9 Critical Factors in Plankton Abundance

Notes for Marine Biology: Function, Biodiversity,

EcologyBy Jeffrey S. Levinton

Spring Phytoplankton Increase(or Spring Diatom Increase)

In midlatitudes, phytoplankton increase inthe spring, decline in summer, and may increaseto a lesser extent in fall

Nutrientsat surface

SpringDiatomIncrease

Zooplankton

Availablesunlight

Winter Spring Summer Fall Winter

Arctic

Temperate

Tropical

Phytoplankton

Phytoplankton

Herbivorezooplankton

PhytoplanktonHerbivorezooplankton

J F M A M J J A S O N D

Month

Mechanisms causing the spring phytoplankton bloom and its

decline

Light and Phytoplankton - concepts

Compensation depth - that depth at whichoxygen produced by a phytoplankton cell inphotosynthesis equals the oxygen consumed inrespiration

Consider a phytoplankton cell held in jarat a certain depth ( = a certain light intensity:

Light and Phytoplankton - concepts 2

Compensation light intensity - the lightintensity corresponding to the compensationdepth

Consider a phytoplankton cell held in jarat a certain depth ( = a certain light intensity:

Before the spring phytoplankton increase:

Water density similar at all depthsWind mixing homogenizes water column

Cause of the spring phytoplankton increase:

Important concepts:

Mixing depth - depth above which all wateris thoroughly mixed, due to wind.

Critical depth - depth above which total oxygencoming from primary production in the water column equals total consumption (respiration)

If: Mixing depth < Critical depth: bloomIf: Mixing depth > Critical depth: no bloom

Total respiration, RTotal photosynthesis, P

Compensationdepth

Mixing depth 1

CriticalDepth

Mixing depth 2

Depth

Oxy

gen

pro

du

ctio

n (

P )

or

resp

irat

ion

( R

)

Mixing depth 1: area P > RMixing depth 2: area P < R

Cause of the spring phytoplankton increase:

Important concepts 2:

Key processes: 1. Water column becomesmore 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 stirredto darker deep waters ----> BLOOM!!

Decline of the Spring Phytoplankton Increase

Why do phytoplankton decline?Water column is STABLEIn shallow water shelf waters: diatoms startsinking from surface water to bottom,which removes nutrients.

Decline of the Spring Phytoplankton Increase 2

Why do phytoplankton decline?

Zooplankton grazing? Has some effect butoften 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 columnbecomes isothermal with depth, wind mixingrestores nutrients to surface waters until conditions are right next spring

Water column exchange in shallow waters and estuaries

In very shallow estuaries, nutrient exchangeor benthic-pelagic coupling, occurs betweenthe 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 combineswith net water flow to the sea and mixing todetermine nutrient regime:1. Freshwater rivers create a net downstream flow2. Tides cause mixing up and down estuary as well as vertical mixing3. 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 estuarybefore entering ocean2. Rate of nutrient input from watershed3. Nutients may be released to coastal zone

Light

Two components of loss in the waterColumn:

Absorption: Molecular absorptionof light energy

Scattering: Light interaction withparticles

Light 2

Penetration into water column varies:with wavelength

Clear Open Ocean Water: Maximumpenetration at 480 nm

Turbid inshore water: Maximumpenetration at 500-550 nm

Light 3

Ultraviolet light strongly attenuatedIn water column:

Inshore waters: Incident light with wlof 380 nm or less is almost attenuatedat depth of 1-2 mClear open ocean water: 20 m maybe required to remove 90% of surfaceincident light

Photosynthesis in Water Column

Phytoplankton species may useChlorophyll a, c and “accessory pigments”, which absorbenergy over the light spectrum

Photosynthesis in Water Column 2

Action spectrum - utilization ofdifferent wavelengths of light bya given species for photosynthesis

Chlorophyll absorbs wavelengths ofmainly > 600 nmAccessory pigments absorb wavelengths< 600 nm

Photosynthesis in Water Column 3

Photosynthesis increases with increasing light intensity to a plateau, Pmax, then decreases

Light intensity (I)

NetPhotosynthesis

Grossphotosynthesis

Respiration

Pmax

Pho

tosy

nthe

tic

rate

Compensation point+_0

Nutrients

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

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

Nutrients 3

Nitrogen - uptake depends upon source

Ammonium ion is taken up the fastestby phytoplankton cells, requires the leastchemical alteration (no reduction), but Nitrate is usually the most abundant sourceOf nitrogen in shallow coastal water columns

Nutrients 4

Nitrogen - New vs. Regenerated ProductionNew Production:Nutrients for primary production may Derive from circulation of nutrients fromBelow the surface waters (upwelling, stormsThat bring deeper waters to the surface)Regenerated Production:Nutrients derive from recycling in surfacewaters from excretion

Nutrients 5Nitrogen - Microbial control

Nitrogen added to ocean from atmospheric nitrogen by nitrogen fixing bacteriaNitrifying 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

Respiration

Dissolvedinorganicnitrogen

Organicnitrogen

Advection,Mixing

External nitrogensources and sinks

Nitrogen Cycle

Nutrients 6Phosphorus - 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 productionDissolvedInorganic

phosphorus

Organicphosphorus

External phosphorussources and sinks

Advection andmixing

Respiration

Phosphorus Cycle

Nutrients 7The limiting nutrient? In ocean,nitrogen is believed to be the mainelement limiting phytoplankton growth, Rather than phosphorus

Important question are these the only limitingnutrients or nutrient elements?

Nutrients 8Silicon - important limiting elementfor diatoms, exact role in controllingphytoplankton growth not well understood

Nutrients 9Iron - important cofactor in oxygenproduction step of photosynthesis

Shown in lab experiment to enhance phytoplankton growth

May be crucial in parts of the ocean (easternequatorial Pacific, parts of Antarctic, northPacific where nitrogen appears not to belimiting factor

Nutrients 10Trace elements such as Mn, Zn, Mo,Co, Cu can be important, but poorly understood

Organic trace substances such as vitaminsimportant, especially for auxotrophic phytoplankton (e.g., many dinoflagellates)

Phytoplankton Succession

Seasonal change in dominance bydifferent phytoplankton speciese.g.: diatoms in early spring followedby dinoflagellates in summer

Phytoplankton Succession 2

Mechanisms poorly understood:

1. Shift in advantage of nutrient uptake,Favoring different cell types2. Species later in season may depend uponsubstances that are not in the water columnin early spring (e.g., auxotrophic species mightfollow autotrophic species)

Microbial Loop1. Bacteria are abundant and take up largeAmounts of nutrients from the water column

2. Bacteria are consumed by ciliates and otherHeterotrophs

3. These heterotrophs are consumed by otherSmaller zooplankton, which incorporates Bacterially derived nutrients into the planktonicFood web

Microbial Loop 2

Microbial loopDOC & POC

Viruses

Bacteria

Microconsumers

DIOC andnutrients

Phytoplankton

Herbivores

Larger consumers

DOC=dissolved organic carbonPOC=particulate organic carbonDIOC=dissolved inorganic carbon

Nutrient uptakeNutrient uptake by phytoplankton cellsvaries with nutrient concentration

Modelling uptake:Need to know (1) nutrient concentration CAnd (2) rate of uptake of nutrients, which we measure indirectly as D, cell doublings/day(3) K is concentration at which cell doublingrate is one half of maximum doubling

Nutrient uptake 2

K

Dmax

Dmax/2

Nutrient concentration

Cel

l dou

blin

gs/d

ay

Nutrient uptake 3

K

K is nutrient concentration at whichhalf of maximum cell doubling rateoccurs - useful measure of phytoplanktonNutrient uptake

Nutrient concentration

Cel

l dou

blin

g ra

te

Nutrient uptake 4

Application of model: Inshore versus open ocean phytoplankton nutrient uptake

Inshore species: live in higher nutrientconcentrations, should be good at uptake at highconcentrations, but may be tradeoff and lowerefficiency 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 lowerconcentrations but tradeoff is inability to dealwith higher concentrations.

Nutrient uptake 6

Application of model: Inshore versus open ocean phytoplankton nutrient uptake

Dmax-1

Dmax-2

K2 K1

1

2

Adapted to highnutrient concentrationAdapted to lownutrient concentration

Nutrient uptake 7

General results for nitrate:

Dmax-1Dmax-2

K2 K1

Adapted to highnutrient concentrationAdapted to lownutrient concentration

Environment K

Inshore 1M

Offshore 0.1-0.2 M

Zooplankton Grazing

Grazing effect: Difference between grazingrate and phytoplankton growth rate

Grazing quite variable, sometimes causes:1. Strong spatial variation in phytoplanktonabundance,2. Cycles of phytoplankton abundanceand decline

Zooplankton Feeding

Zooplankton feeding increases with increasing phytoplankton cell density, up to a plateau

Phytoplankton cell density

Cel

l ing

esti

on r

ate

Copepod feeding response on a diatom

The End