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

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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

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nv-00

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vr-00

10 m

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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