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