déb anne goa 130204 et al. goa janv... · nutrient and phytoplankton responses to external forcing...
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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
Jan 13
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• 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
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• Water temperature, wind,
phytoplankton and zooplankton from
1979
• Nutrients from 1988
STARESOStation de Recherches Sous-Marines
et Océanographiques
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)
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• 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
STARESOStation de Recherches Sous-Marines
et Océanographiques
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
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
1,52009
D J F M A M J J A S O N D
ng C
hl a
l-1
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In unstratified water column (surface temperature < 13,5 °C), surface nutrient enrichment is
strongly controlled by wind stress - annual example : 1999
STARESOStation de Recherches Sous-Marines
et Océanographiques
Control of nutrient availability by wind stress
Goffart et al in prep
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WII takes into account the duration of the period of cold water and the frequency of windy days
during the period of cold water
STARESOStation de Recherches Sous-Marines
et Océanographiques
Definition of a winter index intensity : WII
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Winter surface nutrient enrichment is strongly and linearly controlled by WII (R2 = 0,86)
STARESOStation de Recherches Sous-Marines
et Océanographiques
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
Goffart et al in prep
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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)
STARESOStation de Recherches Sous-Marines
et Océanographiques
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
Goffart et al in prep
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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.
STARESOStation de Recherches Sous-Marines
et Océanographiques
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
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STARESOStation de Recherches Sous-Marines
et Océanographiques
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
Goffart et al in prep
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STARESOStation de Recherches Sous-Marines
et Océanographiques
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.
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Thank you for your attention !
STARESOStation de Recherches Sous-Marines
et Océanographiques