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9 Critical Factors in Plankton
Abundance
Notes for Marine Biology:
Function, Biodiversity, Ecology
By Jeffrey S. Levinton
Patchiness of the Plankton
• Plankton occur in spatially discontinuous
patches, sometimes concentrated at
interfaces between water bodies
Patchiness of the Plankton 2
• Spatial changes in physical conditions
(light, temperature, salinity) - behavioral
responses and population growth/mortality
responses
• Water turbulence and current transport
• Spatially discontinuous levels of grazing
• Localized reproduction
• Social behavior
Patchiness of the Plankton 3
• Phytoplankton - population density
determined by interaction between
turbulence and population growth
• Low turbulence allows population to
increase in any one place
• High turbulence disperses phytoplankton
and density does not increase
Example of spread of plankton with general turbulence (left)
or in a current (right)
Spring Phytoplankton Bloom
Spring diatom increase
Mechanisms causing the spring
phytoplankton bloom and its
decline
Light and Phytoplankton - Concepts
IMPORTANT: LIGHT INTENSITY DECREASES WITH
INCREASING DEPTH
Consider a phytoplankton cell held in jar
at a certain depth ( = a certain light intensity):
Compensation depth:
O2 produced = O2 consumed
Increase of O2: from photosynthesis
Decrease: from respiration
Light and Phytoplankton - Concepts
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
SO…
AS YOU INCREASE DEPTH, LIGHT INTENSITY
DECREASES
EVENTUALLY YOU REACH A DEPTH WHERE
OXYGEN CONSUMED BY A CELL = OXYGEN
PRODUCED BY PHOTOSYNTHESIS (IF CELL IS
KEPT AT THAT DEPTH)
THAT IS THE COMPENSATION DEPTH, WHICH
IS AT THE COMPENSATION LIGHT INTENSITY
A CELL KEPT DEEPER THAN THAT DEPTH
WOULD HAVE NET OXYGEN PRODUCED (LESS
THAN, =, MORE THAN ??) OXYGEN CONSUMED?
SO…
AS YOU INCREASE DEPTH, LIGHT INTENSITY
DECREASES
EVENTUALLY YOU REACH A DEPTH WHERE
OXYGEN CONSUMED BY A CELL = OXYGEN
PRODUCED BY PHOTOSYNTHESIS (IF CELL IS
KEPT AT THAT DEPTH)
THAT IS THE COMPENSATION DEPTH, WHICH
IS AT THE COMPENSATION LIGHT INTENSITY
A CELL KEPT BELOW THAT DEPTH WOULD
HAVE OXYGEN PRODUCED < OXYGEN
CONSUMED
Before the spring phytoplankton
Increase
Water density similar at all depths
Wind mixing homogenizes water column
WINTER-->SPRING: CHANGES IN MIXING DEPTH AND LIGHT
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
produced in the water column equals total
consumed
If: Mixing depth < Critical depth: bloom
If: Mixing depth > Critical depth: no bloom
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, thermocline develops.
2. Surface nutrients are rich and trapped in
surface waters.
3. Phytoplankton cells are no longer stirred
to darker deep waters ----> BLOOM!!
Gulf of Maine
Decline of the Spring
Phytoplankton Increase
Why do phytoplankton (diatoms) decline?
Water column is STABLE
In shallow water shelf waters: diatoms remove
nutrients from water and also start
sinking from surface water to bottom,
which removes nutrients
Decline of the Spring
Phytoplankton Increase
Why do phytoplankton decline?
Zooplankton grazing? Has some effect but
often secondary to sinking
Rejuvenation of Conditions for
the Spring Phytoplankton
Increase
Why do phytoplankton sometimes 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
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
Water Column Exchange in
Shallow Waters and Estuaries
Beach phytoplankton blooms
Water Column Exchange in
Shallow Waters and Estuaries
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
Water Column Exchange in
Shallow Waters and Estuaries
Important factors in nutrient exchange:
1. 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,
especially if flow is high (NY Harbor releases
nutrients to NY Bight, for example)
Light
Two components of loss in the water
Column:
Absorption: Molecular absorption
of light energy
Scattering: Light interaction with
particles
Light
Penetration into water column varies:
with wavelength
Clear open ocean water: Maximum
penetration at 480 nm
Turbid inshore water: Maximum
penetration at 500-550 nm
Light
Ultraviolet light strongly attenuated
In water column:
Inshore waters: Incident light with wavelength
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
Photosynthesis in Water Column
Phytoplankton species may use
Chlorophyll a, c, and “accessory
Pigments,” which absorb
energy over the light spectrum
Photosynthesis in Water Column
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
Nutrients
Nutrients are substances required by plants;
they are resources that can be limited in
supply
Nutrient dependence and use:
autotrophs, auxotrophs, heterotrophs
Nutrients
Nitrogen - what is it used for?
Nitrates NO3 - MOST ABUNDANT SOURCE
USUALLY
Nitrites NO2
Ammonium ion, NH4 - excretion product
recycling from animal excretion in the
water column - TAKEN UP THE FASTEST
Nutrients
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
Nutrients
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
Nitrogen Cycle
Nutrients
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
Phosphorus Cycle
Nutrients
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?
Nutrients
Silicon - important limiting element
for diatoms, exact role in controlling
phytoplankton growth not well
understood
Nutrients
Iron - important cofactor in oxygen
production step of photosynthesis
FERREDOXIN - ELECTRON DONOR,
NITROGENASE IN N-FIXERS
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 (high nutrient - low productivity
(HNLP) - commonly wind delivered
Fe, Si often enter the ocean by
wind-borne particles
Nutrients
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)
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
Microbial Loop
DOC=dissolved organic carbon
POC=particulate organic carbon
DIOC=dissolved inorganic carbon
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
Nutrient Uptake
Nutrient Uptake
K is nutrient concentration at which
half of maximum cell doubling rate
occurs - useful measure of phytoplankton
Nutrient uptake
Nutrient Uptake
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
Open ocean species: lower nutrient
concentrations, should be better at uptake at lower
concentrations but tradeoff is inability to deal
with higher concentrations
Nutrient Uptake
Application of model: inshore versus open ocean
phytoplankton nutrient uptake
Nutrient Uptake
General results for nitrate:
Environment
K
Inshore
1M
Offshore
0.1-0.2 M
Intense and Harmful Algal Blooms
Conditions:
1. A stable water column
2. Input of nutrients
3. Sometimes an initial input of resting stages
Combine: to promote dense harmful phytoplankton
blooms, principally some dinoflagellates and
cyanobacteria
Major problem in Florida, but also common in other
coastal waters
Phytoplankton Succession
Seasonal change in dominance by
different phytoplankton species
e.g.: diatoms in early spring followed
by dinoflagellates, other flagellates in summer
Phytoplankton Succession
Mechanisms poorly understood:
1. Shift in advantage of nutrient uptake, later species
in season may depend upon substances that are not
in the water column in early spring (e.g.,
auxotrophic species might follow
autotrophic species)
2. Stratification - diatoms give way to swimming
flagellates
3. Chromatic adaptation - shifting advantage to
species with differing wavelength specializations
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
Zooplankton Feeding
Zooplankton feeding increases with increasing
phytoplankton cell density, up to a plateau
Copepod feeding response to diatom density
North Sea: grazing results in alternating patches
of phytoplankton and zooplankton - cycles of
abundance
Diurnal Vertical Migration of
Zooplankton
• Zooplankton rise to shallow water at night, sink to
deeper water during the day
• Zooplankters usually start to sink before dawn and
start to rise before dusk
• Internal biological clock that is reinforced by daynight light changes (bring individuals in laboratory
and they maintain daily rhythm for many days
even in a light-constant environment)
DiurnalVertical migration
of planktonic shrimp Sergia lucens
Diurnal Vertical Migration of
Zooplankton
Cause of Vertical Migration?
• 1. Strong light hypothesis - plankton migrate away
from strong light
• Problem: plankton migrate to great depths, below
those where light damage is likely
Diurnal Vertical Migration of
Zooplankton
Cause of Vertical Migration?
• 2. Phytoplankton recovery hypothesis zooplankton migrate downward for a time to allow
phytoplankton to recover
• Problem: Why not cheaters who would stay up to
feed on phytoplankton? Hypothesis requires
cooperation, even among many species
Diurnal Vertical Migration of
Zooplankton
Cause of Vertical Migration?
• 3. Predation hypothesis - zooplankton migrate downward
to avoid visual predation during day
• Dabob Bay, Washington where copepods have a reverse
migration, moving up during the day, apparently in
response to arrow worm predators, which themselves carry
a more typical diurnal vertical migration
Diurnal Vertical Migration of
Zooplankton
Cause of Vertical Migration?
• 4. Energy conservation hypothesis - zooplankton migrate
downward to avoid higher surface temperatures during the
day, which saves energy
• energy budget of a planktonic copepod, produces a benefit
for females - exercise not too costly
• copepods tend to sink beneath the surface after spring
phytoplankton increase -> adaptation to save energy in the
lower temperature deeper waters
• Problem: vertical migration in the absence of vertical
temperature gradient
The End