Freshwater cyanobacterial blooms and 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
nutrient/light uptake
- Accessory pigments (phycoerythrin…) - Multicellular organization (filament, colony) - Synthesis of toxins
defense against predation
… and the winner is:
Predicting cyanobacteria dominance in lakes ? Causes Insufficiently Runoff from fertilized Manure, effluent treated sewage agricultural areas from livestock industries
Runoff from roads
Effects Fertilization of water, chiefly with P Consequences Mass developments of potentially toxic cyanobacteria
Low N/P is not a key parameter The risk is more associated to total P or total N Enhancing factors: Shallow waters Long RT
Most common cyanobacterial toxins ! Cyclic peptides - Microcystins
Hepatotoxicity
- Nodularin ! Alkaloids - Anatoxin –a, -a(S)
Neurotoxicity
- Saxitoxins - Cylindrospermopsins ! Lipopolysaccharides
Hepatotoxicity Potential irritant for any exposed tissue
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 vertebrates Example: 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 Environmental control is little known 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 cyanobacteria Species dependence ⇒ case studies
% cyanobacterial blooms associated to toxin Production : UK : up to 60% Finland : up to 45% Norway: up to 45%
Sweden: up to 53% Denmark : up to 80% Germany : up to 70%
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
Preventive/remedial measures ! Reduction of nutrients: Phosphorus principally (< 10 µg/l) Permissible and dangerous inputs for P and N in lakes Permissible inputs Mean Depth (m) 0.13 > 0.2 > 0.5 > 0.8 > 1.0 > 1.2
> 2.0 > 3.0 > 8.0 > 12.0 > 15.0 > 18.0
Vollenweider/OECD
Reduction of dissolved inorganic nitrogen alone supports the dominance of heterocystic species (Anabaena and Aphanizomenon)
Preventive/remedial measures ! 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…
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. rubescens since 1996-97
The case of Planktothrix rubescens in Lake Bourget 0m
50 m July 99
April 00
July 00
April 01
MCYS-RR (µg/l) 6
10 m 15 m
5 4
20 m
WHO drinking water guideline conc. of 1µg/l
3 2 1
03
-a oû t-9 31 9 -a oû t-9 9 13 -s ep t-9 9 29 -s ep t-9 9 14 -o ct -9 9 03 -n ov -9 9 16 -n ov -9 9 29 -n ov -9 9 07 -d éc -9 9 22 -d éc -9 05 9 -ja nv -0 18 0 -ja nv -0 31 0 -ja nv -0 0 15 -fé vr -0 0
0
July 01
April 02
How to explain P. rubescens bloom since 4 years ?
P +++
24 °C
P +++
P +++
PP+
7 °C
Eutrophic conditions
P +++ 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
Advance of spring bloom & zooplankton development = Advance in population decline & advance of clear water phase
Human pressure = Reduction of P
Advance of P-depleted Surface waters = Sinking of population & the P-depleted zone
Very competitive species for the 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 migration Stability 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
