What do phytoplankton in the McMurdo Dry Valley lakes do when the sun sets?

Introduction

The McMurdo Dry Valleys (MDV) is the largest ice-free area on the Antarctic continent,

and is the study site of the Long Term Ecological Research (LTER) program. Research

on ice-covered lakes in the Taylor Valley has been restricted to the austral spring and

early summer (~October through January) when logistical support has allowed access

to the area. The International Polar Year 2007-2008 (IPY) provided the first opportunity

to study biological adaptation/acclimation by phytoplankton during the transition from

24h sun to the polar night. We present phytoplankton succession and diversity from

West Lobe Bonney (WLB) and Lake Fryxell (FRX) (Fig 1).

Study site

The MCM lakes contain stable water columns resulting

from perennial ice covers, low advective stream inflow,

and strong chemical gradients (Fig 2) which create

distinct environments for phytoplankton populations.

Methods

Changes in under-ice photosynthetically active

radiation (PAR) and primary production (PPR) were

measured from early November through mid-April. During this same period, phytoplankton diversity

was determined in-situ using a submersible

spectrofluorometer (Fig 3) which differentiates the

following groups: Cyanobacteria, Chlorophyta,

Chrysophyta, Cryptophyta.

Results I: PAR, PPR, total chl-a as the sun sets

Water column PAR decreased significantly in March and was no longer measureable by

early April (Fig 4a). PPR reflected changes in PAR and ceased by mid-April (Fig 4b).

Total chl-a did not decrease in direct response to diminished PPR (Fig 4c).

Conclusions

Results II: Phytoplankton group responses as the sun sets

WLB: All groups increased in March and April as PAR decreased, with Chlorophytes,

Chrysophytes and Cryptophytes peaking at the chemocline, while the highest abundance of

cyanobacteria occurred below the chemocline, which may be the result of fall-out from the

permanent ice cover (Fig 5).

FRX: Chrysophyte and Chryptophyte biomass peaked during late Nov/Dec, and increased

again during March/April as PAR started to decrease. The highest abundance of

cyanobacteria occurred at and below the chl-a peak in early April, likely the result of fall-out

from the permanent ice cover. Chlorophytes were the only group to show a decrease during

April (Fig 5).

Results III: Phytoplankton diversity as the sun sets

-1 0 1 2 3

0

2

4

6

8

10

12

14

16

18

0 2 4 6 8 10

Temperature (C)

Dep

th (

m)

Conductivity (mS/cm) Salinity (PSU) FRX

Conductivity

Salinity

Temperature

Ice

-6 -4 -2 0 2 4

0

5

10

15

20

25

30

35

40

0 50 100 150

Temperature (C)

Dep

th (

m)

Conductivity (mS/cm) Salinity (PSU) WLB

Conductivity

Salinity

Temperature

Ice

Phytoplankton diversity in WLB

increased below the chemocline

during March and April (Fig 6).

Amy L. Chiuchiolo1, John C. Priscu1, Rachael M. Morgan-Kiss2

1Dept of Land Resources and Environmental Sciences, Montana State University 2Dept of Microbiology, Miami University

MCM Dry

Valleys

Fig 2. Temperature, salinity and conductivity profiles for each study lake.

• Phytoplankton biomass does not respond to diminishing PAR despite diminished PPR

• Phytoplankton diversity shifted through the season and increased as the polar night

approached

• These data indicate that phytoplankton may shift from photosynthesis to mixotrophy

during the polar night as a mechanism for winter survival, and that fall-out from the ice

cover can influence water column diversity.

Fig 3. Schematic of the bbe Fluoroprobe. Cellular pigments

are excited at 450, 525, 570, 590, and 610 nm and chl-a

emission is measured at 683 nm. Algal groups are

differentiated by light absorption by specific accessory

pigment complexes.

Fig 4. Contour plots of UW PAR (a) and PPR and total chl-a integrated over the photic zone (4-

20m in WLB; 5-12m in FRX).

Fig 1. Location of the MCM Dry Valleys (77 °S, 163°E) and the study lakes in the Taylor Valley.

Fig 6. Photic zone phytoplankton diversity based on in vivo pigment fluorescence. Pi values for Shannon-

Weiner Diversity Index H’ were calculated using the proportion of fluorescence for each group to total chl-

a. Black dots represent data points; white line represents the chemocline.

Phytoplankton diversity in FRX increased

over time in the upper half of the water

column, with the highest diversity found at the

10m chl peak between Feb and Mar (Fig 6).

Fig 5. Phytoplankton concentration in the photic zones of WLB and FRX. Black dots represent data

points; white line represents the chemocline.

WLB FRX

De

pth

(m

)

5

6

7

8

9

1 0

1 1

1 2

0 . 0

0 . 5

1 . 0

1 . 5

2 . 0

2 . 5

F R X in t e g r a te d p h o t ic z o n eP P R

mg

C

m

-2

d-1

0

6

1 2

1 8

2 4

F R X in t e g r a te d p h o t ic z o n eto ta l c h l - a

2 0 0 7 - 2 0 0 8

S e p O c t N o v D e c J a n F e b M a r A p r

mg

m-2

3 23 64 04 44 85 2

F R X u n d e r - ic e P A R

( µ m o l m - 2 s - 1 )

De

pth

(m

)

4

6

8

1 0

1 2

1 4

1 6

1 8

2 0

0 .0

5 .0

1 0 .0

1 5 .0

2 0 .0

W L B in te g ra te d p h o t ic z o n e P P R

m

g C

m

-2

d-1

051 01 52 02 5

W L B in te g ra te d p h o t ic z o n eto ta l c h l- a

2 0 0 7 -2 0 0 8

S e p O c t N o v D e c J a n F e b M a r A p r

mg

m-

2

3 04 05 06 07 08 0

W L B u n d e r - ic e P A R

( µ m o l m - 2 s -1 )

Acknowledgements

Chlorophyta (µg chl-a L-1)

2007-2008

Dec Jan Feb Mar Apr

Depth (m)

5

6

7

8

9

10

11

12

0.0

2.0

4.0

6.0

8.0

Cyanophyta (µg chl-a L-1)

2007-2008

Dec Jan Feb Mar Apr

0.0

0.2

0.4

0.6

0.8

1.0

Cryptophyta (µg chl-a L-1)

2007-2008

Dec Jan Feb Mar Apr

0.0

4.0

8.0

12.0

Chrysophyta (µg chl-a L-1)

2007-2008

Dec Jan Feb Mar Apr

0.0

2.0

4.0

6.0

8.0

10.0

Chlorophyta (µg chl-a L-1)

Dec Jan Feb Mar Apr

Depth (m)

4

6

8

10

12

14

16

18

20

0.0

2.0

4.0

6.0

8.0

Cyanophyta (µg chl-a L-1)

Dec Jan Feb Mar Apr

0.0

0.1

0.2

0.3

Chrysophyta (µg chl-a L-1)

Dec Jan Feb Mar Apr

0.0

2.0

4.0

6.0

8.0

10.0

Cryptophyta (µg chl-a L-1)

Dec Jan Feb Mar Apr

0.0

1.0

2.0

3.0

WLB

FRX

FRX Shannon-Weiner Diversity Index H'

2007-2008

Dec Jan Feb Mar Apr

Depth

(m

)

5

6

7

8

9

10

11

12

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

W L B S h a n n o n -W e in e r D iv e rs ity In d e x H '

2 0 0 7 -2 0 0 8

D e c J a n F e b M a r A p r D

ep

th

(m

)

4

6

8

1 0

1 2

1 4

1 6

1 8

2 0

0 .0

0 .2

0 .4

0 .6

0 .8

1 .0

1 .2

1 .4

Funding was provided by NSF-OPP 0423595 (LTER) and NSF OPP 0631494 (IPY). Special thinks to

Raytheon Polar Services for logistical support during the Polar Night project, and to the 2007-2008

MCM LTER team members for field and lab support.

a a

b b

c c