pools of metabolites
% cyanobacterial blooms associated to toxinProduction :
UK : up to 60% Sweden: up to 53%Finland : up to 45% Denmark : up to 80%Norway: up to 45% Germany : up to 70%
Preventive/remedial measures
!!!! Reduction of nutrients: Phosphorus principally (< 10 µg/l)
Permissible inputs Dangerous inputs
P N P N(g m-2 a-1) (g m-2 a-1) (g m-2 a-1) (g m-2 a-1)
MeanDepth (m)
< 5 < 0.07 < 1.0 > 0.13 > 2.0< 10 < 0.1 < 1.5 > 0.2 > 3.0< 50 < 0.25 < 4.0 > 0.5 > 8.0< 100 < 0.4 < 6.0 > 0.8 > 12.0< 150 < 0.5 < 7.5 > 1.0 > 15.0< 200 < 0.6 < 9.0 > 1.2 > 18.0
Renewal time of 2 m3 m-2 a-1 Vollenweider/OECD
Reduction of dissolved inorganic nitrogen alone supports the dominance of heterocystic species (Anabaena andAphanizomenon)
Permissible and dangerous inputs for P and N in lakes
! In small lakes
- In-lake phosphorus precipitation
- Construction of pre-reservoir to retain P
- Sediment dredging and P binding
- Physical and chemical treatments - Vertical mixing- Copper sulfate !!!
- Biomanipulation- Fish, virus…
Preventive/remedial measures
The case of Planktothrix rubescens in Lake Bourget
Decrease of P from 120 µg/l to 30 µg/l in the last 20 years
BUT
problems with the toxic cyanobacterium P. rubescenssince 1996-97
0 m
50 mJuly 99 April 00 July 00 April 01 July 01 April 02
0
1
2
3
4
5
6
03-ao
ût-99
31-ao
ût-99
13-se
pt-99
29-se
pt-99
14-oc
t-99
03-no
v-99
16-no
v-99
29-no
v-99
07-dé
c-99
22-dé
c-99
05-ja
nv-00
18-ja
nv-00
31-ja
nv-00
15-fé
vr-00
10 m
15 m
20 m
MCYS-RR (µg/l)
The case of Planktothrix rubescens in Lake Bourget
WHO drinking waterguideline conc. of 1µg/l
How to explain P. rubescens bloom since 4 years ?
7 °C
24 °C
P +++
P +++
P +++
P +
P +++
P -
Eutrophic conditions Meso-trophic conditions
P. rubescens is - low light, low temperature, low nutrient adapted- filamentous and toxic and hence little grazed- able to regulate its buoyancy- enhanced by P pulses- …
Differently said:Climatic influence
=Warmer winter & spring
Human pressure=
Reduction of P
Advance of spring bloom& zooplankton development
=Advance in population decline
& advance of clear water phase
Advance of P-depleted Surface waters
=Sinking of population
& the P-depleted zone
Very competitive species forthe new environmental conditions:
low nutrient, low light of metalimnion
Planktothrix rubescens
Low grazing, low viral attack, stable water column
How to survey the development of P. rubescens ?
! Counting filaments
! Use of a fluorimetric probe
Why P. rubescens in lake Bourget and not Léman?1 - Original species diversity ⇒⇒⇒⇒ Competition
Lake Léman> 800 Phytopk species described to date~ 150 phytopk species observed each year
Clearly less for Lake Bourget~ 100 phytopk species
2 - Water column stability (IDH), depth and timming
- Bourget is highly stratified in summer compared to Léman- There is a clear delay of stratification for Léman (> September)- Metalimnion is deeper in Bourget than in Léman
Stability of epilimnion = vertical migrationStability of metalimnion = refuge from continuous entrainment
Conclusions
Still efforts are required to continue the reduction of nutrients (especially P) in small and deep lakes
Probably efforts should be rewarded when P < 10 µg.l-1
In the whole trophic zone ⇒⇒⇒⇒ real P limitation
Particular case: P. rubescens that grows with < 3 µg.l-1
Importance of global change to account for
⇒⇒⇒⇒ Modelisation to predict future changes of lake water quality