230
Exercise 15
Clearance rate and grazmg rate as calculated above are not two different
measurements, but rather are simply different ways of expressing the rate of clearance of particles from a suspension.
EXPERIMENTAL MANIPULATIONS
Zooplankton
Evaluate the effects of several of the following variables on the clearance and grazing
rates by either or both of the food removal techniques. Use control flasks without
zooplankton to determine changes in food levels by natural growth and reproduction.
1. Differences in concentrations of zooplankton, e.g., replicates of zooplankton, at 2, 4,
8, 16, and 32 zooplankters/l.
2. Differences in concentrations of food particles, e.g., 1000, 10,000 and 100,000
cells/ml.
3. Different species of zooplankton, e.g., two or more species of the same genus such
as Daphnia; comparison of a mixture of rotifers, cladocerans, and/or copepods
(separation can be done manually at the end of the experiments prior to radioassay).
4. Different food particles (separate controls for each type):
a. Different species of algae of similar size.
b. Different sized algae and/or bacteria of similar geometry, e.g., spherical.
c. Differently shaped algae, such as a green alga versus a blue-green alga of
filamentous or colonial morphology.
b. Dissect out the guts of several zooplankters and examine them microscopically
to ascertain different apparent digestion rates of different algae.
5. Varying temperatures, e.g., lOoC indicative of hypolimnetic conditions and 25°C
similar to summer epilimnetic temperatures.
6. Clearance rate under bright light versus that under total darkness.
7. Using the radioactive food method, allow introduced zooplankters to feed only 5
to 7 min. Rapidly remove the animals by sieving. Transfer about half of the live
animals to nonradioactive food of the same type and concentration and allow
them to feed for 15 min. Transfer the other half to scintillation vials (enumerate
these animals). After allowing them to feed for 15 min on the nonradioactive food,
remove by sieving the live animals that have now cleared their digestive tracks of
radioactive material and radioassay (enumerate these organisms). Compare the
amount of radioactivity that was assimilated with the amount ingested.
8. Graph all data and compare the results. Answer the following questions as related
to your results.
Questions
1. What does the rate of ingestion of food measured by these methods tell about
the assimilation of food materials? Why?
2. If the "assimilation rate" is determined by procedure 7 above, is this a true rate
of assimilation? What assumptions are made?
3. Among zooplankton that migrate vertically, such as many species of Daphnia,
clearance rates have been observed to increase (double or more) at night when
they migrate toward the epilimnion. Discuss in terms of (a) differences in food
concentrations, (b) differences in food quality at different times of the day, (c)
Exercise 15
Clearance rate and grazmg rate as calculated above are not two different
measurements, but rather are simply different ways of expressing the rate of clearance of particles from a suspension.
EXPERIMENTAL MANIPULATIONS
Zooplankton
Evaluate the effects of several of the following variables on the clearance and grazing
rates by either or both of the food removal techniques. Use control flasks without
zooplankton to determine changes in food levels by natural growth and reproduction.
1. Differences in concentrations of zooplankton, e.g., replicates of zooplankton, at 2, 4,
8, 16, and 32 zooplankters/l.
2. Differences in concentrations of food particles, e.g., 1000, 10,000 and 100,000
cells/ml.
3. Different species of zooplankton, e.g., two or more species of the same genus such
as Daphnia; comparison of a mixture of rotifers, cladocerans, and/or copepods
(separation can be done manually at the end of the experiments prior to radioassay).
4. Different food particles (separate controls for each type):
a. Different species of algae of similar size.
b. Different sized algae and/or bacteria of similar geometry, e.g., spherical.
c. Differently shaped algae, such as a green alga versus a blue-green alga of
filamentous or colonial morphology.
b. Dissect out the guts of several zooplankters and examine them microscopically
to ascertain different apparent digestion rates of different algae.
5. Varying temperatures, e.g., lOoC indicative of hypolimnetic conditions and 25°C
similar to summer epilimnetic temperatures.
6. Clearance rate under bright light versus that under total darkness.
7. Using the radioactive food method, allow introduced zooplankters to feed only 5
to 7 min. Rapidly remove the animals by sieving. Transfer about half of the live
animals to nonradioactive food of the same type and concentration and allow
them to feed for 15 min. Transfer the other half to scintillation vials (enumerate
these animals). After allowing them to feed for 15 min on the nonradioactive food,
remove by sieving the live animals that have now cleared their digestive tracks of
radioactive material and radioassay (enumerate these organisms). Compare the
amount of radioactivity that was assimilated with the amount ingested.
8. Graph all data and compare the results. Answer the following questions as related
to your results.
Questions
1. What does the rate of ingestion of food measured by these methods tell about
the assimilation of food materials? Why?
2. If the "assimilation rate" is determined by procedure 7 above, is this a true rate
of assimilation? What assumptions are made?
3. Among zooplankton that migrate vertically, such as many species of Daphnia,
clearance rates have been observed to increase (double or more) at night when
they migrate toward the epilimnion. Discuss in terms of (a) differences in food
concentrations, (b) differences in food quality at different times of the day, (c)