physical effects on plankton and productivity on the faroe plateau e. gaard, b. hansen, s. k....
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Physical effects onplankton and productivity
on the Faroe Plateau
E. Gaard, B. Hansen, S. K. Eliasen and K. M. H. LarsenFaroese Fisheries Laboratory
Tórshavn, Faroe Islands
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Content of the talk
Plankton variability
Cod growth & recruitment variability
Possible environmental effects
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Location of the Faroe shelf
Shetland
Iceland
Fa roeIs la nds
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The Faroe Shelf ecosystem
• The shelf water is well separated from the surrounding offshore.
• Retention of holo- and meroplankton
• The plankton is different in composition, timing and production from offshore
• An ecosystem. The primary production is the basis for production in the higher trophic levels.
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Phytoplankton biomassSeasonal, 1997-2003
0
2
4
6
8
10
12 C
hlo
rop
hyl
l a (
µg
/l)
1997
J F M A M J J A S O N D
0
2
4
6
8
10
12 C
hlo
rop
hyl
l a (
µg
/l)
1998
J F M A M J J A S O N D
0
2
4
6
8
10
12 C
hlo
rop
hyl
l a (
µg
/l)
1999
J F M A M J J A S O N D
0
2
4
6
8
10
12 C
hlo
rop
hyl
l a (
µg
/l) 2000
J F M A M J J A S O N D
0
2
4
6
8
10
12 C
hlo
rop
hyl
l a (
µg
/l) 2001
J F M A M J J A S O N D
2002
0
2
4
6
8
10
12 C
hlo
rop
hyl
l a (
µg
/l)
J F M A M J J A S O N D
0
2
4
6
8
10
12 C
hlo
rop
hyl
l a (
µg
/l)
2003
J F M A M J J A S O N D
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Nitrate, Faroe shelf1995-2003
0
2
4
6
8
10
12
14 1995
1996
1997
1998
1999
2000
2001
2002
2003
J F M A M J J A S O N D
Nit
rate
(µ
M)
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Index of potential new primary production Spring – mid summer, 1990-2003
0
5
10
15
20
Pri
mar
y p
rod
ucti
on
ind
ex
1990 1992 1994 1996 1998 2000 2002
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Zooplankton in spring
Calanus
Reproduction
Neriticzooplankton
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Zooplankton in late April
50
100
150
200
250
300
350 S
ma
ll c
op
ep
od
s
0 .4 0.6 0.8 1 1.2 1.4 1.6 1.8
Ch lo ro p h yll a
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Cod variability
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Primary production and cod
0
3
6
9
12
15
18
PP
inde
x
0
20
40
60
80
C
od p
rodu
ctio
n(1
000
tonn
es/y
ear)
1990 1992 1994 1996 1998 2000 2002
Primary production
Cod production
0
5
1 0
1 5
2 0
PP
ind
ex
0
1 0
2 0
3 0
4 0
5 0
Re
crui
tme
nt (
mill
.)
1 9 9 0 1 9 9 2 1 9 9 4 1 9 9 6 1 9 9 8 2 0 0 0 2 0 0 2 2 0 0 4
P rim ary p ro d uc tio n
C o d re c ruitm e nt
0
5
10
15
20
PP
ind
ex
0
0 .5
1
1 .5
2
Co
d g
row
th r
ate
(k
g/y
ea
r)
1990 1992 1994 1996 1998 2000 2002 2004
Prim ary production
Cod grow th rate
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What causes the large fluctuations in potential new primary
production ?
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Zooplankton DW & nitrateFaroe shelf, late June
5
6
7
8
9
1 0
1 1
1 2
0 2 5 5 0 7 5
91
92
93
9495
9697
98
99
00
Nitr
ate
loss
(µM
)
Zooplankton biomass (mg dry w eight/ m3)
01
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Modelled chlorophyll concentrations with high exchange rate (k = 4000 m/day),
with & without Calanus off shelf
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Modelled chlorophyll concentrations (high and low exchange rates)
compared with observations from different years
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Air temperature in Tórshavn (January-April) and Primary Production Index
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003
2.0
3.0
4.0
5.0
6.0
Air
tem
pera
ture
(°C
)
0
4
8
12
16
20
Prim
ary
Pro
duct
ion
Inde
x
Air temperaturePP index
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200m
Surface temperature 18. April 2003(from P. Miller, Plymouth Marine Lab)
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Seasonal variations in mean month temperature Off shelf
5m depth
Off shelf100m depth
5
6
7
8
9
10
11
12
13
Jan M arch M ay July Sept Nov
Tem
pera
ture
(°C
)
Shelf w ater
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Heat and freshwater exchanges affect the
shelf temperature, salinity, and density
Cooling in winter
Rain
Horizontalexchange
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Salinity difference across the shelf front
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Density difference across the shelf front
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Potential new primary production anddensity difference across the shelf front in May
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Hypothesis• Horizontal exchange inhibits primary
production
• A large density difference across the shelf front inhibits horizontal exchange (through baroclinic instability, e.g.)
• The air temperature in Jan-April determines the density difference across the shelf front
• Cold winters give good production