Déb Anne Goa 130204 et al. Goa janv... · NUTRIENT AND PHYTOPLANKTON RESPONSES TO EXTERNAL FORCING IN A MEDITERRANEAN COASTAL AREA UNBIAISED BY TERRESTRIAL INPUTS AND LOCAL ACTIVITIES

NUTRIENT AND PHYTOPLANKTON RESPONSES TO EXTERNAL

FORCING IN A MEDITERRANEAN COASTAL AREA UNBIAISED BY

TERRESTRIAL INPUTS AND LOCAL ACTIVITIES (CALVI, CORSICA)

Anne GOFFART 1,*, Jean-Henri HECQ 1 and Louis LEGENDRE 2

1 University of Liège, Belgium & STARESO, Calvi, Corsica, France 2 Laboratoire d'Océanographie de Villefranche (LOV), Villefranche, France

* [email protected]

Jan 13

•  To synthesize a long-term high-resolution study of nutrient and phytoplankton bloom

dynamics performed between 1979 and 2011 at a permanent station in the Bay of Calvi

(Corsica, northwestern Mediterranean)

•  To understand mechanisms controlling the interannual variability of phytoplankton bloom in a

context of climate change

STARESOStation de Recherches Sous-Marines

et Océanographiques

Objectives

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The studied area : the Bay of Calvi, Corsica, Western Mediterranean

•  Open bay and narrow shelf

•  Oligotrophic characteristics

•  Few anthropogenic pressures

•  High control by climate forcing

•  Reference for the WFD

Bay of Calvi

4 kms

•  Water temperature, wind,

phytoplankton and zooplankton from

1979

•  Nutrients from 1988



Time-series : subsurface data

[email protected] [email protected]

STARESO STAtion de REcherches Sous-marines et Océanographiques

de l’Université de Liège

High sampling frequency during phyto- and zooplankton blooms (1 - 7 times per week)

•  The winter-spring phytoplankton bloom

of the Bay of Calvi is characterized by a

very large interannual variability

reaching one order of magnitude from

one year to another



Time-series : results

What factors and/or processes control

this variability ?

•  Water temperature

•  Wind intensity

•  Nutrient availability

•  Zooplankton grazing The blue bars indicate the limits of the period of

cold water (< 13,5 °C)

0,0

0,5

1,0

1,52001

D J F M A M J J A S O N D

ng C

hl a

l-1

0,0

0,5

1,0

1,52007


ng C

hl a

l-1

0,0

0,5

1,0

1,52009


ng C

hl a

l-1

In unstratified water column (surface temperature < 13,5 °C), surface nutrient enrichment is

strongly controlled by wind stress - annual example : 1999



Control of nutrient availability by wind stress

Goffart et al in prep

WII takes into account the duration of the period of cold water and the frequency of windy days

during the period of cold water



Definition of a winter index intensity : WII

Winter surface nutrient enrichment is strongly and linearly controlled by WII (R2 = 0,86)



Control of nutrient availability by WII : time-series results

0

50

100

150

0 1 2 3 4 5 6

Integrated NO3

1988

1997

1998

1999

2000

2001

2002

2006

2007

2008

2009

2010

2011

y = -2,61 + 29,73x R2= 0,86

Inte

grat

ed N

O3 (

wat

er <

13.

5 °C

)

Winter intensity index

1988 – 2011 data


Phytoplankton biomass is strongly controlled by WII : increasing phytoplankton concentrations are

observed for low and intermediate WII, while high WII are associated with lower phytoplankton

biomasses (polynomial regression : R2 = 0,70)



Control of phytoplankton by WII : time-series results

0

10

20

30

40

50

60

70

80

0 1 2 3 4 5 6

Integrated Chl a (< 13.5 °C)1979198619881997199819992001

20022005200620072008200920102011

Winter intensity index

Inte

grat

ed C

hl a

(w

ater

< 1

3.5

°C)

Y = M0 + M1*x + M2*x2 + M3*x3

2,12M0

5,15M1

10,59M2

-2,08M3

0,70R2

1979 – 2011 data


Phyto- and zooplankton biomasses increase concomitantly (linear regression : R2 = 0,70). This

implies that lower phytoplankton concentrations observed at high WII are not due to enhanced

zooplankton grazing.



Is zooplankton grazing responsible for lower phytoplankton biomasses associated with high WII ?

Lower phytoplankton biomasses observed at high WII are probably due

limitation of phytoplankton growth by reduced exposition to light in a

highly mixed water column



Trends in WII over the 1979 – 2011 period

0

1

2

3

4

5

619

80

1982

1984

1986

1988

1990

1992

1994

1996

1998

2000

2002

2004

2006

2008

2010

WII phyto WII without phyto WII

Win

ter

inte

nsity

inde

x

Year

x x 0 0 0




Conclusions

This study, built from three decades of observation acquired in an oligotrophic Mediterranean coastal area, •  shows the combined effects of 2 key natural forcings (wind intensity and water temperature)

on nutrient refueling of surface water,

•  detects consistent patterns in terms of relationships between environmental drivers and response of winter-spring phytoplankton bloom,

•  does not evidence any continuous change in phytoplankton biomass from 1979 but shows a year to year response to climate variation.

Thank you for your attention !



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Déb Anne Goa 130204 et al. Goa janv... · NUTRIENT AND PHYTOPLANKTON RESPONSES TO EXTERNAL FORCING IN A MEDITERRANEAN COASTAL AREA UNBIAISED BY TERRESTRIAL INPUTS AND LOCAL ACTIVITIES