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
Content of the talk
Plankton variability
Cod growth & recruitment variability
Possible environmental effects
Location of the Faroe shelf
Shetland
Iceland
Fa roeIs la nds
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.
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
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)
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
Zooplankton in spring
Calanus
Reproduction
Neriticzooplankton
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
Cod variability
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
What causes the large fluctuations in potential new primary
production ?
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
Modelled chlorophyll concentrations with high exchange rate (k = 4000 m/day),
with & without Calanus off shelf
Modelled chlorophyll concentrations (high and low exchange rates)
compared with observations from different years
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
200m
Surface temperature 18. April 2003(from P. Miller, Plymouth Marine Lab)
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
Heat and freshwater exchanges affect the
shelf temperature, salinity, and density
Cooling in winter
Rain
Horizontalexchange
Salinity difference across the shelf front
Density difference across the shelf front
Potential new primary production anddensity difference across the shelf front in May
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
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