IoE 184 - The Basics of Satellite Oceanography. 7. Ocean Color and Phytoplankton Growth Lecture 7 Ocean Color and Phytoplankton Growth

IoE 184 - The Basics of Satellite Oceanography. 7. Ocean Color and Phytoplankton Growth

Lecture 7Ocean Color and Phytoplankton Growth

This lecture includes the following topics:

1. Chlorophyll and photosynthesis

2. Vertical distribution of phytoplankton in the ocean

3. Estimation of phytoplankton biomass from satellite ocean color observations

4. Estimation of chlorophyll fluorescence from MODIS ocean color observations

5. Estimation of coccolithophores concentration and harmful algal blooms


6. Seasonal cycles of phytoplankton biomass

7. Global phytoplankton biomass and primary production


Green color of plants, including phytoplankton, is a result of plant pigments, primarily chlorophyll a.


Chlorophyll absorbs light energy and stores it in the form of chemical agent ATP. This energy is used for the synthesis of organic matter.



The synthesis of organic matter by plants (primary production) is a basic source of food for all living organisms.




Photosynthesis cannot proceed without light; so, in the deep layer phytoplankton growth is “light limited”

Another requirement for photosynthesis is a sufficient concentration of nutrients (nitrates, phosphates, iron, etc.).

In stratified water nutrients in the upper mixed layer are consumed by phytoplankton; so, phytoplankton growth there is “nutrient limited”. 50

40

30

20

10

0

Depth (m)

Nutrient

Light

Phytoplankton



50

40

30

20

10

0

Depth (m)

Nutrient

Light

Phytoplankton

The growth of phytoplankton occurs in the layer where both light and nutrient concentration are sufficient.

With increase of the upwelling nutrient flux the conditions of phytoplankton growth become better and the maximum of its vertical distribution moves to more shallow layer.



50

40

30

20

10

0

Depth (m)

Nutrient

Light

Phytoplankton







50

40

30

20

10

0

Depth (m)

Nutrient

Light

Phytoplankton



When nutrient flux is very intensive and phytoplankton biomass is high, the maximum of vertical distribution of phytoplankton is located at the surface.

The result is a direct correlation between total phytoplankton (or chlorophyll) concentration in water column (or within the euphotic layer) and in the thin surface layer.

50

40

30

20

10

0

Depth (m)

Nutrient

Light Phytoplankton



Hence, both the surface chlorophyll concentration and the chlorophyll concentration above the penetration depth can be used as a measure of water productivity, i. e., phytoplankton biomass.

50

40

30

20

10

0

Depth (m)

Nutrient

Light Phytoplankton



Vertical distribution of ecosystem characteristics at a typical station in the oligotrophic waters shows deep phytoplankton maximum and nutricline.



Vertical distribution of ecosystem characteristics at a typical station in the eutrophic boreal waters shows shallow phytoplankton maximum and nutricline.



What layer contributes to the color of ocean surface?

50

40

30

20

10

0

De

pth

(m

)

Light Vertical attenuation of sun light (I) with depth (Z) can be described by exponential equation

Iz = I0*exp(-k*Z)

Deeper from the surface - less light is reflected or scattered by phytoplankton cells and contributes to the color of ocean surface.




Iz = I0*exp(-k*Z)Coefficient K is called “attenuation coefficient”; it is measured in 1/m.

The value 1/K is called “attenuation length”, and the layer of this length is called

“penetration depth” (Zpd).

Another depth used in ocean optic is “euphotic depth” (Ze); it is

defined as the depth where the downwelling PAR (Photosynthetically Active Radiation) is reduced to 1% of its value at the surface.

These two values are related by empirical equation Zpd Ze / 4.6




The Csat (averaged concentration

"seen" by a remote sensor) is computed as follows:

∫∫

⋅−

⋅−⋅= Zpd

Zpd

satdzKZ

dzKZzCC

0

0

)2exp(

)2exp()(

Csat is correlated with phytoplankton biomass in water column.




Empirical models are based on direct correlations between normalized water-leaving radiation and chlorophyll concentration.

Semi-analytic models are based on the Inherited Optical Properties (IOPs) of water column, i.e., absorption and backscattering of different water constituents (phytoplankton, suspended sediments, CDOM, etc.).

It is assumed that chlorophyll concentration in phytoplankton is a constant. In practice, chlorophyll content varies within a wide range.

4. Estimation of chlorophyll fluorescence from MODIS ocean color observations.

Recent studies of the College of Oceanic and Atmospheric Sciences at Oregon State University.



p + f + h = 1Light energy not used for photosynthesis is lost as heat and fluorescence.



Light emitted by chlorophyllLight emitted by chlorophyll

Light absorption by algal pigments

Light absorption by algal pigments



Regular method to

calculate Chl fluorescence :

use Fluorescence Line Height



Fluorescence can be used as another measure of chlorophyll, but only in chlorophyll-rich water, because the optical signal produced by chlorophyll absorption substantially exceeds the signal of fluorescence.

ADVANTAGE: Absorption-based algorithms fail in waters where there are other materials that absorb and scatter and are not correlated with chlorophyll

–Sediment–Dissolved organic matter

Chlorophyll fluorescence is specific to chlorophyll

LIMITATION: it also depends on physiology



MODIS successfully estimates FLH from space even in low chlorophyll case 1 waters



ΦP + ΦF + Φh = 1P – photosynthesis; F – fluorescence; H – heat.

Φh (heat) is assumed a constant;

Estimation of chlorophyll concentration

assumes ΦF constant

Estimation of primary production

assumes a predictable relationship

between ΦF and ΦP



Physiological parameters (APR and CFE) vary spatially.



The patterns of variability of phytoplankton physiology estimated from fluorescence can be used in for evaluation of photosynthesis and primary production.


5. Estimation of coccolithophores concentration and harmful algal blooms.

Coccolithophores are small algae containing coccoliths – inorganic carbon structures.

The blooms of coccolithophores result in very intensive water surface backscattering and hinder remote-sensed estimation of phytoplankton biomass.



From typical to coccolithophores backscattering spectra we can now estimate the biomass of these algae.



Optical measurements of MODIS enable estimation of not only chlorophyll a concentration, but also concentrations of pheopigments, which are produced by zooplankton during grazing.

Decreased concentration of pheopigments indicates absence of grazing typical to harmful algal bloom.



This image shows the area of harmful algal bloom near the Pacific coast of Central America.




One of the main goals of remote-sensing observations is the study of seasonal cycles of phytoplankton biomass in different regions of the World Ocean.

In many regions these cycles repeat every year including minor details. This pattern is a result of seasonal oscillations of physical environment.

In high latitudes these oscillations are more pronounced, and the response of phytoplankton is more evident.



Phytoplankton

Zooplankton

Typical pattern of seasonal variations of phytoplankton in temperate latitudes is known since the beginning of 20th century.

The main feature is a short-period (1-2 weeks) "vernal bloom" called in parallel with seasonal cycle of terrestrial plants. The cycle contains the period of exponential growth and then abrupt decrease resulting from grazing of phytoplankton by zooplankton.



50

40

30

20

10

0

Depth (m)

Nutrient

Light

Phytoplankton

The hydrological conditions of start of the spring bloom of phytoplankton were described and explained by Harold Sverdrup in 1953. He attributed the beginning of spring bloom to the formation of seasonal thermocline, when the upper mixed layer is separated from deeper water column and phytoplankton Is retained in illuminated (euphotic) layer.

The strengthening of seasonal thermocline in summer results in nutrient limitation of phytoplankton growth.



50

40

30

20

10

0

Depth (m)

Nutrient

Light

Phytoplankton

Stratification within the euphotic layer is a primary factor controlling phytoplankton growth. We consider two main factors limiting phytoplankton growth: illumination and nutrients.

Light limitation is crucial under low stratification (e. g., winter convection), because algae cells are dispersed by turbulent mixing within deep dark layers.Nutrient limitation is crucial under enhanced stratification (e. g., seasonal thermocline in summer), because nutrients do not penetrate into the euphotic (i. e., well illuminated) upper mixed layer.

The hydrometeorological factors (heat flux, wind, freshwater load with precipitation and river discharge) either increase or decrease the stratification within the euphotic layer.



Typical seasonal cycles of phytoplankton result from the combined effect of seasonal cycles of hydrometeorological factors influencing water stratification within the euphotic layer. The most illustrative is the phytoplankton seasonal cycle in mid-latitudes with two maxima in spring and autumn:



Season Hydrometeorological factors

Stratification Phytoplankton growth

Winter Maximum wind mixing;

Maximum cooling of upper layer

Deep convection

Winter minimum resulting from light limitation

Spring Wind mixing weakens;

Heating of upper layer increases

Formation of seasonal thermocline

Spring bloom

Summer Maximum heating of upper layer; Minimum wind mixing

Maximum stratification

Summer minimum resulting from nutrient limitation

Fall Cooling of upper layer increases; Wind mixing increases

Erosion of seasonal thermocline

Autumn bloom



In high latitudes (cold and windy) winter minimum is more pronounced and summer minimum is less pronounced.

In low latitudes (warm and less windy) winter minimum is less pronounced or absent and summer minimum is more pronounced.

Deviations from typical seasonal pattern of hydrometeorological factors result in local peculiarities of phytoplankton cycle.



Typical seasonal cycles of phytoplankton described by Alan Longharst are given below. He distinguishes eight types of cycle. The figures illustrate pigment concentration (Chl), primary production (P), mixed layer depth (Zm), and the period when the picnocline is illuminated (■ ■).












Near Newfoundland two different water masses (cold Labrador Current and warm Gulf Stream) are separated by frontal zone.

70 W 65 W 60 W 55 W 50 W 45 W

45 N

50 N

70 W 65 W 60 W 55 W 50 W 45 W

45 N

50 N

70 W 65 W 60 W 55 W 50 W 45 W

45 N

50 N

70 W 65 W 60 W 55 W 50 W 45 W

45 N

50 N

February 10-14 April 11-15

July 15-19 October 12-16

40 N

40 N

40 N

40 N

-2

0

2

4

6

8

10

12

14

16

18

20

22

24

30

T (oC)



Four small regions were selected in the zones of influence of the Labrador Current, the Gulf Stream, and over shallow and deep parts of the Newfoundland Bank.

70 W 65 W 60 W 55 W 50 W 45 W 40 W

70 W 65 W 60 W 55 W 50 W 45 W 40 W

40 N

42 N

44 N

46 N

48 N

50 N

52 N

40 N

42 N

44 N

46 N

48 N

50 N

52 N

Newfoundland

Nova Scotia

Labrador

Gulf of

St. Lawrence

The Grand Bank

Flemish Cap

Tail ofthe Bank

Whale Bank

St. Pierre Bank

Cabot Strait

5000 m

4000 m

3000 m2000 m

1000 m200 m

200 m

2000 m

4000 m3000 m

Strait of Belle Isle

Avalon Peninsula

Avalon Channel

1000 m

1

3

2

4

Seasonal patterns were typical to Arctic, coastal, mid-latitude, and subtropical regions.



199719981999

Region 1(Labrador Current)

3

0.1

1.0

10.0

SeaWiFS Chlorophyll (mg/m )

3

Region 2(The Grand Bank)

0.1

1.0

10.0

0.1

1.0

10.0

SeaWiFS Chlorophyll (mg/m )

0.1

1.0

10.0

Region 3(Flemish Pass)

Region 4(Tail of the Bank)

J F M A M J J A S O N D J F M A M J J A S O N D

Seasonal patterns were typical to Arctic, coastal, mid-latitude, and subtropical regions.


6. Seasonal cycles of phytoplankton biomass70W 60W 50W 40W

40N

45N

50N

40N

45N

50N

0.0 0.5 0.7 1.0 1.5 2.0

60W 50W 40W

Chl (mg/m3 )

April 1946-1990

April 1999

April 1979-1986

April 1999

5 m/s

In April 1999 unusual wind pattern (weak wind in northern part and strong wind in southern part) resulted in stronger bloom of phytoplankton. Weak wind in northern (light-limited) zone enhanced stratification; strong wind in southern (nutrient-limited) zone eroded thermocline; both favored phytoplankton growth.



These images show seasonal variations of plant pigment concentration in in the Ligurian Sea.

January 1998,

March 1998, and

August 1998

Subtropical seasonal cycle with summer minimum and higher chlorophyll concentration during winter-spring is evident.



These images show seasonal variations of plant pigment concentration in in the Ligurian Sea.

March 1999,

April 1999, and

May 1999

In 1999 typical to mid-latitudes spring bloom was observed.



8 9 10 11 12 13 14 15

Difference in air temperature between August and November (oC)

1

2

3

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

Ma x

imum

mon

thly

CZC

S c

hlor

ophy

ll (m

g /m

3 )

78/7979/80

80/81

81/82

82/83

83/8484/85

85/86

97/98

98/99

Significant correlation was revealed between air temperature contrast in autumn and the magnitude of spring bloom next spring (CZCS and SeaWiFS data).



1997 1998

0

1

2

3

4

5

Precipitation (mm/day)

-8

-4

0

4

8

-6

-2

2

6

Air temperature anomaly in Nice (oC)

1999

Mean 1961-1990

1997-1999

S O N D J F M A M J J A S O N D J F M A M J J A S O

0

2

4

6

8

Wind speed (m/s)

-1

0

1

2

SST anomaly (oC)

0.1

1

0.2

0.3

0.4

0.50.60.70.80.9

2

SeaWiFS pigment concentration (mg/m3)

1997 1998 1999

Cold and windy autumn of 1998 preceded vigorous spring bloom in spring 1999. Explanation: 1. Deeper winter convection and enrichment of the upper layer with nutrients. 2. Cold winter and warm spring favors formation of seasonal thermocline.



These SeaWiFS monthly composite images averaged over 1997-2001 and NCEP wind data illustrate seasonal variations of California upwelling.

Stronger wind and increased phytoplankton biomass are observed in summer.













































MODIS OCEAN NET PRIMARY PRODUCTION (ONPP)

Authors: Behrenfeld & Falkowski

Net Primary Production

NPP = f (Chl a, PAR, SST)

Integrated over the Euphotic zone (i.e., the depth of 1% of incident

Photosynthatically Available Radiation - PAR)


7. Global phytoplankton biomass and primary production


Chlorophyll a

Input fields (measured by MODIS):



Photosynthetically Available Radiation




Sea Surface Temperature




H – day length (hours);

PAR - Photosynthetically Available Radiation;

Chl – Surface Chlorophyll a concentration;

Zeu - Depth of euphotic zone (power function of Chl);Pb

opt - Optimal Photosynthetic Yield (7-th order polynomial function

of Sea Surface Temperature).

Equation:



Resulting NPP are estimated for 8-day intervals at global grid of 4.5-km resolution.



Documents

IoE 184 - The Basics of Satellite Oceanography. 7. Ocean Color and Phytoplankton Growth Lecture 7 Ocean Color and Phytoplankton Growth