The Deep Chlorophyll Maximum in Lake Superior

M.T. Auer, L.A. Bub and K.D. Elenbaas Department of Civil and Environmental

EngineeringMichigan Technological University

2000 HN210 Chl

seasonal development of the DCM

HN Transect - Chl (µg•L-1)4/28/00 6/22/00 7/30/00 8/25/00 10/20/00

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Reported in both freshwater and marine systems and related to a variety of environmental factors

System Factor Reference

Adriatic Sea Zooplankton grazingNutrient levels, distribution

Revelante and Gilmartin 1995

Baltic Sea Ciliate grazingSedimentation

Kononen et al. 1998

Mediterranean Sea

Zooplankton grazingNutrient levels, distributionPhysical factors

Gould and Wisenburg 1990

Lake Tahoe In situ growthSedimentationZooplankton grazing

Coon et al. 1987

Lake Michigan In situ growthSedimentationShade adaptation

Brooks and Torke 1977; Fahnenstiel and Scavia 1987

Finger Lakes of New York, New York City Reservoirs, et al.

In Lake Superior

Year Investigator Factor

1966 Olson and OdlaugPutnam and Olson

LightTemperatureZooplankton grazing

1975 Watson et al. Factors other than depth-variable settling differences

1978 Munawar and Munawar In situ growth

1983 Fahenstiel and Glime In situ growth

2001 Zhou et al. Light – nutrientsThermocline tilting

The Ecological

Enhanced growth due to conditions

of light, temperature,

nutrients, grazing, particularly

suitable for some or all taxa

The Physical

Accumulation of phytoplankton due

to differences in settling velocities

over the metalimnion

But it comes down to two camps …

Or more simply,

“Is the DCM a great place to live … or are they just hanging around?

The Ecological

For phytoplankton:

• nutrient supply

• temperature optima

• light efficiency

• chlorophyll adaptation

• grazing pressure

chemoautotrophs

nutrient supply

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DIP (µgP∙L-1)

Dep

th (

m)

C:P Ratios (molar)

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Feb On May On May Off Jun On Jun Off Jul On Jul Off

nutrient supply

ELF

Gloeocystis

ELF

Fragilaria

ELF (Enzyme-Labeled Fluorescence) is an alkaline phosphatase substrate which yields a bright yellow-green fluorescent precipitate upon enzymatic cleavage.

ELF-APA Data fromVickie Morris, MTU

nutrient supply

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Algal Biomass

(µgChl·L-1)

De

pth

(m

)

ELF Activity% of cells

81%

61%

65%

0.073

0.037

0.025

APA Vmax

µM/min

temperature optima

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Temperature (°C)

Pri

ma

ry P

rod

uct

ion

(mg

C∙µ

g C

hl∙L

-1∙h

r-1)

Optimum T @ ~15 °CT in DCM ~ 4-5 °C

light efficiency

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Light I ntensity

(µE m-2 s-1)

Spe

cifi

c Pr

oduc

tion

(mg

Chl•

µg

Chl-1

hr-1

)

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300 350 400 450 500 550 600 650 700

1%

lig

ht

level, m

wavelength, nm

HN090 - JD 239

3-5%

1-3%

0.6-1%

chlorophyll adaptation

Shade adaptation does account for some of the DCM, but certainly not all of it. The maximum is also present in the carbon data.

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POC (mg/L)

De

pth

(m

)

carbon data courtesy of Noel Urban

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C:Chl (mg/ug)

De

pth

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July and AugustOffshore Stations

grazing pressure

OPC scans provided by David J. Osterberg, MTU

temperature(0-20 m)

zooplankton(15-30 m)

chlorophyll(25-40 m)

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Chlorophyll (g L-1)

Production (mg C m-3 h-1)

August 1999 HN210

PC

2

PC1

5m22m

50m

Carbon Source Utilization

let’s ask the bacteria

… and furthermore

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BCR(mg C m-3 h-1)

Excretion(mg C m-3 h-1)

Dep

th (

m)

Carbon excretion by actively photosynthesizing phytoplankton is able to satisfy the bacterial carbon requirement (BCR) only in the surface waters.

The Physical

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6 8 10 12

settling velocity (cm∙ s-1)

Depth

(m

)

2

18solid liquidg d

velocity

The Expanding Universe

6/22/00

7/30/00

9/24/00

T °C

Chl (µg/L)

Conversation Overheard at 30m …

Hey! Is this a great place

to live?

Naw. We’re just

hangin’ around.

acknowledgements:

For field support:

Captain & Crew of the R/V Laurentian

For assistance in data analysis:

Jae Jeong, MTU

For data:

Angela Cates, LLOJudy Budd, MTUKim Elenbaas, MTUSarah Green, MTUVickie Morris, MTUDave Osterberg, MTUNoel Urban, MTU

For financial support:

National Science Foundation