EXERCISE 29
Estimates of Whole Lake
Metabolism: Hypolimnetic
Oxygen Deficits and Carbon
Dioxide Accumulation
Measurements of the supply of organic matter to
aquatic ecosystems are complex and require an
elaborate research program continuing over at
least a year. The inputs from the products of
photosynthesis of autotrophic phytoplankton
and of littoral flora must be evaluated, as
well as the inputs from allochthonous organic
matter entering the aquatic ecosystem from the
atmosphere and from the drainage basin.
Assuming that the allochthonous inputs of
organic matter are small in relation to those
synthesized within a lake, and that the lake is
sufficiently large to stratify thermally, the autotrophic productivity can be estimated indirectly
from long-term changes in either hypolimnetic
oxygen deficits or accumulations of DIe. The
assumption is that organic matter, which was
synthesized in the trophogenic zone, sinks into
the hypolimnetic zone and decomposes there.
Changes in oxygen or DIC concentrations of the
hypolimnetic water reflect the rates of loading of
organic matter and of decomposition. However,
the relationships are not so tidy in nature. Several
major reactions in the development of an anaerobic hypolimnion arc related to the decomposition of organic matter [see Wetzel (1983) and
Schindler (1985)]. These reactions are based on a
simplified molecule for organic matter of planktonic material:*
CHzO + Oz ~ COz + HzO (early stages only)
CHzO + 4Fe(OHh + 8H+ ~4Fe2+ + COz
+ I1H zO
2CH z O + S04Z- ~Sz- + 2CO z + 2H z O
2CHzO ~CH4 + COz
COz + NH3 + HzO ~NH4 + + HC03The procedures given in this exercise are
appropriate for lakes where methanogenesis is
the primary decomposition pathway, and the
dominant anion in the hypolimnion is bicarbonate. Other confounding problems include, for
example, decomposition in the epilimnion,
turbulent exchange between the stratified water
layers, and photosynthesis in the hypolimnion.
Nonetheless, a general, direct relationship does
exist between autotrophic productivity and
*The complete Redfield molecule is [(CH2 0)106(NH 3 )16(H 3 P04 )]; see Redfield (1958) and
Vollenweider (1985).
349
Estimates of Whole Lake
Metabolism: Hypolimnetic
Oxygen Deficits and Carbon
Dioxide Accumulation
Measurements of the supply of organic matter to
aquatic ecosystems are complex and require an
elaborate research program continuing over at
least a year. The inputs from the products of
photosynthesis of autotrophic phytoplankton
and of littoral flora must be evaluated, as
well as the inputs from allochthonous organic
matter entering the aquatic ecosystem from the
atmosphere and from the drainage basin.
Assuming that the allochthonous inputs of
organic matter are small in relation to those
synthesized within a lake, and that the lake is
sufficiently large to stratify thermally, the autotrophic productivity can be estimated indirectly
from long-term changes in either hypolimnetic
oxygen deficits or accumulations of DIe. The
assumption is that organic matter, which was
synthesized in the trophogenic zone, sinks into
the hypolimnetic zone and decomposes there.
Changes in oxygen or DIC concentrations of the
hypolimnetic water reflect the rates of loading of
organic matter and of decomposition. However,
the relationships are not so tidy in nature. Several
major reactions in the development of an anaerobic hypolimnion arc related to the decomposition of organic matter [see Wetzel (1983) and
Schindler (1985)]. These reactions are based on a
simplified molecule for organic matter of planktonic material:*
CHzO + Oz ~ COz + HzO (early stages only)
CHzO + 4Fe(OHh + 8H+ ~4Fe2+ + COz
+ I1H zO
2CH z O + S04Z- ~Sz- + 2CO z + 2H z O
2CHzO ~CH4 + COz
COz + NH3 + HzO ~NH4 + + HC03The procedures given in this exercise are
appropriate for lakes where methanogenesis is
the primary decomposition pathway, and the
dominant anion in the hypolimnion is bicarbonate. Other confounding problems include, for
example, decomposition in the epilimnion,
turbulent exchange between the stratified water
layers, and photosynthesis in the hypolimnion.
Nonetheless, a general, direct relationship does
exist between autotrophic productivity and
*The complete Redfield molecule is [(CH2 0)106(NH 3 )16(H 3 P04 )]; see Redfield (1958) and
Vollenweider (1985).
349