Nutrient Dynamics in Lakes

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Nutrient Dynamics in Lakes
Schindler study in Canadian shield lake Nutrient
a chemical element essential for life Limiting
nutrient plant growth is limited by that
nutrient that is least abundant relative to the
needs of the plant     Vallentynes algal bowl
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Nitrogen

• Forms of nitrogen
• Organic nitrogen
• Amino acids, proteins
• DON dissolved organic nitrogen
• urea ? large molecules
• PON particulate organic nitrogen
• living zooplankton
• detritus

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• Forms of nitrogen (cont.)
• Ammonium NH41
• Ammonia gas is very soluble in water
• NH3 H2O ? NH4OH ? NH41 OH-1
• Equilibrium is usually far to right
• NH41 is the most reduced form of N
• Nitrate from geologic sources
• Ammonium from biological sources
• Nitrate usually more abundant except in eutrophic
  lakes

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• Forms of nitrogen (cont.)
• NO, N2O, NOx oxides
• Anthropogenic pollutants

Oxidation states NO3-1 5 NO2-1 3 NO 2 N2O
1 N2 0 ? NH41 -3
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Processes transformations among forms of
nitrogen
1. Nitrogen fixation requires energy to break
triple bond N2 ? NH4, NO3, organic N A.
Abiotic, lightning, not too important B.
Biotic fixation Terrestrial Rhizobium,
symbiont on legumes
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1. Nitrogen fixation (cont.) Biotic fixation
(cont.) Aquatic N fixers Blue-green algae,
60 species Anabena Aphenazonama Nostoc
(streams) Bacteria little known Importance
of N-fix in lakes is highly variable
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Percent contribution of nitrogen fixation to the
total input of nitrogen to lakes
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2. Nitrification oxidation of ammonium
NH41 ? NO2-1 ? NO3-1 reduced 3
3 5 oxdized Nitrosomonas
Nitrobacter Chemoautotrophs O2 must be present
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• Autotrophic immobilization -- nitrogen uptake by
  plants
• DIN ? PON
• Either NH4 or NO3 ? living algae

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• 5. Ammonification -- mineralization of organic
  nitrogen
• ON ? NH4
• Bacteria and fungi -- Decomposition of organic
  material
• Animals Byproduct of protein metabolism,
  excretion

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• 6. Denitrification - oxidation of organic
  matter using NO3 as the oxidizing agent
• e.g., C6H12O6 4NO3 ?
• 6CO2 6H2O 2 N2 574 kcal/mole
• Carbon oxidized (0 to 4)
• Nitrogen reduced (5 to 0)
• Exothermic
• Nitrogen converted to unusable form
• Oxidation of glucose with oxygen produces 686
  kcal/mole
• Facultative anaerobic bacteria, e.g., Pseudomonas
• Anoxic hypolimnetic sediments

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Diffusion
Diffusion
Nitrogen fixation Nitrification
(Nit) Immobilization (I) Autotrophic Heterotroph
ic Ammonification (A) Denitrifiction Runoff Diffu
sion Sedimentation
N
N
Runoff
Runoff
2
2
Denitrification
Denitrification
N
-
fixation
N
-
fixation
Nit
Nit
NO
NH
NO
NH
3
4
3
4
I
I
I
I
I
I
Plant N
Plant N
Excreation (A)
I
Excretion (A)
I
Death
Death
A
A
Animal
Animal
N
N
PON DON
PON DON
Death
Death
Detritus
Detritus
A
Mineralization
Sedimentation
Sedimentation
Sediments
Sediments
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Nitrogen distribution in an oligotrophic lake Mid
summer, thermally stratified Orthograde oxygen
curve
1. DIN low -- Oligotrophic lake 2. NH4 low
throughout water column -- O2 present,
nitrification 3. NO3 low in photic zone --
Autotrophic immobilization 4. NO3 higher in
hypolimniion -- Mineralization -- No autotrophic
uptake
Figure 12-4
?
NO3
O2
Depth
NH4
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Nitrogen distribution in a eutrophic lake Mid
summer, thermally stratified Clinograde oxygen
curve
1. High DIN input -- Eutrophic lake 2. NH4 and
NO3 low in photic zone -- Autotrophic
immobilization 3. NO3 low in aphotic zone --
No oxygen for nitrification -- Denitrification 4.
NH4 high in aphotic zone -- No oxygen for
nitrification -- No uptake
Figure 12-4
?
O2
NH4
Depth
NO3
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Seasonal pattern of nitrate Oligotrophic lake --
low throughout the year Eutrophic lake --
depleted in summer due to immobilization
and denitrification
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Depth - time diagrams -- combine depth and
seasonal distributions Lawrence Lake --
oligotrophic Wintergreen Lake -- eutrophic
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Depth-time diagram of DIN (NO3-N NO2-N)
concentration (mg/L) in Lawrence Lake, MI,
1971-72. From Wetzel (2001, Fig. 12-6).
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Depth-time diagram of NH4-N concentration (?g/L)
in Lawrence Lake, MI, 1971-72. From Wetzel
(2001, Fig. 12-6).
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0
10
1000
15
1300
700
1200
10
1
5
800
2
0
800
0
25
1200
600
200
50
0
0
75
200
500
0
50
100
150
3
100
DEPTH (m)
400
825
1300
4
1400
800
600
5
800
JAN FEB MAR APR MAY
JUN JUL AUG SEP
OCT NOV DEC
Nitrate (NO3-N) concentration (?gN/L) in
Wintergreen Lake, MI, in 1971. From Wetzel
(2001, Fig. 12-8). (Includes small amount of
nitrite.)
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0
5
5
1300
1
1000
2000
1500
2
1000
400
2000
500
400
200
0
500
0
100
3
DEPTH (m)
1000
1500
2000
1500
600
4
3000
1000
3000
1500
800
2400
2000
5000
4000
4000
5
3000
1000
7000
5000
4900
3000
1200
JAN FEB MAR APR MAY
JUN JUL AUG SEP
OCT NOV DEC
Ammonium (NH4-N) concentration (?gN/L) in
Wintergreen Lake, MI, in 1971. From Wetzel
(2001, Fig. 12-8).
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Depth - time diagrams -- combine depth and
seasonal distributions Lawrence Lake --
oligotrophic Wintergreen Lake -- eutrophic
Neither fits exactly the pattern weve described
for ideal lakes. See Fig. 9-7. Lawrence Lake
has a clinograde oxygen curve even though it is
considered to be an oligotrophic lake.
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Phosphorus Forms Processes Distribution Three
special topics
Forms Essentially all phosphate,
PO4-3 Problems 1. PO4-3 combines with many
things 2. Define forms based on function or
analytical techniques
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Functional forms -- what we think is in the
water I. Dissolved a. dissolved inorganic
PO4-3 b. dissolved polyphosphates chains of
PO4 -- algal storage -- detergents c.
Dissolved organic PO4 low molecular weight
(250) very large (5,000,000) leachates
from living organisms
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Functional forms (cont.) II. Particulate a.
Organic living and detritus ATP, ADP, DNA,
RNA, phosphoproteins, phospholipids b.
PO4 adsorbed to organic particles c. PO4
adsorbed to inorganic particles clays,
carbonates, ferric hydroxide d. Particles of
insoluble P compounds Ca3(PO4)2 III.
Gaseous -- phosphene PH3 a. Swamp gas, highly
reducing conditions b. Rapidly oxidized in
presence of O2 c. Not important
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• Light spectrophotometry
• Combine target chemical with another chemical
  that produces a color
• Measure the intensity of the color

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Measuring phosphate concentration (actually SRP,
soluble reactive phosphate)

• Convert the phosphate to phosphomolybdate
• Reduce with ascorbic acid in the presence of
  antimony forms a blue complex that adsorbs
  light at a wavelength of 882 nm
• Measure the color intensity

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Since the PO4 Mo complex absorbs only in the
882 nm range, the better we focus on this range,
the better our accuracy. With a colorimeter,
this is done with a filter. With a
spectrophotometer this is done with a prism and a
grating.
prism
slit
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Rotate the prism to get desired wavelength. The
narrower the slit the higher the precision
i.e., less interference Spec20 20 nm Really
good specs 1 nm
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Standard Curve

• We have to have a way to convert absorbance to
  concentration
• Make a set of solutions with known PO4
  concentrations
• Measure their absorbance
• Make a graph of absorbance versus concentration
• Use this graph (or regression equation) to get
  concentration of samples

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Analytical forms
SRP -- soluable reactive phosphorus
Filterable
TDP -- total dissolved phosphorus
Dissolved organic phosphorus -- DOPTDP-SRP
Sample
Non-filterable
Particulate phosphorus -- PP
Total phosphorus -- TP
PPTP-TDP
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Major questions SRP PO4? not all PO4 is
reactive some polyphosphates may be reactive TP
all phosphorus? not all particulate inorganic
P brought into solution by acid treatment TP
is mostly SRP, DOP, and POP
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What are the most abundant forms of P in
freshwaters?
From Wetzel 2001, Tab. 13-2
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Phosphorus reactions (see figure) I. Reactions
with cations a. Calcium Ca3(PO4)2 ,
Ca5(OH)(PO4)3 highly insoluble b. Redox
metals Fe, Mn, Al P reaction depends on
oxidation state
O2 present O2
absent High redox potential (Eh) Low
redox potential Ferric iron Fe3
Ferrous iron Fe2 Fe(OH)3
Fe2 Precipitates
In solution Complexes with PO4 PO4
PO4 in solution
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Phosphorus reactions (cont.) II. Adsorption to
particles PO4 sticks to everything PO4
inorganic particles --gt PIP PO4 organic
particles --gt POP
III. Autotrophic immobilization Very
rapid Luxury uptake --polyphosphate granules
IV. Heterotrophic immobilization
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Phosphorus reactions (cont.) V. Mineralization
of P POP DOP --gt SRP microbial
decomposition animal excretion
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Precipitation (P) Adsorption and Heterotrophic
Immobilization (H) Autotrophic Immobilization
(A) Mineralization (M) Excretion (E) Death and
Feeding (F) Leaching (L)
Runoff
N
-
M
L
A
L
L
I
L
E
F
Death
H
A
F
F
F
M
M
P
Sedimentation
Sediments
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Phosphorus distribution Thermally
stratified Oligotrophic Orthograde O2 curve
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Phosphorus distribution Thermally
stratified Eutrophic Clinograde O2 curve
?
O2
Depth
What happens at fall turnover?
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12
10
10
12
Depth (m)
10
14
8
8
6
4
2
6
0
4
2
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Dissolved oxygen (mg/L) in Lawrence Lake,
Michigan. 1968. From Wetzel 2001, Figure 9-7.
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Phosphorus in sediment Sediment concentrations
may be much higher than concentrations in the
overlying water
Microzone prevents release of P from sediments
What happens if hypolimnion becomes anoxic?
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Phosphorus in sediment -- rooted
macrophytes Terrestrial plants move nutrients up
from soil. Can aquatic plants do the same thing?
Not like trees Reduced vascular system Reduced or
absent cuticle Foliar uptake of nutrients Roots
for anchorage
High DO, high Eh, low SRP
P?
Eelgrass has been shown to move P from marine
sediments. Not shown in FW lakes
no DO, low Eh, high SRP
Sediments
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The role of animals on the phosphorus cycle 1. A
source of P Juday et al. 1931 -- anadromous
salmon a source of P to headwater lakes in
Alaska Lake Dalnee, Kamchatka 26 of
P decline in salmon reduced NPP, zooplankton,
and fish MDN important to riparian trees in
Alaska (fish, bears) 2. Form of P Kitchell et
al. 1975 -- fish biomass in Wisconsin lakes may
contain up to 50 of epilimnetic P Periods of
fish death stimulate algal production
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The role of animals on the phosphorus cycle
(cont.) 3. Bioturbation worms, chronomids,
burrowing mayflies May increase oxygen in the
sediment, increasing its function as a P
exchange barrier May release P from reduced
sediments 4. Diel migration Translocation of
P from epilimnion May exceed sedimentation of
P