The Open Ocean Environment:
Plankton, Productivity and
Food Webs of the Sea
Chapters 7, 9, and 10
Plankton: Definitions
• Plankton: organisms living in the water
column, too small to be able to swim
counter to typical ocean currents. This term
refers to both animals and plants living in
the water column.
Plankton: Definitions 4
• Size classes
Ultraplankton
< 2 m
Nannoplankton
2-20 m
Microplankton
20-200 m
Macroplankton
200-2000 m
Megaplankton
> 2000 m
Phytoplankton (only plants)
Diatoms
•
•
•
•
•
•
•
•
Occur singly or form chains
Size range of nanno to microplankton
Encased in silica shell consisting of two valves (pillbox)
Usually radially symmetrical
Reproduce asexually by binary fission
Also sexual reproduction
Doubling once or twice per day usually
Dominate the seas over the world.
Phytoplankton
Dinoflagellates
•
•
•
•
•
•
Secrete organic test and have two flagellae
Size range of nanno and microplankton
Asexual and sexual reproduction
Often many life history stages
Many species are heterotrophic
Often abundant in tropics, mid-latitudes in
summer
• A few species are the cause of red tides
Red Tides
Red tides are caused by an
increase in nutrients in
seawater that causes an
increase in dinoflagellate
populations. Species that
cause red tides produce a
neurotoxic substance,
saxitoxin.
This substance is dangerous to
people. Shellfish (filter
feeders) ingest the
dinoflagellates and the toxin.
Their tissues also become
toxic.
Diatoms
Coccolithophore
Dinoflagellates
Flagellate Isochrysis
Zooplankton
Copepods
Females of different species with eggs
Zooplankton
Crustaceans - Euphausids (Krill)
Zooplankton
Gelatinous Zooplankton - Cnidaria
Note muscular bell and tentacles
Zooplankton
Gelatinous Zooplankton - Cnidaria
By-the-wind-sailor
Vellela
Porpita (ca. 10 cm wide) Physophora
(50 mm high)
Siphonophores
Zooplankton
Gelatinous Zooplankton - Ctenophores
Patchiness of the Plankton
• Plankton rarely distributed homogeneously
in the water column
• Plankton occur in spatially discontinuous
patches, sometimes distinct aggregations
• WHY?
Diurnal Vertical Migration of
Zooplankton
• Zooplankton rise to shallow water at night, sink to
deeper water during the day
• Found in many different groups of zooplankton
• Zooplankters usually start to sink before dawn,
and start to rise before dusk
• Cycle is probably an internal biological clock that
must be reinforced by day-night light changes
0
Night
100
200
300
400
0
Day
Twilight
1
2
Distance (km)
Vertical migration of planktonic shrimp Sergia lucens
3
Review Questions:
1)Why does diurnal vertical migration occur?
2) Do plankton have high or low Reynolds numbers?
Explain what a Reynolds number is.
3) How does plankton’s Reynolds number influence
feeding in copepods?
4) Define the term drag in marine hydrodynamics. What
adaptations do plankton have to decrease drag?
5) Why might it be important for plankton to stay in
surface waters and not sink to great depths?
6) In some marine environments, copepods have strong
diurnal migration and it others it is very weak. Why??
Critical Factors in Plankton
Abundance
Chapter 9
Spring Phytoplankton Increase
(or Spring Diatom Increase)
In midlatitudes, phytoplankton increase in
the spring, decline in summer, and may
increase to a lesser extent in fall.
Question: What factors may cause these
seasonal increases and declines?
Nutrients
at surface
Spring
Diatom
Increase Available
sunlight
Zooplankton
Winter
Spring
Summer
Fall
Winter
Variations on the Spring
Phytoplankton Increase
The spring phytoplankton peak and the
later zooplankton peak are shortest and
sharpest in high latitudes, becoming
indistinct in the tropics
Arctic
Phytoplankton
Herbivore
zooplankton
Temperate
Phytoplankton
Tropical
Herbivore
zooplankton
Phytoplankton
J F MA M J J A S O N D
Month
Productivity and Food Webs in
the Sea
Chapter 10
©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.
3. 2% transferred from sunlight to primary
producers; 10% transferred from primary
producers, herbivores and up.
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
Note: Great potential of upwelling areas
due to combination of high primary
production,
higher food chain efficiency, lower
number
of trophic levels
Why does lower number of trophic
levels increase potential of production?
Oceanic Food Webs
Open ocean,
gyre centers
Many
trophic
levels
Stable, low nutrient
Shelf,
upwelling
Few trophic
levels
Turbulent, high nutrient
North
America
North
America
Asia
North
Atlantic
North
Pacific
Africa
South
America
South
Atlantic
Indian
Ocean
South
Pacific
Antarctica
Primary production
(mg C/m2/day)
<100
100-150
150-250
>250
Satellite Images and
marine sciences
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
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
sun
Satellite
Irradiance
Color
scanner
Radiance
Phytoplankton
Satellite image of world productivity, from SeaWiFS satellite
What’s in this image?
How do sensors collect
satellite images?
What do the colors
represent?
How do we get these
colors?
Images are always collected
in black and white first.
A computer then adds color.
Gulf stream image with pixels
A satellite
image of this
Red tide event
helped
researchers
determine the
concentrations
of diatoms
and toxins in
the water.