BULLETIN OF MARINE SCIENCE, 35(3): 503-509, 1984
DETRITAL DISSOLVED AND PARTICULATE ORGANIC
CARBON FUNCTIONS IN AQUATIC ECOSYSTEMS
Robert G. Wetzel
ABSTRACT
The conceptual relationships of pool sizes and metabolism of detrital dissolved (DOq and
particulate (POC) organic carbon in aquatic ecosystems are evaluated in relation to trophic
dynamics of organisms. In spite of the relatively refractory composition of detrital DOC, its
large pool size and slow degradation can dominate ecosystem metabolism and provide an
inherent stability to carbon flow through trophic dynamic structural components via nutrient
regeneration and bacterial metabolic scavenging of poe detritus through dissolved organic
matter to gaseous phases. Metabolism of DOC and the particulate detrital organic by-products
of producers by bacterial decomposition provides this stability.
For a number of years, limnologists and oceanographers have analyzed the
instantaneous pool sizes of organic carbon in the pelagic zones of aquatic ecosystems. In terms of ecosystem dynamics, there is no energetic difference between
dissolved and particulate fractions of organic matter (Wetzel et al., 1972; Wetzel,
1975; 1983). Detritus is all dead organic carbon, distinguishable from living
organic and inorganic carbon, lost by non-predatory means from any trophic level
(egestion, excretion, secretion, etc.) or from sources external to the ecosystem that
enter and cycle in the system (allochthonous organic carbon). The high concentrations of dissolved, relative to particulate, organic matter was noted over fifty
years ago in numerous lakes (Birge and Juday, 1926). Since that time, much
attention has been devoted to determining the gross and specific chemical composition of detrital dissolved and dead particulate organic matter. Because much
of the dissolved organic matter is chemically complex, with its origin in structural
components of plants, its metabolism was often assumed to be minor or insignificant. Nearly all research attention has been devoted to the trophic dynamics
of material and energy fluxes of the living particulate organic matter and recep.t
detrital POM.
.
In this brief discussion, I emphasize that traditional, animal-dominated trophic
dynamic functions generally do not constitute much of the total quantitative
metabolism of many aquatic ecosystems. This idea has not been accepted despite
clear supporting evidence. Lack of recognition has impeded understanding of the
regulation of growth and productivity of aquatic ecosystems. We argued long ago
(Wetzel et al., 1972; Rich and Wetzel, 1978) that detrital dissolved organic matter
is a primary regulating component in the fluxes of carbon through all aquatic
ecosystems via nutrient regeneration and the bacterial metabolic scavenging of
detrital POC through dissolved organic matter to gaseous phases. This organic
reserve serves as an energetic store upon which the metabolism of ecosystems
depends for stability, and upon which the less stable trophic dynamic functions
are superimposed.
DOC:POC Relationships
Several recent reviews have delineated quantitative aspects of dissolved (DOC)
to particulate organic carbon (POC) relationships in aquatic ecosystems (Menzel,
1974; Wetzel, 1975; 1983; Schlesinger and Melack, 1981; Meybeck, 1982). The
503
504
BULLETIN OF MARINE SCIENCE, VOL. 3S. NO.3.
Table I. Approximate concentrations
merous sources cited in the text)
1984
of dissolved organic carbon and DOC:POC ratios (from nu-
DOC (mg liter')
Rivers, world
Lakes
Oceans
Mean
Range
5.75
c.IO
<I
2-30
1-20
<1-10
OOC:POC ratio
c.6:1
c.IO:1
C'IO:1
inputs of DOC and POC to rivers and lakes are a function of both magnitude of
erosive runoff and proximity of the water body to regions of high organic productivity-terrestrial
and particularly wetland and littoral regions. Quantities of
detrital DOC and POC of rivers are high and variable, but their annual loading
to rivers represents only about 1 to 2% of the net primary productivity of the
adjacent terrestrial ecosystems (Meybeck, 1982). The DOC:POC ratios of rivers
vary from 1:1 to 10:1, with a mean of6:1 (Table 1).
Much of the DOC input to lakes is from terrestrial, wetland and littoral sources,
in contrast to planktonic production. Littoral contributions to the whole of course
vary with the geological, morphometric, nutrient, and other characteristics of
lakes. Most natural lakes are small and shallow ( < 15 m in depth); the surrounding
highly productive wetlands and littoral regions represent major sources of DOC
loading (Wetzel, 1979). Allochthonous POC of streams is generally a small ( < 10%)
portion of the annual organic carbon loading. Thus, contributions of DOC and
POC from wetland and littoral portions of lake ecosystems can quantitatively
dominate ecosystem inputs of total organic carbon.
In contrast, river and littoral inputs, largely as DOC, to oceans are relatively
small «10%) in comparison to DOC and POC inputs from pelagic primary
production. Most primary production in oceans is metabolized in the water column. Particulate organic carbon from river sources may account for only some
10% of the carbon flux to the sediments (Menzel, 1974; Schlesinger and Melack,
1981). In lakes, less of the organic carbon of both allochthonous and autochthonous origins is metabolized in the water column; benthic metabolism usually
predominates (Wetzel, 1975; 1983).
Organic Carbon Pool Sizes and Relative
Rates of Mineralization
DOC concentrations of pelagic regions of oceans and many lakes exhibit a
marked constancy in distribution both vertically within the water column and
seasonally (Wetzel et aI., 1972; Menzel, 1974; Wangersky, 1978; Wetzel, 1983).
Brief periods of DOC increase or decrease occur in concert with certain events,
such as major phytoplankton or macrophyte population declines, major precipitation events and loading, etc. These events, however, are relatively short-lived
(days) and on an annual basis usually cause small deviations from the overall
constancy. Within a given lake, the quantities of DOC and POC, and their ratios,
are quite constant from year to year, in spite of significant annual variations in
meteorology and internal and external loadings (Table 2). Deviations from this
constancy occur in anoxic regions where dense populations of microorganisms
can develop (e.g., bacteria in the metalimnetic-hypolimnetic
interface strata where
DOC:POC ratios can be 1:1; Wetzel, 1983) and in running water with variable
loading and erosion patterns.
505
WETZEL: DETRITAL FUNCTIONS IN ECOSYSTEMS
Table 2. Annual mean concentrations of Doe and poe of Lawrence Lake, southwestern Michigan,
compensating for volume differences in 1-m water strata·
Year
DOC
mean g C m-'
POe
mean gC m-'
DOC:POC
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
15-yr ave .
30.96
34.75
27.82
26.73
20.21
25.17
23.26
25.89
25.49
21.56
22.13
28.72
37.07
30.90
29.39
27.34
2.04
2.66
2.19
2.08
1.73
1.91
2.06
2.14
2.26
2.07
2.73
2.38
2.42
1.85
2.24
2.18
15.2
13.1
12.7
12.9
11.7
13.2
11.3
12.1
11.3
10.4
8.1
12.1
15.3
16.7
13.1
12.5
• Sampled at I-m depth intervals at weekly or biweekly intervals; N - 260 to 520 per year; total N - 4.160 each for DOC and for
POCo
In most standing fresh waters, however, DOC concentrations are large and
relatively constant. The size of the DOC pool is approximately 10 times that of
detrital poe, and the detrital poe is 5 to 10 times larger than the living poe of
all organisms from bacteria to fish and mammals. Of course pool sizes of detrital
DOC, dead poe, and living poe mean little without consideration of the largely
biologically-mediated rates of turnover, utilization, and mineralization of both
living and dead organic matter. It is common to see statements of the importance
of detrital metabolism, but often its significance is erroneously minimized or
dismissed as insignificant simply because it has not been adequately quantified.
The essence of my arguments is that true understanding of the total ecosystem
metabolism is impossible without quantitative evaluation of metabolic fluxes of
both the detrital and living components within ecosystems. Only by considering
all of these components can comprehensive mass balance evaluations of aquatic
ecosystems evolve.
If we consider the relative rates of decomposition of organic substrates, it is
obvious that labile, energy-rich substrates are turned over very rapidly, as often
as 5 to 10 times per day under optimal conditions (Table 3). Instantaneous concentrations oflabile DOC are continually very low and availability may be limited
to heterotrophic microorganisms. Bacteria of pelagic regions are often chronically
substrate limited (starved) but the frequency of substrate encounter is usually
adequate to maintain their enzymatic capabilities to utilize many organic substrates.
Even though much more recalcitrant to degradation than labile substrates, the
more complex dissolved organic pool constituting most of the DOM is degraded
slowly. Rates of degradation of this recalcitrant pool range from I to 5% per day
(Saunders et al., 1980). Although the mineralization rates are slow, the total
ecosystem processing ofthe refractory DOM is very large because of the relatively
massive size of the refractory DOM pool.
Using a representative lake ecosystem as an example, if we assume on an annual
average that the total living and non-living poe is 0.1 or 10% of DOC (Table
506
BULLETIN OF MARINE SCIENCE, VOL. 35, NO.3,
Table 3. Relative rates of decomposition
(modified from Saunders et aI., 1980)
Decomposition
(% day-')
Dissolved
500-1,000
25
2.5
I
Particulate
10
I
0.03
1984
of organic substrates in productive lake and river systems
Type of subslrale
Simple small molecules
Extracellular algal and bacterial products; labile leaf leachate
Refractory components of extracellular products
Refractory leaf leachate
Phytoplankton, zooplankton, submersed plants, algae in littoral
zone
Leaves in littoral zone and streams, residues of zooplankton
Secondary residues of zooplankton, emergent macrophyte structural components
I), that phytoplankton POC is on the average 0.5 or 50% of total POC (Wetzel,
1983), phytoplankton POC represents 0.05 or 5% of DOC. If phytoplankton grow
at a mean rate of 0.2 or 20% per day and the DOC turns over at 0.0 I or I% per
day, then the amount of carbon flow in phytoplankton net primary productivity
and DOC mineralization is the same.
Using the oligotrophic Lawrence Lake, Michigan, ecosystem as an example, the
pool size of the DOC (mean 27 g m-2, Table 2; 1.356 x 106 g C lake-I), and a
mineralization rate of I% per day results in an annual average carbon flux for
DOC of 13,560 g C day-I for the lake. Assuming a 10% per day average mineralization rate for POC (Table 3), the annual average carbon flux for POC of the
water column only is 10,813 g C day-l lake-I. Considering only the pelagic, the
total detrital carbon flux of DOC and POC of 24,373 g C day-l lake-1 compares
to an annual mean (14-year average, Wetzel, 1983) of net phytoplankton productivity of 4,960 g C day-l lake-I. In this case, carbon flux through the phytoplankton is approximately 17% of the total pelagic carbon flux, because much
DOC entering, and respired in, the pelagic zone has its source in the littoral region
and surrounding wetlands.
Quantitative evaluations ofthe complex organic carbon budgets offresh waters
lead to the conclusion that most of the synthesized organic carbon, nearly totally
from phytoplankton and sessile plant photosynthesis in lakes and from both
autochthonous and allochthonous plant production in rivers, enters the detrital
phases of POM and DOM without entering the digestive system of animals. The
assumption that most synthesized organic matter (e.g., of phytoplankton origin)
is consumed and mineralized by animals, or that dissolved and particulate organic
matter is largely assimilated by bacteria which are then ingested and mineralized
by animals, are processes that are simply not demonstrated to quantitatively
overshadow bacterial and fungal decomposition. The quantitative significance of
processes of simple autotrophic synthesis, growth, reproduction and autolysis with
subsequent mineralization by bacteria, followed by direct growth and autolysis
of bacteria, must be determined simultaneously with analyses of biochemical
degradation of organic matter by animals. I suggest that in addition to the refractory DOM that is mineralized directly by bacteria, most of the newly synthesized organic matter on an annual basis is mineralized without entering animals.
Of that usually very much smaller portion of the total newly synthesized organic
matter entering animals, much of the organic matter is egested in dissolved and
particulate form, without being metabolized and mineralized, to enter the detrital
WETZEL: DETRITAL FUNCTIONS
507
IN ECOSYSTEMS
A11ochtl1onouo OOM·!'OM
Autochthonous
~~
DIssolved Org8nic Carbon
Detntus
Primary
Parllculate
Organic
Matter Production
Bactena
Not Gr91.~~.
04""'O"v
{[
Dead Particulate Organic Matter
E eslmn
~----------------------\
,
,
I
I
Indirect Regulatory Roles
In C Cycles and
-roo
DETRITAL DYNAMIC
,
Gut Contents of Detntovores
0.',.,
••O'I'.'u.
l
I
I
I
:
·Oetntus Food Chain"
:~
IOdu_1
I
I
I
I
I
All Non-Predatory
(~L,,"••••
Losses
ft·. UJld.r •• ,IIl'l"'·)
g Special
cases of detntus food
chains already recognized
h
Recuperative
Food ChaIn"
I
STRUCTURE
losses
Perm~nent Sedimentation
PrecipitatIon
Export
""Bal
I
CD .J:
TROPHIC DYNAMIC STRUCTURE
I
I
t
Losses
Emigration
Figure I. Generalized integration of the trophic and detrital dynamics of aquatic ecosystems (from
Wetzel et aI., 1972).
pool for subsequent partial or total bacterial degradation. We elaborated the basic
constructs of these bacterially-dominating metabolic processes by evaluating the
detrital dynamic structure (Fig. 1) as dominating functional components interacting with the trophic dynamic structure (Wetzel et al., 1972; Rich and Wetzel,
1978). Similar conceptual discussions of detrital-dominated
interrelationships
were developed subsequently for marine ecosystems (Pomeroy, 1974; Williams,
1981; Sorokin, 1981). Although meager, evidence from the marine pelagic regions
indicates that the coupling organic matter synthesis and utilization by animals is
tighter, i.e., more efficient, than in fresh waters. Because inputs of photosynthate
and resulting POM and DOM are largely of phytoplanktonic origin in the open
ocean, a greater portion of the organic matter input is diverted, as POM directly
and as DOM through bacterial POM, to animal ingestion and degradation (Menzel, 1974) than is the case in most fresh waters. In the more productive coastal
marine areas with their increased pelagic and benthic productivity and allochthonous organic loading, the detrital, non-animal dominance in metabolism increases, analogous to its prevalence in fresh waters. Benthic bacterial metabolism
of organic matter increases in proportion to the whole for the water column, and
totally dominates in shallow coastal regions.
The importance and general domination of detrital dynamics is often avoided
despite increasing quantitative evidence in its support. As we enter an obviously
vigorous period in which methodology for assaying in situ rates of bacterial metabolism and the chemical composition of detritus is rapidly improving, several
points can be raised as essential to quantitatively understand the true functional
relationships. (1) Bacterial rates of the entire spectrum of DOC from the most
508
BULLETIN OF MARINE SCIENCE, VOL. 35, NO.3,
1984
labile to the most refractory compounds must be comprehensively evaluated.
Even though the rates of turnover of relatively refractory DOM may be low, the
pool size is so large within the ecosystem that this metabolism simply cannot be
quantitatively ignored. In lakes, the benthic detrital metabolism can dominate
and both aerobic and anaerobic carbon flux rates of detrital DOC and poe must
be evaluated quantitatively. (2) The rates of detrital dynamics and trophic rate
functions must be evaluated on an annual basis. Excessive emphasis has been
given to plant-animal trophic rates during optimal growth periods, often in temperate regions in which biotic metabolism exhibits large seasonal differences.
Much of the bacterial detrital metabolism and mineralization exhibits much less
metabolic periodicity and is highly adapted to conditions (e.g., cold, dark) that
are adverse to higher organisms. The only meaningful quantitative scale of comparison of organic carbon metabolic rates is over one complete year. (3) As new
trophic functions are evaluated, their significance can only be effectively compared
if the other heterotrophic processing rates are known. For example, the current
exciting findings that suggest significant protozoan and micro flagellate heterotrophy must be quantitatively compared with the efficacy of bacterial heterotrophyl
of the dominating DOC pool.
Metabolic Stability and Unity in
Aquatic Ecosystems
Living pelagic, and many littoral, non-bacterial organisms of aquatic ecosystems
generally undergo rapid metabolic responses and oscillations in growth and reproduction in an opportunistic series of competitive responses to changes in
availability of constraining nutritional and physical factors regulating their growth.
The integrated functioning of an ecosystem as a whole would be quite energetically
unstable if these oscillating trophic dynamic responses dominated the ecosystem
metabolism. The overall ecosystem metabolism depends to a large extent on the
detritus components of DOC and dead poe that form the primary store of
utilizable energy. The dominant detrital organic carbon reserve is not rapidly
metabolized.
The relatively slow utilization rate of the detrital reservoir imparts metabolic
stability to aquatic ecosystems. Stability is afforded, in part, by the relative refractory chemical structure of the organic substrates and in part by displacement
of much organic matter to anoxic parts of the system, particularly to the sediments.
Nutrient regeneration within the pelagial of lakes largely depends upon detrital
metabolism.
Few unifying ecological concepts presently exist in aquatic ecology. I suggest
that the detrital dynamics and its couplings to the trophic dynamic structure
pervades all aquatic ecosystems in a fundamentally analogous manner. Although
relatively small quantitative differences are emerging, the metabolic dominance
of the photosynthetic-DOe-bacterial
decomposition pathway and the stability it
affords appears to permeate all natural ecosystems. Rather than endlessly seeking
the differences among operational details of ecosystems, the search for such unifying metabolic similarities is a much more meaningful avenue in quantitative
ecology.
I The utilization
of dissolved organic substrates by animals is generally quantitatively very small (Stephens. 1981). Assimilation of
DOC by heterotrophy and photoheterotrophy of algae is very low, even under specialized conditions, in comparison to photosynthetic
inorganic carbon fixation (Droop, 1974; Neilson and Lewin, 1974; Wetzel, 1983).
WETZEL:
DETRITAL
AJNCfIONSINECOSYSTEMS
509
ACKNOWLEDGMENTS
I particularly acknowledge the stimulating discussions of this subject with P. H. Rich and G. L.
Godshalk. D. W. Menzel and R. E. Moeller offered much critical insight on the manuscript. Support
for many of the data leading to these concepts has been provided by the U.S. Department of Energy
(EY-75-S-02-1599, COO-I 599-242) and the National Science Foundation. Contribution No. 534, W.
K. Kellogg Biological Station of Michigan State University.
LITERATURE
CITED
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Droop, M. R. 1974. Heterotrophy of carbon. Pages 530-559 in W. D. P. Stewart, ed. Algal physiology
and biochemistry. Univ. of Califomia Press, Berkeley.
Meybeck, M. 1982. Carbon, nitrogen, and phosphorus transport by world rivers. Amer. J. Sci. 282:
401-450.
Menzel, D. W. 1974. Primary productivity, dissolved and particulate organic matter, and the sites
of oxidation of organic matter. Pages 659-678 in E. D. Goldberg, ed. The sea. V. Marine chemistry.
J. Wiley & Sons, Inc., New York.
Neilson, A. H. and R. A. Lewin. 1974. The uptake and utilization of organic carbon by algae: an
essay in comparative biochemistry. Phycologia 13: 227-264.
Pomeroy, L. R. 1974. The ocean's food web, a changing paradigm. BioScience 24: 499-504.
Rich, P. H. and R. G. Wetzel. 1978. Detritus in lake ecosystems. Amer. Nat. 112: 57-71.
Saunders, G. W., K. W. Cummins, D. Z. Gak, E. Pieczynska, V. Straskrabova and R. G. Wetzel.
1980. Organic matter and decomposers. Pages 341-392 in E. D. LeCren and R. H. LoweMcConnell, eds. The functioning of freshwater ecosystems. Cambridge Univ. Press, Cambridge.
Schlesinger, W. H. and J. M. Melack. 1981. Transport of organic carbon in the world's rivers. Tellus
33: 172-187.
Sorokin, Y. I. 1981. Microheterotrophic organisms in marine ecosystems. Pages 293-342 in A. R.
Longhurst, ed. Analysis of marine ecosystems. Academic Press, New York.
Stephens, G. C. 1981. The trophic role of dissolved organic material. Pages 271-291 in A. R.
Longhurst, ed. Analysis of marine ecosystems. Academic Press, New York.
Wangersky, P. J. 1978. Production of dissolved organic matter. Mar. Ecol. 4: 115-220.
Wetzel, R. G. 1975. Limnology. W. B. Saunders Co., Philadelphia. 743 pp.
--.
1979. The role of the littoral zone and detritus in lake metabolism. Arch. Hydrobiol. Beih.
Ergebn. Limnol. 13: 145-161.
--.
1983. Limnology, 2nd ed. Saunders College Publ., Philadelphia. 860 pp.
--,
P. H. Rich, M. C. Miller and H. L. Allen. 1972. Metabolism of dissolved and particulate
detrital carbon in a temperate hard-water lake. Mem. 1st. ltal. Idrobiol. 29 Suppl.: 185-243.
Williams, P. 1. leB. 1981. Incorporation of micro heterotrophic processes into the classical paradigm
of the planktonic food web. Kieler Meeresforsch., Sonderh. 5: 1-28.
DATEACCEPTED: May I, 1984.
ADDRESS: W. K. Kellogg Biological Station, Michigan State University, Hickory Corners, Michigan
49060.