Presentation (ASLO 2015 Granada, Spain) - Impacts of Ocean Acidification on the Mediteranean seagrass Posidonia oceanica and its epiphytic community (2015)

Impacts  of  ocean  acidifica0on  on  the  Mediterranean  seagrass  

Posidonia  oceanica  and  its  epiphy0c  community  

Cox  TE,  Ga=uso  J-­‐P,  Diaz  V,  Delille  J,  Schenone  S,  Gazeau  F  

ASLO  mee0ng,  Granada,  Spain,  February  24th  2015    

Background  

•  Posidonia  oceanica  is  the  dominant  seagrass  that  provides  important  

ecosystem  services  in  the  Mediterranean  Sea,  covers  25,000  to  50,000  km2  

•  Its  distribu0on  area  in  the  Mediterranean  Sea  has  significantly  decreased  in  

the  last  few  decades  as  a  consequence  of  anthropogenic  pressure,  with  

important  ecological  and  economical  losses  

•  in  situ  studies  at  the  proximity  of  CO2  vents  show  that  Posidonia  might  

beneficiate  from  increased  CO2  availability  while  calcareous  epiphytes  could  

disappear  

•  Necessity  to  provide  experimental  evidence  under  carefully  controlled  pH  

condi0ons  as  projected  for  the  coming  decades  

Sampling  and  processing  

•  60  shoots  of  Posidonia  oceanica  were  collected  at  12  m  in  the  Bay  of  Villefranche  in  February  2014  

•  In  the  laboratory,  immerged  for  48  h  in  a  tank  containing  calcein  and  fed  ad  libitum  

•  5  shoots  a=ached  together  and  placed  in  12  x  8.4  L  transparent  cylinders  containing  sandy  

sediment  

•  Acclima0on  period  of  1  week  and  4  weeks  of  perturba0on  

Isochrysis  

150  μmol  photon  m-­‐2  s-­‐1  

Calcein  80  g  L-­‐1  

5  weeks  

Ambient

Pure  CO2  

Treatment#1

P   P   P  

pH  stat  

pHT  Temp  

pHT  Temp  

pHT  Temp  

Treatment#2

14-­‐15  °C  

Light:  9.5  L:14.5  D  6x  39  W    

85  50-­‐60  20-­‐25  

μmol  photons  m-­‐2  s-­‐1  

Experimental  set-­‐up  

Experimental  condi0ons  

Physiological  measurements  

•  Respira0on  and  net  primary  produc0on  measured  on  3  occasions  (before  pH  change,  acer  2  weeks,  and  acer  4  weeks)  over  1  h  incuba0ons  

•  Net  calcifica0on  measured  using  the  alkalinity  anomaly  technique  over  7  h  incuba0ons  

•  PE  curves  measured  on  leaf  fragments  at  the  end  of  the  experiment  at  9  irradiance  levels  

Epiphytes  surface  cover  &  %CaCO3  

ZooScan (©Hydroptics)

Crustose  coralline  algae  Bleached  crustose  coralline  algae  Other  Rhodophyta  Filamentous  brown  algae  Bryozoa  Porifera      Hydrozoa    Polychaeta  Foraminifera  

9  «  groups  »  

Pictures  at  2400  dpi  processed  with  ImageJ  

•  Content  (%)  of  CaCO3  in  epiphytes  es0mated  using  the  weight  loss  acer  

acidifica0on  method  

•  Epiphytes  were  scrapped  and  dried  at  40  °C  for  48  h,  weighted,  acidified  with  4  mL  

of  5%  HCl,  rinsed  twice  with  deionized  water,  dried  again  at  40  °C  for  48  h  and  re-­‐

weighted.  

Background  metabolic  rates  

GPP CRNCP

Calcific

ation

0.00

0.01

0.02

0.030.81.01.21.41.61.8

AmbientTreatment#1Treatment#2

µm

ol g

DW

-1 m

in-1

•  No  significant  difference  between  treatments  before  acidifica0on    

Metabolic  rates  rela0ve  change  

Week 2

Week 4

Week 2

Week 4

Week 2

Week 4

-20

0

20

40

60

Rel

ativ

e ch

ange

(%)

a a

b

Week 2

Week 4

Week 2

Week 4

Week 2

Week 4

a a a

Week 2

Week 4

Week 2

Week 4

Week 2

Week 4

AmbientTreatment#1Treatment#2

a a

b

GPP CR NCP

•  Significant  increase  (2-­‐way  RM  ANOVA)  of  GPP  and  NCP  at  pHT  7.3  

(Treatment#2)  

•  No  difference  in  terms  of  CR  

•  In  all  treatments,  a  clear  decrease  of  calcifica0on  is  visible  

•  At  pHT  7.7  (Treatment#1):  Net  calcifica0on  ~0  

•  At  pHT  7.3  (Treatment#1):  Net  dissolu0on  

•  Due  to  high  variability,  only  Treatment#2  is  significantly  different  

(2-­‐way  RM  ANOVA)  

 W

eek 2

Wee

k 4

Wee

k 2

Wee

k 4

Wee

k 2

Wee

k 4

-200

-100

0

a

a

b

Rel

ativ

e ch

ange

(%)

Calcification

PE  curves  

0 50 100 150-2

0

2

4

6

8

10AmbientTreatment#1Treatment#2

Irradiance (µmol photon m-2 s-1)

NPP

(µm

ol O2 g

DW-1

min-1

)

Jassby  &  Pla=  (1976)        P  =  Pmax  ·∙  tanh  (α  ·∙  E/Pmax)    •  α  is  the  slope  or  the  efficiency  with  

which  a  plant  harvests  light    •  Pmax  is  the  maximum  photosynthe0c  

produc0on  and  represents  the  light-­‐saturated  or  carbon-­‐limited  region  of  photosynthesis    

Clear  increase  in  NPP  (much  higher  Pmax)  at  a  pH  of  7.3,  with  no  effect  at  7.7  

Epiphytes  surface  cover  and  %CaCO3  

•  Epiphytes  total  surface  cover  significantly  decreases  with  pH  

•  ~40%  dissimilarity  between  ambient  and  pHT  7.3  •  ~20%  dissimilarity  between  ambient  and  pHT  7.7  •  Most  of  this  dissimilarity  is  a=ributable  to  

crustose  coralline  algae  that  decrease  in  %cover  with  pH  decrease  

•  CaCO3  loss  between  Ambient  and  pHT  7.3  is  evident  

-2 -1 0 1 2-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5AmbientTreatment#1Treatment#2

Axis 1

nMDS Stress:  0.06  

Rela0ve  growth  with  calcein  

Ambient

Treatm

ent#1

Treatm

ent#2

0.0

0.2

0.4

0.6

0.8

1.0

R/G

ratio

a

ab

b

•  Significant  difference  between  ambient  and  pHT  7.3  

•  No  significant  effect  on  epiphyte  growth  at  pHT  7.7  

•  Pictures  taken  on  a  epifluorescence  binocular  

•  Post-­‐treatment  with  ImageJ  •  Transforma0on  to  RGB  pictures  •  Rela0ve  growth  was  calculated  

as  the  ra0o  between  red  and  green  pixels  

Summary  and  conclusions  (1)  

•  P.  oceanica  O2  metabolism  is  not  

significantly  affected  by  a  pH  level  

projected  for  the  end  of  the  century  

•  A  a  pHT  of  7.3,  not  foreseen  in  the  

coming  decades,  effects  are  much  

clearer  with  a  beneficial  effect  on  O2  

metabolism  

•  Missing  data:  C/N  and  δ13C  of  the  

youngest  leaves  +  phenols  

Summary  and  conclusions  (2)  •  Epiphytes,  and  especially  coralline  algae,  

appear  sensi0ve  at  a  pH  level  that  will  likely  

be  reached  by  2100  

•  Epiphytes  play  major  roles  in  seagrass  

meadows  being  the  major  food  of  source  for  

many  organisms.  This  could  therefore  lead  to  

important  ecosystem  changes  

•  No  overgrowth  of  filamentous  brown  algae  

as  observed  in  other  studies  with  higher  

nutrient  concentra0ons  

•  At  pH  7.3,  a  highly  detrimental  effect  on  

epiphytes  (mostly  calcifiers)  was  observed  

confirming  observa0ons  from  CO2  vents  

Ambien

t  7.7  

7.3  

What’s  next?  •  Reproduce  experiments  in  other  periodsand  sites  

(different  light/temperature,  nutrient  condi0ons)  

•  Long-­‐term  (>  several  months),  community  and  mul0-­‐

stressor  experiments  s0ll  required!  

Please  a=end  the  talk  from  E.  Cox  Wednesday  17:45    “In  situ  studies  of  the  impacts  of  ocean  acidifica0on:  observa0ons,  CO2  vents,  and  FOCE  experiments  

Thank  you  for  your  a=en0on  

Acknowledgements  •  A.  Elineau  for  help  with  the  ZooScan  •  C.  Rouvière  for  help  with  the  epifluorescence  binocular