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P/B RATIOS
Exercise 12
The ratio of production rate (P) to mean biomass (li) gives an estimate of the turnover
of the population. P/B ratios can be useful for comparative purposes of population
growth in response to environmental conditions and perturbations. In the case of a
cohort, the cohort P/B ratio is equal to the cohort production divided by the mean
cohort biomass (Waters, 1969, 1977; Benke, 1984). The annual P/B ratio is the
annual production divided by the mean biomass of the entire 12-month period, even
though the generation under study may have been present for less than a year. Mean
annual biomass is simply the mean of all monthly average biomass values.
For a univoltine species, the annual production rate is equal to that of the cohort.
The cohort p/li ratio refers to the estimate of production rate and biomass over the
duration of the generation under study (i.e., the CPI). Cohort p/li ratios are commonly
ca. 5 (ranging between 2 and 8), regardless of voltinism (Waters, 1969, 1987; Benke,
1984). Benthic animals fall into two major groups: (1) animals that live their entire
life history in the aquatic habitat (e.g., crustaceans), exhibit an Allen curve that is
not truncated, and have an expected cohort Pili ratio of ca. 5 to 7; and (2) animals
that terminate their CPI in pupation/emergence, exhibit an Allen curve that is
truncated, and have a cohort p/li ratio of about 2 to 5 (Waters, 1987). Annual p/li
varies approximately with voltinism (the CPI). For a bivoltine species, annual
production is equal to that of the first generation plus the cohort production of the
second generation. Cohort p/li ratios of the two generations are usually reported
separately. The units of annual p/li are inverse time (year- 1 ) and the reciprocal
equals turnover time, i.e., the amount of time needed to replace the biomass of the
population.
EXERCISES
OPTION 1. FIELD ANALYSES (ALTERNATIVE A)
1. In a lake or reservoir, locate a representative transect of the basin extending from the
littoral to the profundal zone of the open water.
2. At regular intervals of depth along the transect, collect duplicate or multiple quantitative
samples of the sediment with two or more different samplers (e.g., an Ekman and corers).
3. Record the depth, vegetation (if any), type of sediment, and any other information that is
germane to the distribution of the organisms (e.g., wave action in the area).
4. Determine the temperature and dissolved oxygen profiles (Exercises 1 and 6).
5. Sieve the samples with the finest sized mesh screens that time will permit to remove a
major portion of the sediments.
6. In the laboratory, sort the samples. A comparison of manual-visual sorting to a flotation
method on samples from the same depth should be made. Use flotation methods and dyes,
as discussed earlier, to facilitate quantitative sorting.
7. Identify the organisms to the most specific level possible under the conditions of time and
experience. Enumerate the organisms of each category.
8. Determine the biomass ofthe benthic organisms by dry weight and organic weight analyses.
9. Analyze your data in relation to gradients of depth, temperature, dissolved oxygen
concentration, sediment composition, and other factors.
10. Answer the questions following Option 5.
P/B RATIOS
Exercise 12
The ratio of production rate (P) to mean biomass (li) gives an estimate of the turnover
of the population. P/B ratios can be useful for comparative purposes of population
growth in response to environmental conditions and perturbations. In the case of a
cohort, the cohort P/B ratio is equal to the cohort production divided by the mean
cohort biomass (Waters, 1969, 1977; Benke, 1984). The annual P/B ratio is the
annual production divided by the mean biomass of the entire 12-month period, even
though the generation under study may have been present for less than a year. Mean
annual biomass is simply the mean of all monthly average biomass values.
For a univoltine species, the annual production rate is equal to that of the cohort.
The cohort p/li ratio refers to the estimate of production rate and biomass over the
duration of the generation under study (i.e., the CPI). Cohort p/li ratios are commonly
ca. 5 (ranging between 2 and 8), regardless of voltinism (Waters, 1969, 1987; Benke,
1984). Benthic animals fall into two major groups: (1) animals that live their entire
life history in the aquatic habitat (e.g., crustaceans), exhibit an Allen curve that is
not truncated, and have an expected cohort Pili ratio of ca. 5 to 7; and (2) animals
that terminate their CPI in pupation/emergence, exhibit an Allen curve that is
truncated, and have a cohort p/li ratio of about 2 to 5 (Waters, 1987). Annual p/li
varies approximately with voltinism (the CPI). For a bivoltine species, annual
production is equal to that of the first generation plus the cohort production of the
second generation. Cohort p/li ratios of the two generations are usually reported
separately. The units of annual p/li are inverse time (year- 1 ) and the reciprocal
equals turnover time, i.e., the amount of time needed to replace the biomass of the
population.
EXERCISES
OPTION 1. FIELD ANALYSES (ALTERNATIVE A)
1. In a lake or reservoir, locate a representative transect of the basin extending from the
littoral to the profundal zone of the open water.
2. At regular intervals of depth along the transect, collect duplicate or multiple quantitative
samples of the sediment with two or more different samplers (e.g., an Ekman and corers).
3. Record the depth, vegetation (if any), type of sediment, and any other information that is
germane to the distribution of the organisms (e.g., wave action in the area).
4. Determine the temperature and dissolved oxygen profiles (Exercises 1 and 6).
5. Sieve the samples with the finest sized mesh screens that time will permit to remove a
major portion of the sediments.
6. In the laboratory, sort the samples. A comparison of manual-visual sorting to a flotation
method on samples from the same depth should be made. Use flotation methods and dyes,
as discussed earlier, to facilitate quantitative sorting.
7. Identify the organisms to the most specific level possible under the conditions of time and
experience. Enumerate the organisms of each category.
8. Determine the biomass ofthe benthic organisms by dry weight and organic weight analyses.
9. Analyze your data in relation to gradients of depth, temperature, dissolved oxygen
concentration, sediment composition, and other factors.
10. Answer the questions following Option 5.