WATER QUALITY: Technical Information--Briefing paper on "The ecological role(s) of aquatic microorganisms in lakes and reservoirs" 

In Reply Refer To:                              August 26, 1980
EGS-Mail Stop 412

QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 80.23

Subject: WATER QUALITY: Technical Information--Briefing paper on 
         "The ecological role(s) of aquatic microorganisms in 
         lakes and reservoirs" by Bruce L. Kimmel, Biological 
         Station and Department of Zoology, University of 
         Oklahoma, Norman, Oklahoma

The enclosed discussion of "The ecological role(s) of aquatic 
microorganisms in lakes and reservoirs" was prepared at the 
request of the Quality of Water Branch by Dr. Bruce Kimmel, 
Assistant Director, University of Oklahoma Biological Station. 
Bruce has been extremely helpful in assisting with our training 
course "Fundamentals of Lake Limnology" which is held at the 
Biological Station.

This briefing paper is a continuation of the series initiated by 
the Quality of Water Branch in August, 1974. Please circulate it 
as widely as possible in all District and project offices. 

                           R. J. Pickering
                           Chief, Quality of Water Branch

Enclosure

Key words: water quality, information, briefing paper, 
           microorganisms, bacteria.

Superceded memoranda: none

WRD Distribution: A, B, FO-L, PO

      THE ECOLOGICAL ROLE(S) OF AQUATIC MICROORGANISMS 
                   IN LAKES AND RESERVOIRS

                             by

                       Bruce L. Kimmel 1/

ABSTRACT

Microorganisms form vital links in the structure of aquatic 
ecosystems by virtue of their roles in organic matter production 
and decomposition, nutrient uptake and regeneration, foodweb 
transfers, and biogeochemical transformations. By the nature of 
their biological activities, autotrophic and heterotrophic 
microorganisms are sensitive indicators of the ecological and 
water-quality status of aquatic environments. However, only by 
thoroughly understanding how microorganisms function in healthy 
aquatic ecosystems can we hope to recognize or accurately predict 
their responses to water-quality changes.

INTRODUCTION

Much attention in the field of water quality is focused on 
microorganisms as indicators of pollution (for example, fecal 
coliform and fecal streptococci bacteria), threats to public 
health (for example, pathogenic bacteria and viruses), or symptoms 
of water-quality deterioration (for example, nuisance algal 
blooms). Because characteristics common to water quality problems 
are readily noticeable, the public often receives the mistaken 
impression that the presence of microorganisms signals the demise 
of a body of water. As an example, the term "algae" may connote 
surface scums and foul odors to the layman, while "bacteria" 
implies disease-causing contamination. In fact, such extreme 
conditions are rare, and usually occur only in systems which have 
been altered significantly by excessive loading of nutrients, 
organic matter, and/or toxic contaminants. Under typical 
circumstances, microorganisms such as algae and bacteria form 
vital links in the structure of aquatic ecosystems, and indeed are 
the operative factors in organic matter production, decomposition, 
and nutrient regeneration. As professionals in the field of water 
quality and communicators of water-quality information to other 
agencies and the general public, it is important to maintain a 
perspective in regard to the ecological significance of aquatic 
microorganisms. The purpose of this discussion, therefore, is to 
review the ecological roles of aquatic microorganisms in lakes and 
reservoirs.

1/ Biological Station and Department of Zoology, University of 
Oklahoma, Norman, Oklahoma 73019.

AN OVERVIEW OF ECOSYSTEM STRUCTURE

Before specifically addressing aquatic microorganisms and their 
ecological roles, let us briefly consider the basic structure of 
lentic (that is, lake-like or standing-water) ecosystems. 
Photosynthetic production usually forms the organic matter base of 
aquatic foodwebs. Organic matter formed photosynthetically by 
primary producers (for example, algae) is consumed by herbivores 
(for example, zooplankton), which are fed upon by higher consumers 
(carnivores; for example, fish). Predatory transfers, the 
ingestion and assimilation of living primary producers by 
herbivores and herbivores by carnivores, collectively are referred 
to as the "grazer pathway". All of these 'food web levels produce 
excreted or defecated material and dead organisms which are 
utilized by "decomposers" (for example, bacteria and fungi). 
Decomposers promote the breakdown of organic materials to 
inorganic compounds, and therefore, generally are assigned the 
role of nutrient regeneration. Non-predatory transfers, which 
involve the use of non-living organic matter by decomposers, 
collectively are termed the "detritus pathway" (Fig. 1).

Although the flow of energy and materials through both grazer and 
detritus pathways is recognized, the relative importance of the 
two pathways in various types of ecosystems appears not to be. It 
is often generalized that the grazer pathway dominates ecosystem 
energy flow in grasslands and pelagic (open-water) systems, while 
the detritus pathway is more important in forests, marshes, and 
streams. However, this generalization should not be accepted 
unquestioningly. Although the magnitude of detritus accumulation 
is obvious in some terrestrial and semi-terrestrial systems in 
which litter (for example, fallen leaves and dead marsh grass) is 
readily visible, it is much less apparent in the open waters of 
lakes and reservoirs where the "litter" (for example, dead algal 
cells and zooplankton fecal pellets), as well as many of the 
producers and consumers, is of microscopic size.

This discussion considers primarily those aquatic microorganisms 
which fall into the "Producer" and "Decomposer" categories 
described above; however, as will be seen, some of these 
microorganisms also perform roles assignable to "Herbivores" and 
"Consumers . Our discussion will focus on two fundamental aspects 
of the ecological roles of microorganisms in lakes and reservoirs:

l) Algae as primary producers, and bacteria as decomposers and 
   nutrient regenerators; and
2) Bacteria and fungi as particle producers and potential food 
   sources for higher consumers.

ORGANIC MATTER PRODUCTION AND DECOMPOSITION

Algae and certain bacteria produce organic matter by the process 
of photosynthesis, which is the formation of organic compounds 
from carbon dioxide (C02) and other inorganic nutrients by way of 
light energy and the chlorophyll pigment in plants. Some bacteria 
produce organic compounds by chemosynthesis, in which energy-
yielding chemical reactions, rather than sunlight, provide the 
energy necessary for C02 fixation. Both photosynthesis and 
chemosynthesis result in the formation of organic material from 
inorganic components, and therefore are considered to be 
autotrophic (primary production) processes.2/  Although some 
bacteria are autotrophic, most are heterotrophic and like other 
heterotrophs (for example, herbivores and higher consumers) they 
grow by assimilating organic materials previously synthesized by 
autotrophs. The metabolic processes of all organisms are energy 
consumptive, and require adenosine triphosphate (ATP, formed 
during cellular respiration) as their "energy currency". All 
living organisms, in order to continue living, must conduct 
cellular respiration, which is in some ways the reverse of 
photosynthetic carbon fixation (Fig. 2). Organic matter 
decomposition, a collective term describing the net conversion of 
organic material back to inorganic compounds (for example, C02, 
H20, and nutrients), occurs by virtue of the respiratory 
activities of those heterotrophic microorganisms using the organic 
material for growth.

Carbon dioxide and oxygen are reactants in the photosynthesis-
respiration equilibrium (Fig. 2), and the balance or imbalance of 
these processes within a water body can have profound chemical and 
biological consequences for the entire system. The photosynthetic 
production of oxygen is restricted to the upper, lighted regions 
of the water column (the euphotic zone). Respiration is not 
directly light-dependent, but is dependent on temperature. 
Respiration and organic matter decomposition occur through the 
water column, and depending on the amount of organic matter 
present and the extent of wind-mixing, can deplete the dissolved 
oxygen supply in aphotic (unlighted) layers. The chemical and 
biological conditions associated with such oxygen depletion (for 
example, increased nutrient release from sediments, exclusion of 
oxygen-dependent fauna, and less complete decomposition of 
sedimented organic matter) are marked, and often characteristic of 
lake trophic status. In oligotrophic (nutrient-poor, unproductive) 
lakes, insufficient organic matter is produced for decomposition 
processes to affect available oxygen levels to any significant 
extent. In contrast, organic matter decomposition in eutrophic 
(nutrient-rich, productive) lakes can rapidly deplete dissolved 
oxygen in aphotic parts of the water column.

MICROBIAL NUTRIENT TRANSFORMATIONS AND REGENERATION

Thus far, organic matter production and decomposition have been 
discussed primarily in terms of carbon dioxide, water, and oxygen 
exchanges. However, living-organisms are not composed entirely of 
C, H, and 0, but also contain small amounts of many other 
elements. Table 1 compares the elemental composition of living 
freshwater plants to the environmental availability of elements 
for plant

2/ However, most chemosynthesis results from energy released in 
the oxidation of reduced compounds (S-2, NH4+, CH4) derived from 
the decomposition of previously formed organic matter, and thus is 
actually a special type of secondary production. A notable 
exception, where chemosynthesis appears to be a true autotrophic 
process, is represented by the recently discovered Galapagos Rift 
hydrothermal vent ecosystems. Here, chemosynthetic bacteria fix 
C02 with energy derived from the oxidation of geothermically-
reduced sulfur compounds emitted from the ocean floor vents (see 
Karl and others, 1980).

Table l.--Concentrations of essential elements for plant growth in 
the living tissues of freshwater plants (demand), in mean world 
river water (supply), and the approximate ratio of elemental 
concentrations required to those available (that is, demand:supply 
ratio)(after Vallentyne, 1974).

                          DEMAND BY     SUPPLY IN    DEMAND:SUPPLY
ELEMENT         SYMBOL    PLANTS (%)    WATER (%)        RATIO

Oxygen            0        80.5           89                  1
Hydrogen          H         9.7           11                  1
Carbon            C         6.5            0.0012         5,000
Silicon          Si         1.3            0.00065        2,000
Nitrogen          N         0.7            0.000023      30,000
Calcium          Ca         0.4            0.0015       < 1,000
Potassium         R         0.3            0.00023        1,300
Phosphorus        P         0.08           0.000001      80,000
Magnesium        Mg         0.07           0.0004       < 1,000
Sulfur            S         0.06           0.0004       < 1,000
Chlorine         Cl         0.06           0.0008       < 1,000
Sodium           Na         0.04           0.0006       < 1,000
Iron             Fe         0.02           0.00007      < 1,000
Boron             B         0.001          0.0000l      < 1,000
Manganese        Mn         0.0007         0.0000015    < 1,000
Zinc             Zn         0.0003         0.000001     < 1,000
Copper           Cu         0.0001         0.000001     < 1,000
Molybdenum       Mo         0.00005        0.0000003    < 1,000
Cobalt           Co         0.000002       0.000000005  < 1,000

~

growth (that is, demand-vs-supply). Phosphorus, nitrogen, and 
carbon are the least available elements relative to plant growth 
requirements, and thus are most likely to limit biological 
productivity in aquatic systems.

Although essential nutrients are incorporated into organic 
compounds during organic matter synthesis, they are regenerated as 
a result of excretion processes and organic matter decomposition. 
Organic matter decomposition (Fig. 3) occurs by (1) hydrolysis of 
large (high molecular weight) organic polymers into smaller (low 
molecular weight) compounds, and (2) mineralization of these 
smaller organic molecules to inorganic compounds (for example, 
H2S, NH4+, and P04 ). Bacterial decomposition of organic materials 
accounts for a large fraction of the total nutrient regeneration 
which occurs; however, organisms such as phytoplankton, 
zooplankton, and fish excrete NH4+, P04 , and low molecular weight 
organic compounds, and therefore are also important nutrient 
regenerators.

In an oxygenated environment, organic matter exists in a 
chemically-reduced state (low Eh, high potential energy) relative 
to most of its inorganic components (Fig. 2). The oxidation of 
certain mineralization products (for example, S-2 S!Q>S04 -2, NH4+ 
Q>NO2Q ->N03, and CH4Q>C02) provides the energy for bacterial 
chemosyntheses. The basic types of microbially-mediated 
transformations important in carbon, nitrogen, and sulfur cycling 
are summarized in Tables 2, 3, and 4; and integrated into the 
overall nutrient cycling patterns for these elements in lake 
systems in Figures 4, 5, and 6.

Phosphorus does not undergo the oxidation-reduction 
transformations characteristic of C, N, and S cycles (Fig. 7). 
Inorganic phosphorus occurs in aquatic environments primarily as 
orthophosphate (P04 -3), and is incorporated directly by algae and 
bacteria into phospholipids, nucleic acids, and ATP. Although 
phosphorus turnover time (the time required for phosphorus 
assimilation and regeneration) is often only a matter of minutes 
in surface waters during the summer (Lean, 1973), sedimentation of 
organic particles and precipitation of inorganic phosphorus with 
iron and manganese complexes can cause phosphorus depletion in the 
mixed layers of lakes and reservoirs. Naturally low levels of 
available phosphorus in aquatic environments, the essentiality of 
phosphorus for plant growth, and the high phosphorus concentration 
in certain by-products of our modern society (for example, 
fertilizers, detergents, and municipal sewage) make phosphorus an 
especially important element in regard to water quality. In short 
supply, phosphorus availability can severely limit biological 
productivity, while an overabundance of phosphorus can result in 
nuisance conditions concomitant with accelerated eutrophication.3/ 
Since phosphorus is often the least available inorganic nutrient 
in aquatic environments (that is, relative to demand, see Table 
1), extensive efforts have been undertaken in the past decade to 
minimize phosphorus loading of natural waters.

3/ Eutrophication is a natural process involving the slow 
accumulation of nutrients and organic matter, and gradually 
increased productivity in an aquatic system. Accelerated 
euthrophication results from excessive nutrient loading from the 
inflow of municipal wastes and/or drainage from fertilized 
agricultural land, and often is associated with nuisance algal 
blooms, taste and odor problems, and severe oxygen depletion 
accompanied by fish kills.

Although biogeochemical cycles often are discussed individually by 
element for the sake of simplicity. it should be realized that in 
nature they are inseparable. Carbon, nitrogen, sulfur, phosphorus, 
and other essential elements are assimilated simultaneously during 
organic matter synthesis, and similarly, are released 
simultaneously during organic matter decomposition. Because the 
availability of certain elements (for example, P, Fe, and Mo) 
influences the uptake or transformations of others (C, N), the 
elemental cycles are highly interdependent.

BACTERIA AS PARTICLE PRODUCERS AND POTENTIAL
FOOD SOURCES FOR HIGHER TROPHIC LEVELS

The photosynthetic conversion of inorganic compounds to organic 
matter is only the initial process in a sequence of trophic 
interactions collectively referred to as "the foodweb". The most 
familiar foodweb sequence (primary producer > grazer > higher 
consumer; the grazer pathway) may present an oversimplified view 
of trophic interactions in lakes and reservoirs by ignoring non-
predatory transfers (primary producer > detritus > bacteria > 
grazer > higher consumer; the detritus pathway). More 
specifically, the conversion of dissolved organic compounds 
(derived from algal extracellular excretion or cell lysis, 
zooplankton excretion, and/or allochthonous sources 4/) to organic 
particles by way of bacterial uptake and growth represents a 
potentially important mechanism for reintroducing otherwise 
unavailable (that is, unharvestable) dissolved organic matter to 
the foodweb (Fig. 8). For pelagic marine systems, Pomeroy (1974) 
indicated that the producer-detritus-bacteria-grazer sequence 
could provide at least 30 percent more energy to higher trophic 
levels than estimated from consideration of only a producer-grazer 
pathway.

However, whether bacterial secondary production provides a 
significant source of particulate organic matter for planktonic 
grazers is a controversial question at present. Free-living 
(unattached) bacteria are probably too small (<l um) to be 
efficiently harvested by most zooplankton; however, bacteria 
colonizing suspended detritus or otherwise associated with 
aggregates of detritus, algal cells, and/or silt particles may be 
grazed readily by virtue of their greater effective particle size. 
Planktonic ciliates (a type of Protozoa) may help bridge the 
"particle-size gap" between free-living bacteria and grazers by 
ingesting bacteria and then becoming prey for filter-feeding 
zooplankton (see Porter and others, 1979). The occurrence of 
attached-vs-unattached bacteria in pelagic environments appears to 
be highly variable, both temporally and among systems, as numerous 
investigators have reported contradictory results and conclusions.

______________
4/ The term "allochthonous" refers to organic matter originating 
from outside the lake (e.g., material from the watershed). In 
contrast, "autochthonous" material is that produced within the 
lake (for example, phytoplankton production).

Although algal photosynthesis is the primary mechanism of organic 
matter production in most lakes and reservoirs, the potential 
significance of heterotrophic microorganisms as both secondary 
producers (by conversion of dissolved organic matter to a 
particulate form) and consumers (by respiration) of organic matter 
should be recognized. Indeed, bacterial production may be of 
particular ecological importance in many reservoirs for the 
following reasons:

l) Reservoirs usually receive large quantities of allochthonous 
organic materials by tributary inflow from their watersheds. Some 
labile (biologically available) fraction of this allochthonous 
input may provide major supplements of organic substrates for 
bacterial production.

2) Erosion products from the watershed (for example, suspended 
silt and clay particles) are potential sites for adsorption of 
dissolved organic compounds and bacterial attachment/aggregation, 
and thereby may constitute a harvestable source of organic matter 
for planktonic grazers. Alternatively, such microbial-detrital 
aggregates may sediment rapidly from the water column and result 
in the transport of organic matter and nutrients to benthic 
communities.

3) In turbid, well-mixed reservoirs, phytoplankton production is 
often limited more by available light than by nutrients. In 
contrast to photosynthesis, bacterial production is not directly 
light-dependent and therefore, may form a predominant fraction of 
the particulate organic matter supply available to grazers in 
light-limited systems. Especially for reservoir foodwebs, in which 
bacteria may be of particular importance, an improved 
understanding of algal and bacterial production and respiration 
processes, and the relative availability of these primary and 
secondary particle producers (that is, phytoplankton and bacteria, 
respectively) to grazers is required.

SUMMARY

The classical trophic-dynamics approach (Lindeman, 1942) persists 
as an ecological paradigm even though the nondiscrete nature of 
trophic levels was recognized by Lindeman and is well known to 
most ecologists. Although useful as a conceptual framework, the 
trophic level approach may lead to a simplistic view of energy and 
materials flow in nature (Wetzel and others, 1972; Rigler, 1975; 
Goldman and Kimmel, 1978). Most aquatic organisms can be 
considered as article producers (by either autotrophic or 
heterotrophic production of protoplasm), consumers (assimilation 
of dissolved or particulate organic compounds), and decomposers 
(by conversion of organic materials to inorganic materials by 
respiration and excretion). Nutrient regeneration, the classical 
role of bacteria and fungi, takes place at virtually every level 
of the foodweb. Figures 9 and 10 summarize contrasting views of 
energy and nutrient flow in lakes and reservoirs. Particularly for 
reservoirs, our present level of understanding of ecosystem 
dynamics is inadequate to determine the relative accuracy of 
either view.

Microorganisms, by virtue of their pivotal roles in organic matter 
production and decomposition, foodweb transfers by dissolved to 
particulate organic matter conversions, nutrient uptake and 
regeneration, and biogeochemical transformations, are essential to 
the ecological functioning of aquatic systems. By the nature of 
their biological activities, autotrophic and heterotrophic 
microorganisms are sensitive indicators of the ecological and 
water-quality status of aquatic environments. However, only by 
thoroughly understanding how microorganisms function in healthy 
aquatic ecosystems can we hope to recognize or accurately predict 
their responses to water quality changes or other environmental 
disturbances.

REFERENCES

Brock, T. D., 1966, Principles of microbial ecology: Englewood 
Cliffs, New Jersey, Prentice-Hall, 306 p.

Campbell, R., 1977, Microbial ecology: London, Blackwell 
Scientific Publications, 148 p.

Goldman, C. R., and Kimmel, B. L., 1978, Biological processes 
associated with suspended sediment and detritus in lakes and 
reservoirs, in Cairns, J., Benfield, E. F., and Webster, J. R., 
(eds.), Current perspectives on river-reservoir ecosystems: North 
American Benthological Society, Publication 1, p. 19-44.

Karl, D. M., Wirsen, C. 0., and Jannasch, H. W., 1980, Deep-sea 
primary production at the Galapagos Hydrothermal Vents: Science, 
v. 207, p. 1345-1347.

Kuznetsov, S. I., 1959, Die Rolle der Mikroorganismen im 
Stoffkreislauf der Seen: Berlin, VEB Deutscher Verlag der 
Wissenschdften, 301 p.

Lean, D. R. S., 1973, Phosphorus dynamics in lake water:  Science, 
v. 197, p. 678-680.

Lindeman, R. L., 1942, The trophic-dynamic aspect of ecology: 
Ecology, v. 23, P- 399-41 8.

Pomeroy, L. R., 1974, The ocean's food web: A changing paradigm: 
Bioscience, v. 24, p. 499-504.

Porter, K., Pace, M., and Battey, J., 1979, Ciliate protozoans as 
links in freshwater planktonic food chains: Nature, v. 277, p. 
563-565.

Rigler, F. H., 1975, The concept of energy flow and nutrient flow 
between trophic levels, in van Dobben, W. H., and Lowe-McConnell, 
R. H., (eds.), Wageningen, W. Junk B. V. Publ., (eds.), Unifying 
concepts in ecology, p. 15-26.

Sorokin, Y. I., and Kadota, H., (eds.), 1972, Techniques for the 
assessment of microbial production and decomposition in 
freshwaters: London, Blackwell Scientific Publications, IBP 
Handbook No. 23, 112 p.

Vallentyne, J. R., 1974, The algal bowl: Lakes and man: Ottawa, 
Canada Department of the Environment, Miscellaneous Special 
Publication 22, 186 p.

Wetzel, R. G., Rich, P. H., Miller, M. C., and Allen, H. L., 1972, 
Metabolism of dissolved and particulate detrital carbon in a 
temperate hard-water lake: Memorie dell'Istituto Italiano di 
Idrobiologia, 29 Suppl., p. 185-243.

Additional Recommended Reading

Deevey, E. S., 1970, Mineral cycles: Scientific American, v. 223, 
p. 148-158.

Delwiche, C. C., 1970, The nitrogen cycle: Scientific American, v. 
223, p. 136-146.

Hutchinson, G. E., 1970, The biosphere: Scientific American, v. 
223, p. 44-53.