Phosphorus and Nitrogen

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Phosphorus and Nitrogen

• But firsta few more examples of stratification
  and hypoxia

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Phosphorus

• Why study P?
• Biomolecules
• ADP and ATP
• nucleic acids
• phospholipids (cell membranes)
• apatite (bones)     

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Central Basin
Eastern Basin
Western Basin
ZAVG 6 m
Thermocline
18 m
24 m
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Central Lake Erie Observation Buoy
www.glerl.noaa.gov
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Temporary Stratification in Western Lake Erie
S
Dissolved Oxygen (mg/L)
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Forms of Phosphorus

• Total P DIP DOP PP
• DIP (lt5) dissolved inorganic phosphorus
• PO43- polyphosphates
• DOP dissolved organic phosphorus -- often
  organic colloids less quickly available
• PP particulate phosphorus -- often largest
  percentage of P in lakes (gt70)
• Algae, animals, detritus, suspended sediments
• We usually measure soluble reactive phosphate
  (SRP) which is DIP and some DOP

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Phosphorus and Lake Classification
The productivity of a lake is often determined by
its P loading and its volume (mean depth)
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(No Transcript)
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Limiting nutrient

• Theoretically, phosphorus is usually the most
  limiting nutrient in freshwater systems as
  determined by Ecological stoichiometry
• Ratios of elements in plankton and other organisms

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Stoichiometry gives the recipe for
phytoplankton
2 1/4 cups sifted cake flour2 teaspoons baking
powder1/2 teaspoon salt1/2 pound Butter 2 cups
sugar4 large egg yolks2 teaspoons vanilla1 cup
sour cream4 large egg whites
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Recipe for phytoplankton is the Redfield Ratio

• In the 1950s Alfred Redfield found in the deep
  ocean an average phytoplankton composition (by
  number of atoms) of 
•  C       H      O      N     P    S Fe
  106    263   110   16    1     0.7 0.01

Note that C, H, O, and N are required in greater
proportion than P. Why then are these NOT the
generally nutrient limiting?
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C       H      O      N    P     S Fe
106    263   110   16    1     0.7 0.01

• In freshwater systems P is usually limiting
  because the amount of P available to primary
  producers is much less than the amount required
  relative to the other elements.
• P makes up only 1 of organic matter which
  implies that if nothing else is limiting, then
  increasing P can theoretically generate gt100X the
  weight of added P in algae

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The Thieving Baker
Suppose you were a baker and wanted to sabotage a
rival baker by stealing supplies from his
storehouse. You can carry 50 lbs. of any
ingredient with you. What do you steal in order
to prevent him from making the most cakes?
2 1/4 cups sifted cake flour2 teaspoons baking
powder1/2 teaspoon salt1/2 pound Butter 2 cups
sugar4 large egg yolks2 teaspoons vanilla1 cup
sour cream4 large egg whites
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2 1/4 cups sifted cake flour2 teaspoons baking
powder1/2 teaspoon salt1/2 pound Butter 2 cups
sugar4 large egg yolks2 teaspoons vanilla1 cup
sour cream4 large egg whites
i.e. If you have plenty of everything else, then
with only ½ teaspoon of salt, you can bake a cake.
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Sources of Phosphorus

• Weathering of calcium phosphate minerals,
  especially apatite Ca5(PO4)3OH from sediments
  of ancient oceans. There are no important
  gaseous sources of P.
• Anthropogenic P is now often much greater than
  natural inputs of P in many watersheds
• Sewage, agriculture, etc.
• Increased production of algae due to increased
  Anthropogenic P input is cultural
  eutrophication               
• Anthropogenic P may come from
• point sources (think of a pipe)
• nonpoint sources (diffuse, like agriculture
  runoff)

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Point and Nonpoint sources
thinkquest.org
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External vs. Internal P Loading

• Loading refers to input of a nutrient per unit
  time
• External loading refers to sources outside the
  lake (as in previous slide)
• If all external sources of P were removed, a lake
  would continue to grow algae for many years.
  This is because P is recycled within the lake.
  This recycling is termed Internal Loading

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Sediment Oxygen profile
Diffusion Barrier
P diffusion
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Internal P Loading

• P may be recycled in the food web several times
• Phytoplankton are extemely efficient at absorbing
  any P that is released by excretion or
  decomposition
• Eventually P will be lost from lake either by
  outflow or by sedimentation to the lake bottom.
• P is bound in lake sediments under oxic
  conditions, but may be regenerated from sediments
  under anoxic conditions (iron and microbes play
  an important role)
• Deep lakes with oxic hypolimnia and long WRT may
  retain 70-90 of incoming P in the sediments
• Lakes with Anoxic hypolimnia retain only half as
  much P as lakes with oxic hypolimnia
• Therefore external loading may result in a
  positive feedback loop that multiplies
  eutrophication.

lakes.chebucto.org/DATA/PARAMETERS/TP/popup.html
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?phytoplankton
?external P loading
? decomposition
? regeneration of P from sediments
? hypoxia
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Bioturbation
With Mayflies
Without Mayflies
J. Chaffin
Physical resuspention by organisms living in oxic
sediments may also increase the regeneration of
Phosphorus from sediments into the overlying water
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Phosphorus Remediation

• Eutrophication can be ugly high algal biomass
  (sometimes toxic), hypoxia, fish kills, foul
  smells
• One answer is to reduce P loading by
• Removing P from waste water (tertiary treatment)
• Diverting waste water (see Lake Washington)
• Using natural or constructed wetlands to trap P
• Using buffer strips to trap agricultural runoff
• Using pumps to aerate the hypolimnion

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Wastewater Treatment
Addition of alum to precipitate P
www.defra.gov.uk
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Buffer Strips
www.epa.gov/owow/nps/Section319III/OH.htm
NRCS
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Hypolimnion Aeration
content.cdlib.org/xtf/data
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A look at P in Ohios L. Erie Tributaries (from
P. Richards)
Study completed in 1995 showed almost all trends
improving
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20 year trends 1975-1995
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Water flow has not changed in Maumee R., has
increased in some others
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Decreased SS in Maumee River may be due to
Conservation tillage, other rivers have
increasing suburban development
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TP unchanged in Maumee R. Increasing in others
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SRP loading has increased in all rivers
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SRP/TP ratio has increased in all rivers.
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The Nitrogen Cycle

• Sources of Nitrogen
• N is abundant on earth, but only about 2 is
  available to organisms as reactive nitrogen
  (NOx, NHx, Org N)
• N is made available by Nitrogen-fixation and by
  fertilizer production
• Gaseous N2 ? NO3
• Reactive N can be recycled through the biota
  until it is eventually lost to the atmosphere
  through denitrification

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K. Schulz
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• Nitrogen inputs to lakes
• Atmospheric deposition from combustion of fossil
  fuels dryfall (NO3,NH4, Organic N)
• Atmospheric deposition has doubled every 34 yrs
• Watershed inputs
• Terrestrial systems are generally N-limited,
  therefore most N is retained on land and not
  exported to via streams to lakes

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Nitrogen Transformations

• NH4 (ammonium) uptake by algae.
• NH4 ? PON (Particulate Organic Nitrogen)
• No ? in oxidation state - not a redox reaction.
• takes least amount of energy, therefore preferred
  by algae.
• High concentrations of NH4 in aerobic aquatic
  conditions are usually an indication of pollution
  by sewage or feedlot runoff.
• Most other reactions are mediated by bacteria
• Ammonification NH4 production
• decomposition of PON ? NH4

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Nitrogen Transformations

• Nitrification
• NH4 (ammonium) 3/2 O2 ? NO2 (nitrite) 2H
  H2O then,
• NO2 1/2 O2 ? NO3 (nitrate)
• NO2 usually does not build up
• NO3 is the final product of nitrification. It
  may build up in conditions where there is much
  NH4 being produced, oxygen is present, but there
  is little vegetation to take up NO3
• (fish aquaria)
• Assimilative nitrate reduction
• NO3- uptake by algae
• algal uptake of N to make more cells. NO3 ORG-N
  (NH3)
• needs light, done with O2 present.

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Nitrogen Transformations

• Denitrification (dissimilatory nitrate reduction)
• CHO NO3- H ? CO2 N2 H2O
• must be anaerobic
• sediments, anoxic hypolimnia.
• Nitrogen fixation
• N2 ? ORG-N (NH3).
• Difficult to break triple bond of N2 therefore
  energetically expensive
• May be conducted by Cyanobacteria (Bluegreen
  algae) under bright sunlight
• Or by bacteria in sediments, coupled with other
  reactions.
• Both cases require anoxic conditions for reaction
  to occur

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Nitrogen fixation in Cyanobacteria

• Nitrogen fixation occurs in special cells called
  heterocysts, but
• Not all cyanobacteria have heterocysts or can fix
  nitrogen
• Some cyanobacteria can fix nitrogen without
  heterocysts

www.bio.purdue.edu/people/faculty/sherman/ShermanL
ab
www.micrographia.com/specbiol/bacteri/bacter/bact0
200/anabae03.jpg
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Nitrogen Cycle in Lakes
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• Redfield ratios of ocean phytoplankton (by number
  of atoms)
•  C       H      O      N     P    S Fe
  106    263   110   16    1     0.7 0.01
• Hecky et al. compiled data from lakes around the
  world to see if the ratios held true for lakes
  (as well as the ocean)

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NP ratio
Lakes with NP ratio gt 22 are considered to be
P-limited
Note also CN and CP ratios. If they are higher
than the Redfield ratio it means that algal cells
are making do with less N or P than they would
like.
Mean NP ratio 24
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NP 13, 11 for Lakes Victoria and Albert
Some large tropical lakes can be severely
nitrogen limited
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Most lakes are P deficient relative to N
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What about CP ratios? CP is usually above the
Redfield ratio meaning that algae is usually
P-limited relative to C
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What about NP ratios? NP is also usually above
the Redfield ratio meaning that algae is usually
N-limited relative to C
Therefore, freshwater phytoplankton is usually
both N and P limited. Ie. The cells are making
do with less than optimal N and P. But, P is
more limiting than N
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Finally, recent studies show that the Redfield
NP ratio of 16 is not a universal biochemical
optimum for phytoplankton, but rather an average
of ratios for many different species.
Christopher A. Klausmeier, Elena Litchman, Tanguy
Daufresne and Simon A. Levin Nature 429,
171-174(13 May 2004)