1. 9 Critical Factors in Plankton Abundance
Notes for Marine Biology: Function, Biodiversity, Ecology
By Jeffrey S. Levinton

2. 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

3. Nutrients
at surface
Spring
Diatom
Increase
Available
sunlight
Zooplankton
Winter Spring Summer Fall Winter

4. Month Arctic Phytoplankton Herbivore zooplankton Temperate
Tropical
Herbivore
zooplankton
Phytoplankton
J F M A M J J A S O N D
Month

5. Mechanisms causing the spring phytoplankton bloom and its decline

6. Light and Phytoplankton - concepts
Consider a phytoplankton cell held in jar
at a certain depth ( = a certain light intensity:
Compensation depth - that depth at which
oxygen produced by a phytoplankton cell in
photosynthesis equals the oxygen consumed in
respiration

7. Light and Phytoplankton - concepts 2
Consider a phytoplankton cell held in jar
at a certain depth ( = a certain light intensity:
Compensation light intensity - the light
intensity corresponding to the compensation
depth

8. Before the spring phytoplankton increase:
Water density similar at all depths
Wind mixing homogenizes water column

9. Cause of the spring phytoplankton increase:
Important concepts:
Mixing depth - depth above which all water
is thoroughly mixed, due to wind.
Critical depth - depth above which total oxygen
coming from primary production in the water
column equals total consumption (respiration)
If: Mixing depth < Critical depth: bloom
If: Mixing depth > Critical depth: no bloom

10. Total photosynthesis, P
Total respiration, R
Total photosynthesis, P
Mixing depth 1: area P > R
Mixing depth 2: area P < R
Oxygen production ( P ) or respiration ( R )
Compensation
depth
Mixing
depth 1
Critical
Depth
Mixing
depth 2
Depth

11. Cause of the spring phytoplankton increase:
Important concepts 2:
Key processes:
1. Water column becomes
more stable in spring as sun heats
water from above.
2. Surface nutrients are rich and trapped in
surface waters.
3. Phytoplankton cells are no longer stirred
to darker deep waters ----> BLOOM!!

12. Decline of the Spring Phytoplankton Increase
Why do phytoplankton decline?
Water column is STABLE
In shallow water shelf waters: diatoms start
sinking from surface water to bottom,
which removes nutrients.

13. Decline of the Spring Phytoplankton Increase 2
Why do phytoplankton decline?
Zooplankton grazing? Has some effect but
often secondary to sinking

14. Rejuvenation of conditions for the Spring Phytoplankton Increase
Why do phytoplankton increase again in Fall?
In fall and winter: water cools, water column
becomes isothermal with depth, wind mixing
restores nutrients to surface waters until
conditions are right next spring

15. Water column exchange in shallow waters and estuaries
In very shallow estuaries, nutrient exchange
or benthic-pelagic coupling, occurs between
the bottom and the water column, fueling
more phytoplankton growth

16. Water column exchange in shallow waters and estuaries 2
Beach phytoplankton blooms

17. Water column exchange in shallow waters and estuaries 3
In estuaries, the spring freshet combines
with net water flow to the sea and mixing to
determine nutrient regime:
1. Freshwater rivers create a net downstream flow
2. Tides cause mixing up and down estuary as
well as vertical mixing
3. Nutients may be released to coastal zone

18. Water column exchange in shallow waters and estuaries 4
Important factors in nutrient exchange:
Residence time - time water remains in estuary
before entering ocean
2. Rate of nutrient input from watershed
3. Nutients may be released to coastal zone

19. Light Two components of loss in the water Column:
Absorption: Molecular absorption
of light energy
Scattering: Light interaction with
particles

20. Light 2 Penetration into water column varies: with wavelength
Clear Open Ocean Water: Maximum
penetration at 480 nm
Turbid inshore water: Maximum
penetration at nm

21. Light 3 Ultraviolet light strongly attenuated In water column:
Inshore waters: Incident light with wl
of 380 nm or less is almost attenuated
at depth of 1-2 m
Clear open ocean water: 20 m may
be required to remove 90% of surface
incident light

22. Photosynthesis in Water Column
Phytoplankton species may use
Chlorophyll a, c and “accessory
pigments”, which absorb
energy over the light spectrum

23. Photosynthesis in Water Column 2
Action spectrum - utilization of
different wavelengths of light by
a given species for photosynthesis
Chlorophyll absorbs wavelengths of
mainly > 600 nm
Accessory pigments absorb wavelengths
< 600 nm

24. Photosynthesis in Water Column 3
Photosynthesis increases with
increasing light intensity to a
plateau, Pmax, then decreases

25. + _ Pmax Photosynthetic rate Net Gross Photosynthesis photosynthesis
Compensation point
Respiration
Light intensity (I)

26. Nutrients Nutrients are substances required by plants.
They are resources that can be limited in
supply
Nutrient dependence and use:
Autotrophs, auxotrophs, heterotrophs

27. Nutrients 2 Nitrogen - what for? Nitrates NO3 - Nitrites NO2 -
Ammonium ion, NH4 - excretion product
recycling from animal excretion in the
water column

28. Nutrients 3 Nitrogen - uptake depends upon source
Ammonium ion is taken up the fastest
by phytoplankton cells, requires the least
chemical alteration (no reduction), but
Nitrate is usually the most abundant source
Of nitrogen in shallow coastal water columns

29. Nutrients 4 Nitrogen - New vs. Regenerated Production New Production:
Nutrients for primary production may
Derive from circulation of nutrients from
Below the surface waters (upwelling, storms
That bring deeper waters to the surface)
Regenerated Production:
Nutrients derive from recycling in surface
waters from excretion

30. Nutrients 5 Nitrogen - Microbial control
Nitrogen added to ocean from atmospheric
nitrogen by nitrogen fixing bacteria
Nitrifying bacteria convert NH4 to
NO2, others convert NO2 to NO3
Denitrifying bacteria convert N03 to NH4
Nitrate reducing bacteria return NO3 to
atmosphere

31. Nitrogen Cycle Atmospheric nitrogen Denitrification Nitrogen fixation
Primary production
Dissolved
inorganic
nitrogen
Organic
nitrogen
Respiration
Advection,
Mixing
External nitrogen
sources and sinks
Nitrogen Cycle

32. Nutrients 6 Phosphorus - occurs dissolved in water
mainly as phosphate PO4
Also can find particulate phosphorus,
some dissolved P in organic molecules
Phosphorus required for synthesis of ATP,
source of energy of cellular reactions

33. Phosphorus Cycle Primary production Advection and mixing Dissolved
Inorganic
phosphorus
Organic
phosphorus
Respiration
Advection and
mixing
External phosphorus
sources and sinks
Phosphorus Cycle

34. Nutrients 7 The limiting nutrient? In ocean,
nitrogen is believed to be the main
element limiting phytoplankton growth,
Rather than phosphorus
Important question are these the only limiting
nutrients or nutrient elements?

35. Nutrients 8 Silicon - important limiting element
for diatoms, exact role in controlling
phytoplankton growth not well
understood

36. Nutrients 9 Iron - important cofactor in oxygen
production step of photosynthesis
Shown in lab experiment to enhance
phytoplankton growth
May be crucial in parts of the ocean (eastern
equatorial Pacific, parts of Antarctic, north
Pacific where nitrogen appears not to be
limiting factor

37. Nutrients 10 Trace elements such as Mn, Zn, Mo,
Co, Cu can be important, but poorly
understood
Organic trace substances such as vitamins
important, especially for auxotrophic
phytoplankton (e.g., many dinoflagellates)

38. Phytoplankton Succession
Seasonal change in dominance by
different phytoplankton species
e.g.: diatoms in early spring followed
by dinoflagellates in summer

39. Phytoplankton Succession 2
Mechanisms poorly understood:
Shift in advantage of nutrient uptake,
Favoring different cell types
2. Species later in season may depend upon
substances that are not in the water column
in early spring (e.g., auxotrophic species might
follow autotrophic species)

40. Microbial Loop 1. Bacteria are abundant and take up large
Amounts of nutrients from the water column
2. Bacteria are consumed by ciliates and other
Heterotrophs
3. These heterotrophs are consumed by other
Smaller zooplankton, which incorporates
Bacterially derived nutrients into the planktonic
Food web

41. Microbial Loop 2 Larger consumers Herbivores Microbial loop DOC & POC
Phytoplankton
Viruses
DIOC and
nutrients
Bacteria
Microconsumers
DOC=dissolved organic carbon
POC=particulate organic carbon
DIOC=dissolved inorganic carbon

42. Nutrient uptake Nutrient uptake by phytoplankton cells
varies with nutrient concentration
Modelling uptake:
Need to know (1) nutrient concentration C
And (2) rate of uptake of nutrients, which
we measure indirectly as D, cell doublings/day
(3) K is concentration at which cell doubling
rate is one half of maximum doubling

43. Nutrient uptake 2 Dmax Cell doublings/day Dmax/2 K
Nutrient concentration

44. Nutrient uptake 3 K is nutrient concentration at which
half of maximum cell doubling rate
occurs - useful measure of phytoplankton
Nutrient uptake
Cell doubling rate K
Nutrient concentration

45. Nutrient uptake 4 Application of model: Inshore versus open ocean
phytoplankton nutrient uptake
Inshore species: live in higher nutrient
concentrations, should be good at uptake at high
concentrations, but may be tradeoff and lower
efficiency at low nutrient concentrations

46. Nutrient uptake 5 Application of model: Inshore versus open ocean
phytoplankton nutrient uptake 2
Open ocean species: live in lower nutrient
concentrations, should be better at uptake at lower
concentrations but tradeoff is inability to deal
with higher concentrations.

47. Nutrient uptake 6 Application of model: Inshore versus open ocean
phytoplankton nutrient uptake
Adapted to high
nutrient concentration 1
Dmax-1
Adapted to low
nutrient concentration 2
Dmax-2 K2 K1

48. Nutrient uptake 7 General results for nitrate: Environment K Inshore
Offshore M
Adapted to high
nutrient concentration
Dmax-1
Adapted to low
nutrient concentration
Dmax-2 K2 K1

49. Zooplankton Grazing Grazing effect: Difference between grazing
rate and phytoplankton growth rate
Grazing quite variable, sometimes causes:
1. Strong spatial variation in phytoplankton
abundance,
2. Cycles of phytoplankton abundance
and decline

50. Zooplankton Feeding Zooplankton feeding increases with increasing
phytoplankton cell density, up to a plateau
Cell ingestion rate
Phytoplankton cell density
Copepod feeding response on a diatom

51. The End