352
Exercise 29
portion of the bicarbonate is present as "volatile" ammonium carbonate of metabolic
origin and as nonvolatile bicarbonates, half of which are of metabolic origin [cf.,
discussion in Wetzel (1983)]. The other half is bound CO 2 as CaC0 3 , most of which
becomes incorporated into the sediments. The sum of these CO 2 inputs is assumed to
be an estimate of lake metabolism. When the CO 2 accumulation is calculated for the
entire hypolimnetic volume, and this accumulation is divided by the volume of the
epilimnion, the resulting quantity is termed the relative assimilation intensity. The
relative assimilation intensity factor is expressed as mg C0 2 /1 of epilimnion/month.
The method first corrects DIC production in the hypolimnion by an evaluation of
sediment carbonate dissolution to estimate "metabolic DIC." The metabolic DIC then
is corrected for methane production. A critical assumption of this method is that
methanogenesis is the sole mode of anaerobic metabolism in the hypolimnion, i.e., iron,
sulfate, and nitrate reductions and fermentative reactions other than methanogenesis
are relatively minor both in contributions to decomposition or to alkalinity
accumulation in these water strata. This assumption is not true, particularly in
softwater lakes. Nonetheless, the method does provide a proximate estimate of whole
lake metabolism.
Procedures
1. Collect water samples at close intervals of depth in a stratified lake soon after the
onset of stratification and determine the following parameters (can be incorporated
directly with determinations of the oxygen deficit):
a. Ammonium ion concentrations (see Exercise 7).
b. Concentrations of CO 2 and HC0 3 - (see Exercise 8).
c. Dissolved oxygen concentrations (see Exercise 6).
Repeat after time intervals of about three of four weeks, preferably throughout the
period of stratification.
2. Determine the temperature of water at meter intervals.
3. From the bathymetric map of the lake under study, determine the are·a of water
strata of close depth intervals and the volume of water in these strata (cf.,
Exercise 1).
4. Establish the hypolimnion of the lake during the period from vertical profiles of
temperature [see Fig. 6.3 of Wetzel (1983, p. 75)].
Calculations
1. Make a table of the concentrations of the parameters needed for each of the
hypolimnetic strata for the sampling dates as shown in the following example
(Table 29.1).
2. The following assumptions are made in the calculations:
a. The molecular ratio of ammonium bicarbonate NH4 +: HC0 3 - = 18.04:61.02.
Then, HC0 3 - of ammonium bicarbonate is estimated as
HCO - = (NH4 +)(61.02)
3
18.04
= (3.38)(NH4 +)
=/1
Exercise 29
portion of the bicarbonate is present as "volatile" ammonium carbonate of metabolic
origin and as nonvolatile bicarbonates, half of which are of metabolic origin [cf.,
discussion in Wetzel (1983)]. The other half is bound CO 2 as CaC0 3 , most of which
becomes incorporated into the sediments. The sum of these CO 2 inputs is assumed to
be an estimate of lake metabolism. When the CO 2 accumulation is calculated for the
entire hypolimnetic volume, and this accumulation is divided by the volume of the
epilimnion, the resulting quantity is termed the relative assimilation intensity. The
relative assimilation intensity factor is expressed as mg C0 2 /1 of epilimnion/month.
The method first corrects DIC production in the hypolimnion by an evaluation of
sediment carbonate dissolution to estimate "metabolic DIC." The metabolic DIC then
is corrected for methane production. A critical assumption of this method is that
methanogenesis is the sole mode of anaerobic metabolism in the hypolimnion, i.e., iron,
sulfate, and nitrate reductions and fermentative reactions other than methanogenesis
are relatively minor both in contributions to decomposition or to alkalinity
accumulation in these water strata. This assumption is not true, particularly in
softwater lakes. Nonetheless, the method does provide a proximate estimate of whole
lake metabolism.
Procedures
1. Collect water samples at close intervals of depth in a stratified lake soon after the
onset of stratification and determine the following parameters (can be incorporated
directly with determinations of the oxygen deficit):
a. Ammonium ion concentrations (see Exercise 7).
b. Concentrations of CO 2 and HC0 3 - (see Exercise 8).
c. Dissolved oxygen concentrations (see Exercise 6).
Repeat after time intervals of about three of four weeks, preferably throughout the
period of stratification.
2. Determine the temperature of water at meter intervals.
3. From the bathymetric map of the lake under study, determine the are·a of water
strata of close depth intervals and the volume of water in these strata (cf.,
Exercise 1).
4. Establish the hypolimnion of the lake during the period from vertical profiles of
temperature [see Fig. 6.3 of Wetzel (1983, p. 75)].
Calculations
1. Make a table of the concentrations of the parameters needed for each of the
hypolimnetic strata for the sampling dates as shown in the following example
(Table 29.1).
2. The following assumptions are made in the calculations:
a. The molecular ratio of ammonium bicarbonate NH4 +: HC0 3 - = 18.04:61.02.
Then, HC0 3 - of ammonium bicarbonate is estimated as
HCO - = (NH4 +)(61.02)
3
18.04
= (3.38)(NH4 +)
=/1