Phytoplankton Community Structure in the HNLC Subarctic

Pacific Ocean is determined by Neocalanus flemingeri and N. plumchrus. **

Michael Dagg1, Suzanne Strom2

and Hongbin Liu3

1

Louisiana Universities Marine Consortium Louisiana, USA

2

Shannon Point Marine Laboratory Washington, USA

3

Hong Kong University of Science and Technology Clear Water Bay,

Kowloon, Hong Kong

** submitted DSR, March 2008revised August 2008

(from Levitus: World Ocean Atlas 1994)

-

Low phytoplankton concentrations, -

iron-limited growth (especially large cells)-

Small phytoplankton –

microzooplankton

grazers-

Large grazers consume microzooplankton and large phytoplankton-

Most important large grazers are Neocalanus spp.

The mortality of large phytoplankton remains the least understood component of the lower

trophic-level dynamics in the SPO

Here we re-evaluate the contribution of N. flemingeri and

N. plumchrus to the control of phytoplankton biomass and community structure in

the

subarctic

Pacific Ocean.

Surface

Mixed layer

Thermocline

Permanent halocline

N. plumchrus/flemingeri zone

N. cristatus and E. bungii zone

from Mackas, D.L., H. Sefton, C. B. Miller and A. Raich. 1993. Vertical habitat partitioning by large calanoid copepods in the oceanic subarctic Pacific during spring. Prog. Oceanogr. 32: 259-294.

0 m

35-45 m

100-120 m

5/18/01

copepods bottle-1

0 1 2 3 4 5

App

aren

t gro

wth

rate

(d-1

)

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

< 5 μm5-20 μm> 20 μm

Chlorophyll concentration:< 5 μm = 0.577 μg l-1

5-20 μm = 0.143 μg l-1> 20 μm = 0.051 μg l-1

5/18/01

copepods bottle-1

0 1 2 3 4 5

App

aren

t gro

wth

rate

(d-1

)

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

< 5 μm5-20 μm> 20 μm

Chlorophyll concentration: Clearance rate:< 5 μm = 0.577 μg l-1 -0.068 L cop-1 d-1

5-20 μm = 0.143 μg l-1 0.088 L cop-1 d-1

> 20 μm = 0.051 μg l-1 0.208 L cop-1 d-1

Exp 1 - 5/18/01

phytoplankton ciliates dinoflagellates

< 5

chl

5-20

chl

> 2

0 ch

l>

20 C

oscin

o>2

0 G

ram

m>

20 P

seud

o 1

> 20

Pse

udo

2

< 20

cil

20-3

0 cil

> 30

cil

< 20

din

o20

-30

dino

Cle

aran

ce ra

te (L

cop

-1 d

-1)

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0N. flemingeri CVN. plumchrus CV

Additional experiments with counts

Chlorophyll concentration (μg l-1)< 5 μm 5-20 μm >20 μm total0.27

0.05

0.05

0.37

1.00

0.24

0.06

1.300.18

0.13

3.41

3.73

Exp 2 - 5/24/01

phytoplankton ciliates dinoflagellates

< 5

chl

5-20

chl

> 2

0 ch

l

< 20

cil

20-3

0 cil

30-4

0 cil

40-5

0 cil

> 50

cil

< 20

din

o20

-30

dino

> 30

din

o

Cle

aran

ce R

ate

(L c

op-1

d-1

)

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

N. flemingeri CVN. plumchrus CV

Exp 3 - 5/13/03

phytoplankton ciliates dinoflagellates

< 5

chl

5-20

chl

> 2

0 ch

l

< 20

cil

20-3

0 cil

> 30

cil

< 20

din

o20

-30

dino

Cle

aran

ce R

ate

(L c

op-1

d-1

)

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0N. flemingeri CVN. plumchrus CV

Exp 4 - 5/27/01

phytoplankton ciliates dinoflagellates

< 5

chl

5-20

chl

> 2

0 ch

l

< 20

cil

20-3

0 cil

30-4

0 cil

40-5

0 cil

> 50

cil

< 20

din

o20

-30

dino

30-4

0 di

no40

-50

dino

50-6

0 di

no60

-70

dino

> 70

din

o

Cle

aran

ce ra

te (L

cop

-1 d

-1)

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

N. flemingeriN. plumchrus

Exp 2 –

0.37 μg l-1Exp 3 –

1.30 μg l-1Exp 4 –

3.73 μg l-1

Clearance rate

Reference

Comments0.12

Frost et al., 1983

high copepod concentrations0.04

Landry and

Lehner-Fournier, 1988

high copepod concentrations0.112

Dagg, 1993

total chlorophyll removal0.04 -

1.08

Dagg

and Wyman, 1983

gut pigment converted to clearance 0.17 -

0.94

Gifford, 1993

cell counts0.24 -

0.96

Tsuda

et al., 2006

total ambient chlorophyll 0.12 -

0.60

Dagg

and

Walser, 1987

small cell (12 mm) and high concs0.32 -

0.38

Landry and

Lehner-Fournier, 1988

mesocosm -

total chlorophyll0.12

Landry et al., 1993a

mesocosm

-

total chlorophyll0.42, 0.45, 0.17

Landry et al., 1993

mesocosm

-

Nitzschia,

centrics, ciliates0.00 -

2.40

this study

Same general pattern observed for N. cristatus CV

< 5 μm> 20 μm

From Liu, Dagg and Strom. 2005. J. Plankton Res. 27: 647-662

Neocalanus cristatus CVChlor < 5 μm = 0.39 μg l-1

Chlor 5-20 μm = 0. 06 μg l-1

Chlor > 20 μm = 0.07 μg l-1

Chlorophyll size FloCAM size (μm ESD) fraction (μm)

< 5 5-20 > 20 11-20 21-30 31-40 41-50 51-60 >60

Cle

aran

ce ra

te (L

cop

-1 d

-1)

-1

0

1

2

3

4

n = 4 replicates

FloCAM showed same general pattern

N. plumchrus + N. flemingeri CV

0 50 100 150 200 250 300 350 400 450

0-15 m15-30 m30-50 m

0-15 m15-30 m30-50 m

0-15 m15-30 m30-50 m

0-15 m15-30 m30-50 m

0-15 m15-30 m30-50 m

0-15 m15-30 m30-50 m

0-15 m15-30 m30-50 m

0-15 m15-30 m30-50 m

0-15 m15-30 m30-50 m

0-15 m15-30 m30-50 m

12

34

56

78

910

number ind/m3

May 1984 -

10 stations: mean abundance = 152 CVs m3(from Mackas

et al. 1993)

N. plumchrus + N. flemingeri CV

0 100 200 300 400 500 600

0-1010-25 m

25-500-10

10-25 m25-500-10

10-25 m25-500-10

10-25 m25-500-10

10-25 m25-500-10

10-25 m25-50

1a1b

2a2b

3a3b

# ind/m3

June 1987 –

mean abundance = 204 ind m3(from Mackas

et al. 1993)

Inner Shelf Station29-May-01, 23:53

Number per m30 200 400 600 800

Dep

th (m

)

0

25

50

75

100

Neocalanus cristatusCIV Neocalanus spp. CV

Outer Shelf Station19-May-01, 00:10

Number per m30 200 400 600 800

0

25

50

75

100J. Napp, C. Baier, & J. Lanksbury, unpublished

Vertical Distribution

Middle Shelf Blue Water (GAK4)26-May-01, 23:54

Number per m 3

0 200 400 600 800

Dep

th (m

)

0

25

50

75

100

Neocalanus cristatus CIV Neocalanus spp. CV

Middle Shelf Bloom (4IW)25-May-01, 23:52

Number per m 3

0 200 400 600 8000

25

50

75

100

(J. Napp, C. Baier, & J. Lanksbury, unpublished)

Other abundance data

Miller, 1993; Goldblatt

et al., 1999; Kobari

et al., 2003 report average abundances to depths somewhat greater than the seasonal thermocline

but compressing these populations into the upper 50 or 25 m results in CV concentrations similar to those observed by Mackas

et al. (1993): 60 to 260 m-3

(Miller, 1993); 200 to 400 m-3

(Goldblatt

et al., 1999) and 180 to 510 m-3

(Kobari

et al., 2003).

Underway pumping of surface water by Kawamura (1990) commonly showed surface concentrations between 200-600 CVs m-3, and > 800 m-3

on several occasions

N. flemingeri and

N. plumchrus concentrations in surface waters are typically several hundred m-3, and population distributions are highly patchy, both horizontally and vertically.

Growth rates of phytoplankton

Growth rate (d-1)

reference

comments

0.54, 0.98, 1.33

Noiri

et al., 2005

Grow-out experiments at 5, 8 and 13oC0.1, 0.4

Noiri

et al., 2005

Fe conc

< 0.5 nM, 8 and 13oC0.0 -

0.5

Tsuda

et al., 2005

SEEDS enrichment experiment-early0.5 -

0.9

Tsuda

et al., 2005

SEEDS enrichment experiment-max growth0.2 -

0.5

Tsuda

et al., 2005

SEEDS enrichment experiment-post bloom0.3 -

0.5 Saito et al., 2005

SEEDS -

outside Fe enriched patch0.2 -

0.9

Boyd/Harrison, 1999

eastern SPO -

spring0.3 -

0.8

Boyd/Harrison, 1999

eastern SPO -

summer0.3 -

1.0

Booth et al., 1993

eastern SPO0.1 -

0.7 Landry et al., 1993b

eastern SPO0.2 -

0.6

Rivkin

et al., 1999

eastern SPO0.0 -

0.8

Strom/Welschmeyer, 1991

eastern SPO0.24

Boyd et al., 1999

eastern SPO

Population clearance rate

0.8

0.6

0.6

0.4

0.4

0.4

0.2

0.2

0.2

0.2

0.2

Copepod concentration (CV m-3)

0 100 200 300 400 500

Cle

aran

ce ra

te (L

cop

-1 d

-1)

0.0

0.5

1.0

1.5

2.0

population clearance rate (d-1)

• growth rates of large diatoms in this Fe limited system are 0.2 -

0.4 d-1

• growth rates of large ciliates and dinoflagellates??

• other copepodid

stages must contribute also

Other considerations: top down effect by N. flemingeri/plumchrus

Post-ontogenic

descent

Annual phytoplankton max (Yoo

et al., 2008; Clemons and Miller, 1984; Boyd and Harrison, 1999; Parsons and Lalli, 1988)

a higher fraction of phytoplankton growth is consumed by microzooplankton

(Strom and Welschmeyer, 1991)

N. cristatus moves up into the surface layer after the descent of N. flemingeri and N. plumchrus so its contribution to surface layer grazing will increase at this time (Mackas

et al., 1993; Goldblatt

et al., 1999)

Other mesozooplankton, especially Oithona spp. and Pseudocalanus spp., increase in surface water at this time, although their grazing capacity is small (Liu et al., 2008).

Phytoplankton growth may be further reduced by onset of low light (fall/winter) season

Conclusions

Neocalanus spp. exert a strong structuring effect on the nano-and pico-

food webs of the N. Pacific Ocean, tending to drive the food web towards a small-cell dominated system by

a)

selective removal of large cellsb)

cascade effect

These effects are strongest:•

at low food concentrations when copepod clearance rates are highest, and

•

under nutrient or food limiting conditions when growth rates of prey are lowest

The END