Transcript 1 Sounding the Deep

9 Critical Factors in Plankton
Abundance
Notes for Marine Biology:
Function, Biodiversity, Ecology
By Jeffrey S. Levinton
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
Nutrients
at surface
Spring
Diatom
Increase Available
sunlight
Zooplankton
Winter
Spring
Summer
Fall
Winter
Arctic
Phytoplankton
Herbivore
zooplankton
Temperate
Phytoplankton
Tropical
Herbivore
zooplankton
Phytoplankton
J F MA M J J A S O N D
Month
Mechanisms causing the spring
phytoplankton bloom and its
decline
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
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
Before the spring phytoplankton
increase:
Water density similar at all depths
Wind mixing homogenizes water column
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
Oxygen production ( P ) or respiration ( R )
Total respiration, R
Total photosynthesis, P
Mixing depth 1: area P > R
Mixing depth 2: area P < R
Compensation
depth
Mixing
depth 1
Depth
Critical
Depth
Mixing
depth 2
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!!
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.
Decline of the Spring
Phytoplankton Increase 2
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 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 2
Beach phytoplankton blooms
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
Water column exchange in
shallow waters and estuaries 4
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
Light
Two components of loss in the water
Column:
Absorption: Molecular absorption
of light energy
Scattering: Light interaction with
particles
Light 2
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 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
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
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
Photosynthesis in Water Column
3
Photosynthesis increases with
increasing light intensity to a
plateau, Pmax, then decreases
Photosynthetic rate
Pmax
Gross
photosynthesis
+
0
_
Net
Photosynthesis
Compensation point
Respiration
Light intensity (I)
Nutrients
Nutrients are substances required by plants.
They are resources that can be limited in
supply
Nutrient dependence and use:
Autotrophs, auxotrophs, heterotrophs
Nutrients 2
Nitrogen - what for?
Nitrates NO3 Nitrites NO2 Ammonium ion, NH4 - excretion product
recycling from animal excretion in the
water column
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
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
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
Atmospheric nitrogen
Denitrification
Nitrogen fixation
Primary production
Dissolved
inorganic
nitrogen
Organic
nitrogen
Respiration
Advection,
Mixing
External nitrogen
sources and sinks
Nitrogen Cycle
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
Primary production
Dissolved
Inorganic
phosphorus
Organic
phosphorus
Respiration
Advection and
mixing
External phosphorus
sources and sinks
Phosphorus Cycle
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?
Nutrients 8
Silicon - important limiting element
for diatoms, exact role in controlling
phytoplankton growth not well
understood
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
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)
Phytoplankton Succession
Seasonal change in dominance by
different phytoplankton species
e.g.: diatoms in early spring followed
by dinoflagellates in summer
Phytoplankton Succession 2
Mechanisms poorly understood:
1. 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)
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 2
Larger consumers
Herbivores
Microbial loop
DOC & POC
Viruses
Bacteria
Phytoplankton
DIOC and
nutrients
Microconsumers
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
Cell doublings/day
Nutrient uptake 2
Dmax
Dmax/2
K
Nutrient concentration
Nutrient uptake 3
K is nutrient concentration at which
half of maximum cell doubling rate
occurs - useful measure of phytoplankton
Nutrient uptake
K
Nutrient concentration
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
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.
Nutrient uptake 6
Application of model: Inshore versus open ocean
phytoplankton nutrient uptake
1 Adapted to high
nutrient concentration
Adapted to low
2
nutrient concentration
Dmax-1
Dmax-2
K2
K1
Nutrient uptake 7
General results for nitrate:
Environment
K
Inshore
1M
Offshore
0.1-0.2 M
Adapted to high
nutrient concentration
Adapted to low
nutrient concentration
Dmax-1
Dmax-2
K
K
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
Cell ingestion rate
Zooplankton feeding increases with increasing
phytoplankton cell density, up to a plateau
Phytoplankton cell density
Copepod feeding response on a diatom
The End