Inorganic Nutrients
103
Questions
1. In the vertical profile of a stratified lake, you may observe an inverse relationship between
concentrations of nitrate and ammonium nitrogen. What are the probable biotic factors
responsible for such a relationship?
2. Nitrate and ammonium concentrations are often very low in the epilimnion. Why? What
alternatives do phytoplanktonic algae have for a source of inorganic nitrogen under these
conditions?
3. Nitrite concentrations are usually very low in natural waters. Why?
4. What are potential pathways for loss and transformation of nitrogen from and within lakes?
List all of the sources for nitrogen you can think of for a lake ecosystem. How is nitrogen lost
from the ecosystem?
5. What does the concentration of soluble reactive phosphate mean in terms of utilization by
photosynthetic organisms?
6. If concentrations of soluble reactive phosphate were very low, would this mean that algae are
likely to be phosphate-limited? Why?
7. What alternative phosphate sources do rooted aquatic plants have?
8. What conclusions can you draw from your data on the differences between concentrations of
total phosphorus?
9. In stratified lakes, phosphorus concentrations commonly increase in the hypolimnion. What
mechanisms are likely to cause this pattern? Why?
10. Were there any relationships between your observed concentrations of phosphorus and
those of dissolved oxygen?
11. In stratified lakes, one commonly observes a reduction in the epilimnion and an increase in
the hypolimnion of concentrations of dissolved silica. Discuss probable causes for such
relationships.
12. The calcium concentrations of stratified, hard water lakes often are reduced markedly in the
epilimnion but increase in the hypolimnion. Why do you think this stratification occurs?
13. Magnesium and sodium concentrations are relatively "conservative" in lakes. What is meant
by this statement and why?
14. Discuss you observed measurements of specific conductance. If an increase were observed
with increasing depth, what would this indicate?
15. Of what potential value are measurements of total "dissolved" residues if the water were to
be used for domestic purposes?
Apparatus and Supplies
1. Ammonium nitrogen:
a. 50-ml graduated cylinders.
b. Vortex mixer.
c. Reagents: Na3P04 (or K 3 P04), phenol, NaOH, nitroprusside, sodium hypochlorite (fresh
commercial bleach), and NH4CI.
d. Pipets and propipets.
e. Spectrophotometer(s), with 1-, 4-, or 5-, and lO-cm path length cells.
2. Nitrate-nitrite nitrogen:
a. Cadmium reduction columns (see Fig. 7.2).
b. Cadmium filings, cupric sulfate, 2N HCI, and glass wool.
c. Reagents: NH 4 CI, Na 2 B 4 0 7 , disodium dihydrate EDTA, sulfanilamide, N-(1-naphthyl)
ethylenediamine dihydrochloride, KN0 3 , and NaN0 2 .
d. 50-ml graduated cylinders, pipets, and propipets for distribution.
e. Spectrophotometer(s), with 1-,4-, or 5-, and lO-cm path length cells.
3. Phosphorus:
a. Clean (nonphosphate containing detergents!) 130-ml screw-capped polyethylene bottles.
b. Sulfuric acid (sp. gr. 1.82), ammonium paramolybdate, L-ascorbic acid, potassium
103
Questions
1. In the vertical profile of a stratified lake, you may observe an inverse relationship between
concentrations of nitrate and ammonium nitrogen. What are the probable biotic factors
responsible for such a relationship?
2. Nitrate and ammonium concentrations are often very low in the epilimnion. Why? What
alternatives do phytoplanktonic algae have for a source of inorganic nitrogen under these
conditions?
3. Nitrite concentrations are usually very low in natural waters. Why?
4. What are potential pathways for loss and transformation of nitrogen from and within lakes?
List all of the sources for nitrogen you can think of for a lake ecosystem. How is nitrogen lost
from the ecosystem?
5. What does the concentration of soluble reactive phosphate mean in terms of utilization by
photosynthetic organisms?
6. If concentrations of soluble reactive phosphate were very low, would this mean that algae are
likely to be phosphate-limited? Why?
7. What alternative phosphate sources do rooted aquatic plants have?
8. What conclusions can you draw from your data on the differences between concentrations of
total phosphorus?
9. In stratified lakes, phosphorus concentrations commonly increase in the hypolimnion. What
mechanisms are likely to cause this pattern? Why?
10. Were there any relationships between your observed concentrations of phosphorus and
those of dissolved oxygen?
11. In stratified lakes, one commonly observes a reduction in the epilimnion and an increase in
the hypolimnion of concentrations of dissolved silica. Discuss probable causes for such
relationships.
12. The calcium concentrations of stratified, hard water lakes often are reduced markedly in the
epilimnion but increase in the hypolimnion. Why do you think this stratification occurs?
13. Magnesium and sodium concentrations are relatively "conservative" in lakes. What is meant
by this statement and why?
14. Discuss you observed measurements of specific conductance. If an increase were observed
with increasing depth, what would this indicate?
15. Of what potential value are measurements of total "dissolved" residues if the water were to
be used for domestic purposes?
Apparatus and Supplies
1. Ammonium nitrogen:
a. 50-ml graduated cylinders.
b. Vortex mixer.
c. Reagents: Na3P04 (or K 3 P04), phenol, NaOH, nitroprusside, sodium hypochlorite (fresh
commercial bleach), and NH4CI.
d. Pipets and propipets.
e. Spectrophotometer(s), with 1-, 4-, or 5-, and lO-cm path length cells.
2. Nitrate-nitrite nitrogen:
a. Cadmium reduction columns (see Fig. 7.2).
b. Cadmium filings, cupric sulfate, 2N HCI, and glass wool.
c. Reagents: NH 4 CI, Na 2 B 4 0 7 , disodium dihydrate EDTA, sulfanilamide, N-(1-naphthyl)
ethylenediamine dihydrochloride, KN0 3 , and NaN0 2 .
d. 50-ml graduated cylinders, pipets, and propipets for distribution.
e. Spectrophotometer(s), with 1-,4-, or 5-, and lO-cm path length cells.
3. Phosphorus:
a. Clean (nonphosphate containing detergents!) 130-ml screw-capped polyethylene bottles.
b. Sulfuric acid (sp. gr. 1.82), ammonium paramolybdate, L-ascorbic acid, potassium