Cole et al. 1994 Science 265:1568-1570

Why this pattern?

Cole et al. 1994 Science 265:1568-1570

Directly measured

Autumn

Full Seasonal Data

Summer

Tropical Africs

Lec 6: Nutrients and Nutrient cycling

I. Storages and cyclingII. Nutrient loading (more next lecture)

III. PhosphorusIV. NitrogenV. Other elements

1

I. Storages and cycling A. Energy versus nutrients -Energy flows -Nutrients cycle

B. Closed system 1. Rate = cycles/time a. as rate increases, productivity increases b. total N or P versus the amount of inorganically available N or P 2. Pathways - In a closed system all the nutrients cycle within the system

C. Open system

- Boundaries

1. Rate 2. Pathways (e.g. internal cycling vs. nutrient loading) 3. Residence time: time spent cycling before being lost from the system a. residence time = amount of nutrient in the system/amount in output b. in an open system nutrient use depends on recycling rate and retention by the system

(residence time) c. inputs and outputs do not necessarily balance 2

II. Nutrient Loading

A. Estimates of critical amounts of nutrients for eutrophication (especially used for N and P)B. Amount of nutrient input per time and lake area

called aerial loading

C. Used to develop models of nutrient effects in lakes

D. Must determine:1. Volume of inflow and outflow2. Concentration of nutrient in effluents and influents3. Volume of lake4. Loss rates to sediments

3

A. Except under polluted conditions, the only significant inorganic form of Phosphorus is Orthophosphate (PO4

–3)

B. Phosphorus often is a limiting nutrient in freshwater habitats

C. Generally, >90% of Phosphorus is in or adsorbed to living or dead organisms

D. Phosphorus is unique among the major inorganic nutrients in that its oxidation is not an important energy source (P always occurs in the oxidized form)

III. Phosphorus (P)

4

C : N :P106:16:1

100-1000C:10N:1P

6C:4N:1P

1st to become limiting

2nd to become limiting

rarely limiting

III. A. P as a Limiting NutrientElemental composition in plants (w/ balanced growth)

Presence in environment

Composition of sewage effluents

** Luxury Uptake

5

• Weathering of Rock (Apatite)

• P adsorbs to particles

III. B. Phosphorus Cycle

6

A. Particulate P1. Organisms

2. Rocks, soil, sedimentsIgneous rocks are associated with low P

Sedimentary rocks are associated with high P

3. Adsorbed

B. Dissolved P1. Orthophosphate (PO4

–3)

2. Polyphosphates (from detergents)

3. Organic phosphates (mostly colloidal)

Total Phosphorus must take into account all forms of P, including that incorporated into suspended matter and organisms.

III. C. Forms of Phosphorus

7

A. Precipitation (Wet and Dry)Non-populated areas <30 ug/LUrban-Industrial areas >100 ug/L Range 0.01-0.1 g/m2/year

B. Ground Water 20 ug/L

C. Runoff (fertilizers) varies

Lake requirements ~ 0.07 g/m2/year: >0.13 g/m2/year may result in eutrophication if mean depth < 5m

III. D. Sources of Phosphorus

8

Phosphate inliving plant andanimal tissue

Phosphatedissolved in water

CompensationDepth

AphoticZone

PhoticZone

Phosphate in mud

Thermocline

In epilimnion, P rapidly is taken up by algae

In sediments, P is removed by rooted vegetation and benthos

III. E. Distribution of Phosphorus

9

Generalized P Profiles in Lakes of Low and High Productivity

P, oC, O2

O O2

OLIGOTROPHIC EUTROPHIC

O O2

PSPT PS PT

PS = Soluble phosphorus

PT = Total phosphorus

Dep

th

Dep

th

P, oC, O210

Phosphorus in Sediments• Depends on O2 supply

• O2 depends on trophic status and basin morphology

• P is retained by the oxidized microzone• Breakdown of the oxidized microzone releases P

(also Fe, Mn)• P, Fe, and Mn concentrations are related• P released from sediments under anoxic

conditions (+ feedback of internal cycling)• P also may be released from sediments by rooted

vegetation and benthos

11

Productivity Total PUltraoligotrophic <5 ug/LOligo-Mesotrophic 5-10Meso-Eutrophic 10-30Eutrophic 30-100Hypereutrophic >100

P generally is regarded as more important than other nutrients except in marine costal waters and under high P conditions.

III. F. Epilimnetic Phosphorus and Lake Productivity

*Note areal loading rate; influence of depth

12

Jan 2004

0.01

0.10

1.00

10.00

FR

R01

0F

RR

020

FR

R03

0N

EC

010

FR

R04

0C

HC

010

FR

R05

0F

RR

060

PE

C01

0M

IC01

0

FR

R07

0C

RC

010

CG

C01

0F

RR

080

CG

C02

0F

RR

090

CG

C03

0

FR

R10

0C

GC

040

FR

R11

0H

EL0

10 1

mH

EL0

10 4

mH

EL0

20 1

mH

EL0

20 3

mH

EL0

30 1

mH

EL0

30 4

mH

EL0

40 1

mH

EL0

40 4

m

TN

or

TP

(m

g/L

)

0

20

40

60

80

N:P

Rat

io

TN TP TN:TP

 

Generalized relationship between water clarity (Secchi depth) and algal concentration (Chl a). (OECD 1982).

Hensley Reservoir and Fresno River Data

A. P levels often positively correlated

with aquatic productivity

B. Noxious algal blooms

C. Hypolimnetic Oxygen Deficits

D. P is difficult to remove from water

III. G. Phosphorus and Water Quality

13

P Loading & OxygenP Loading & Phytoplankton(Lake Washington, Seattle)

Hypolimnetic O2 deficit

14

A. Generally considered to be the 2nd most important nutrient in lakes in terms of limiting the rate of primary production (Phosphorus being 1st)

B. Occurs in many forms and energy states (gas, organic and inorganic)

Lithosphere 97.6%

Atmosphere 2.3%

Hydrosphere + Biosphere 0.1%

C. Important both as a nutrient and

(in some forms) for its toxicity to organisms

IV. Nitrogen

15

A. Dissolved molecular Nitrogen (N2)

B. Organic Nitrogen•Proteins Highest Energy•Amino acids•Amines•Humic compounds

C. Inorganic Nitrogen•NH4

+ Ammonium

•NO2– Nitrite

•NO3– Nitrate Lowest Energy

IV. A. Forms of Nitrogen

16

IV. B. Nitrogen Sources and Losses

A. Sources1. Precipitation (wet & dry)2. Nitrogen Fixation3. Runoff

B. Losses1. Outflow2. Denitrification (NO3 => N2)3. Sediments

17

IV. B. Nitrogen FixationA. N2 gas to ammonium, very expensive energetically (Chemical “fixation” of molecular nitrogen (breaking the

triple covalent bond) in the laboratory requires 500OC and 100+ atmospheres of pressure)

B. Only bacteria known to fix nitrogen

C. Nitrogenase sensitive to O2, a variety of adaptations protect it

D. Lightning also fixes N2 to NO3- in the atmosphere

E. Nitrogen-fixing cyanobacteria can be very important in lake N cycles 18

A. Nitrification- oxidation of ammonium to nitrite (Azotobacter) and nitrite to nitrate (Nitrobacter)

B. Denitrification- using NO3- as an electron

acceptor for oxidation of carbon, yields N2O and N2. Drives N loss from environment. Under very low redox, can go to ammonium

C. Remineralization (ammonification)OrgN => NH4

+

IV. C. N Cycling

19

Jan Mar May Jul Sep Nov

5

4

3

2

1

0

1 1

1 10

10

10

10

100

100

250

500

500

1000

100

0 125

0 125

0

A

Jan Mar May Jul Sep Nov

Month

5

4

3

2

1

0

Dep

th (

m)

1

20

100

500 500

500 1

000 1000

1000

150

0

1500

1500 3000

B

Nitrate(NO

3)

Ammonium(NH

3)

IV. D. N Distribution in a Lake

20

N, o C, O2

NH4+ NO3

–O O2 NH4

+ NO3– O

O2

OLIGOTROPHIC EUTROPHIC

N, o C, O2

Dep

th

Dep

th

Generalized N Profiles in Lakes of Low and High Productivity

21

pH NH4+ : NH4OH

6 3000:1 7 300:1 8 30:1 9.5 1:1

Least toxic

Most toxic

IV. E. Toxic Forms of Nitrogen A. Nitrate/Nitrite – concentrations in drinking water >10 mg/l can cause the disease Methemoglobinemia in infants (a problem in some agricultural areas) (NO2 binds to hemoglobin more strongly than O2)

-Can be converted to carcinogenic nitrosamines in the stomach

B. Ammonia (especially in the form NH4OH) is toxic to many organisms

• Amount of NH4+

vs. NH4OH is pH dependent:

22

A. Silicon1. Key element in diatom frustules

2. Can become limiting in lakes

B. Iron1. Ferric, Fe3+, oxidized; ferrous, Fe2+ reduced

2. Iron oxidation by microorganisms important chemoautotrophic pathway, but also will happen abiotically, so must occur at oxic/anoxic interface

3. Oxidized iron precipitates with phosphate, but dissociates again in anoxic conditions

V. Silicon, Iron, etc.

23

0

1

2

3

Sili

ca (

mg

L-1)

1953 1954 1955 1956

Year

1

10

100

1000

Ast

erio

nella

(ce

lls m

L-1)

Annual Silicon Cycle in a Lake

24