10 Productivity and Food Webs
in the Sea
Notes for Marine Biology:
Function, Biodiversity, Ecology
By Jeffrey S. Levinton
©Jeffrey S. Levinton 2001
Productivity vs biomass
Biomass the mass of living material
present at any time, expressed as grams
per unit area or volume
Productivity is the rate of production of
living material per unit time per unit area
or volume
Productivity
Primary productivity - productivity due to
Photosynthesis
Secondary productivity - productivity due to
consumers of primary producers
Food Chain
Food chain - linear sequence showing
which organisms consume which other
organisms, making a series of trophic levels
Food web - more complex diagram showing
feeding relationships among organisms, not
restricted to a linear hierarchy
Food Chain Abstraction
Food chain
Food Web
Adult
herring
Herring
Young herring
arrowworm
Copepod
amphipod
sand eel
Larger
copepod
Barnacle Mollusk
larvae larvae
Phytoplankton
Small
copepods
cladocerans
euphausid
Phytoplankton
tunicate
Transfer Between Trophic Levels
Transfer from one trophic level to the
next is not complete:
1. Some material not eaten
2. Not all eaten is converted with 100%
efficiency
Transfer Between Trophic Levels 2
Budget for ingested food (use energy units):
I=E+R+G
I amount ingested
E amount egested
R amount respired
G growth (partitioned between somatic
growth and reproduction)
Transfer Between Trophic Levels 3
Incomplete transfer up a food chain:
Measure by food chain efficiency:
E = amount extracted from a trophic level
amount of energy supplied to that level
Often in range of as little as 10%
Transfer Between Trophic Levels 4
Use food chain efficiency to calculate
energy available to highest trophic level:
P = BEn
B = primary production
P = production at highest level
E = food chain efficiency
N = number of links between trophic levels
Transfer Between Trophic Levels 4
Use food chain efficiency to calculate
energy available to highest trophic level:
P = BEn
Let E = .1, B = 1, n = 2,3,4
If n = 2, P = ?
Transfer Between Trophic Levels 4
Use food chain efficiency to calculate
energy available to highest trophic level:
P = BEn
Let E = .1, B = 1, n = 2,3,4
If n = 2, P = ?
P = 1 x (0.1)2 = 1 x 0.01 = 0.01
Transfer Between Trophic Levels 4
Use food chain efficiency to calculate
energy available to highest trophic level:
P = BEn
Let E = .1, B = 1, n = 2,3,4
If n = 3, P = ?
P = 1 x (0.1)3 = 1 x 0.1 x 0.1 x 0.1 = 0.001
Transfer Between Trophic Levels 5
Use food chain efficiency to calculate
energy available to highest trophic level:
P = BEn
With 5 trophic levels, a change of E from
0.1 to 0.2 magnifies P by a factor of 16
Oceanic Food Webs
Food webs in the oceans vary
systematically in food chain efficiency,
number of trophic levels, primary
production
Oceanic Food Webs
Food Chain
Primary
Trophic
Food
Potential
Type
Productivity Levels
Chain
Fish
Efficiency Production
gCm-2y-1
mgCm-2y-1
Oceanic
50
5
10
0.5
Shelf
100
3
15
340
Upwelling
300
1.2
20
36,000
Oceanic Food Webs
Food Chain
Primary
Trophic
Food
Potential
Type
Productivity Levels
Chain
Fish
Efficiency Production
gCm-2y-1
mgCm-2y-1
Oceanic
50
5
10
0.5
Shelf
100
3
15
340
Upwelling
300
1.2
20
36,000
Oceanic Food Webs
Food Chain
Primary
Trophic
Food
Potential
Type
Productivity Levels
Chain
Fish
Efficiency Production
gCm-2y-1
mgCm-2y-1
Oceanic
50
5
10
0.5
Shelf
100
3
15
340
Upwelling
300
1.2
20
36,000
Oceanic Food Webs
Food Chain
Primary
Trophic
Food
Potential
Type
Productivity Levels
Chain
Fish
Efficiency Production
gCm-2y-1
mgCm-2y-1
Oceanic
50
5
10
0.5
Shelf
100
3
15
340
Upwelling
300
1.2
20
36,000
Oceanic Food Webs
Note: Great potential of upwelling areas
due to combination of high primary production,
higher food chain efficiency, lower number
of trophic levels
Oceanic Food Webs
Open ocean,
gyre centers
Many
trophic
levels
Stable, low nutrient
Shelf,
upwelling
Few trophic
levels
Turbulent, high nutrient
Measuring Primary Productivity
Gross primary productivity - total carbon fixed
during photosynthesis
Net primary productivity - total carbon fixed
during photosynthesis minus that part which
is respired.
Measuring Primary Productivity 2
Net Primary productivity most interesting:
gives that part of the production available to
higher trophic levels
Measuring Primary Productivity 3
Oxygen technique Principle - relies upon fact that
oxygen is released during photosynthesis
CO2 + 2H2O ---> (CH2O)n + H2O + O2
Measuring Primary Productivity 4
Oxygen technique 2 - there is an addition from
photosynthesis and a subtraction from
respiration
Measuring Primary Productivity 5
Oxygen technique 3 Measurement of oxygen:
Winkler technique - chemical titration of
Oxygen
Polarographic oxygen electrode -
Measuring Primary Productivity 6
Oxygen technique 4 Light-Dark bottle technique:
Light bottle gives oxygen from photosynthesis
minus oxygen consumed in respiration
Dark bottle gives oxygen consumed from
Respiration
Measuring Primary Productivity 7
Oxygen technique 5 Light-Dark bottle technique:
Start light and dark bottles with water sample, wait
a short amount of time
At end of experiment: oxygen in light minus that in
dark bottle gives you gross photosynthesis
Measuring Primary Productivity 8
Oxygen technique 6 Light-Dark bottle technique:
Get change of oxygen, but need to convert
to units of carbon. If phytoplankton are
synthesizing sugars, multiply change in
oxygen atoms by 375 to get equivalent in
carbon
Oxygen technique - effect of depth
Measuring Primary Productivity 9
Radiocarbon technique Principle: carbon is taken up by cells
During photosynthesis, so if you label
that carbon you can trace it as it is
incorporated into cells during photosynthesis.
Measuring Primary Productivity 10
Radiocarbon technique 2 Method: add bicarbonate to solution
With phytoplankton that is labeled with
14C
Measuring Primary Productivity 11
Radiocarbon technique 3 Method: add bicarbonate to solution
With phytoplankton that is labeled with
14C
Incubate phytoplankton in the radiocarbon
Solution
Then filter phytoplankton and count radiocarbon
Taken up by phytoplankton, using a scintillation
counter
Measuring Primary Productivity 12
Radiocarbon technique 4 Calculation:
1. Know the amount of bicarbonate that was
in container
2. Know the amount of radiolabeled bicarbonate
you added and the amount that was taken up by
Phytoplankton
 allows calculation of amount of bicarbonate
taken up in photosynthesis
Measuring Primary Productivity 13
Radiocarbon technique 5 Correction:
14C is taken up more slowly than much
more common stable isotope 12C. Therefore,
--> need to multiply results by 1.05 to get
amount in photosynthesis
Measuring Primary Productivity 14
Radiocarbon technique 6 What you get with this measure:
Carbon incorporation into phytoplankton
(net photosynthesis)
Measuring Primary Productivity 15
Compare Oxygen technique with radiocarbon:
Oxygen technique - used where primary
production is high in estuaries, shelf
Radiocarbon technique - useful where primary
production is low such as open ocean
Measuring Primary Productivity 16
Compare Oxygen technique with radiocarbon 2:
Oxygen technique tends to give higher estimates
of primary production, perhaps because
cells are leaking sugars during photosynthesis,
resulting in loss of radiocarbon when cells
are filtered and counted
Measuring Primary Productivity 17
Satellite Approaches:
Satellites can use photometers specific to
wavelength to measure chlorophyll,
Seawater temperature
Need ground truthing to get relationship
Between chlorophyll concentration and
primary production; varies with region
Measuring Primary Productivity 18
Satellite Approaches 2:
Satellites can use photometers specific to
wavelength to measure chlorophyll,
Seawater temperature
Need ground truthing to get relationship
Between chlorophyll concentration and
primary production; varies with region
sun
Satellite
Irradiance
Color
scanner
Radiance
Phytoplankton
Geographic Variation of Productivity
1. Continental shelf and open-ocean upwelling
Areas are most productive
2. Convergences and fronts often are sites of
rise of nutrient rich deep waters (e.g., shallow
water seaward of slope
3. Central ocean, gyre centers are nutrient poor,
low primary production
North
Atlantic
North
Pacific
Indian
Ocean
South
Atlantic
Antarctic
South
Pacific
Satellite image of world productivity, from SeaWiFS satellite
The End