FEMS Microbiology Ecology 45 (1987) 89-97
Published by Elsevier
89
FEC 00109
Epilithic metabolism of dissolved organic carbon
in boreal forest rivers
T i m E. F o r d a n d M a u r i c e A. L o c k
School of Antmal Biology, Umverstty College of North Wales, Bangor, Gwvnedd, U.K.
Received 7 October 1986
Revision received 7 January 1987
Accepted 8 January 1987
Key words: Epilithic metabolism; Organic carbon; (Colloidal); (Microcalorimetry); (River)
1. SUMMARY
The utilisation of colloidal and dissolved
organic carbon (CDOC) by boreal fiver epilithon
was investigated. The technique of microcalorimetry was used to determine the metabolic response
of the epilithon in terms of both absolute levels
and utilisation of material of apparent molecular
weight (AMW) above and below 1000. Short-term
perfusions were used to measure absolute levels of
metabolic heat output for comparison with other
studies, and long-term perfusions were used to
study the relative utilisation of different molecular
weight fractions. The results indicated low levels
of metabolic heat output from boreal river epilithon with a shorter lag period in the utilisation
of CDOC than we have found in a temperate river
system. Long-term perfusions indicated a considerable increase in metabolic heat output on removal of high-molecular-weight material. We discuss these results in terms of a 'competitive access
model' whereby slowly metabolised high-molecular-weight compounds saturate adsorption sites on
the epilithon surface, resulting in a low metabolic
heat output. When these compounds are removed,
Correspondence to: Tim E. Ford. at present address Laboratory of Microbial Ecology, Harvard Umversity, 40 Oxford
Street, Cambridge, MA 02138. U.S.A.
'labile' low-molecular-weight molecules may more
readily adsorb to or diffuse into the epilithon
matrix.
2. I N T R O D U C T I O N
In fiver systems CDOC frequently represents
40-50% of the total carbon standing stock [1].
Despite its large contribution to organic carbon
budgets, very little is known about its incorporation into river food webs. Studies on the uptake of
CDOC have been primarily restricted to specific
sub-sets, such as leachates from, e.g., leaf litter,
soil, algae, all at greatly elevated concentrations
[2-6]. Although these studies provided a relative
measure of potential utilisation, information on
the fate of the CDOC pool in the river and stream
water itself can only come from direct investigations. A direct approach, however, is hampered by
a lack of knowledge of the chemical components
[7]. Approximately 20% by weight of the CDOC
pool has been classified as specific compounds
(carbohydrates, amino acids, carboxylic acids and
hydrocarbons). Thus, in order to answer the basic
question 'is all this material potentially available
as a source of carbon to the river biota', an
alternative approach to classification/characterisation is clearly required.
0168-6496/87/$03.50 © 1987 Federation of European Microbiological Societies
90
One alternative approach to characterisation
has been to determine the apparent or nominal
molecular weight spectrum, usually through the
use of ultrafiltration [8-11]. The C D O C pool is
divided into a set of A M W fractions using ultrafiltration membranes. Lock and Ford [12] have
previously used this approach, combined with the
technique of flow microcalorimetry [13], to determine the relative contribution of two such fractions ( < 1000 and > 1000 AMW) to the metabolism (as measured by heat output) of a temperate
river epilithon. Comparison with other measurements of community metabolism has shown that
microcalorimetry provides a useful measurement
of metabolic state. Turnover time of [14C]glucose
and [14C]glutamate, and incorporation of [14C]acetate correlated closely with metabolic heat output
from an arctic river epilithon [14]. The epilithon is
the attached microbial community of streams and
rivers and is thought to play a major role in
energy transduction [15]. The temperate river studies showed that the epilithon was greatly buffered
to changes in the composition of the river water.
In many cases, several days were necessary to
achieve a significant decrease in the metabolic
heat output of the epilithon after removal of all
organic material, evidence of a considerable lag in
utilisation of the CDOC.
This paper describes studies to measure utilisation of C D O C by river epilithon from boreal
stream and river systems. These systems contain a
considerable pool of organic carbon (10-15 mg
C - 1 - 1 ) [16] and are highly stained with humic
material. Very little is known about the fate of this
potentially large source of organic energy and,
particularly, the degree to which it is utilised by
the epilithon. In order to improve our understanding of C D O C utilisation in these systems, the
technique of microcalorimetry was used to follow
the metabolic response of river epilithon to waters
of widely differing molecular weight spectra and
individual fractions of the organic carbon pool.
3. M A T E R I A L S A N D M E T H O D S
3.1. The study area
These studies were conducted in the Matamek
region of Quebec, Canada, on six pristine streams
and rivers draining typical northern boreal forest.
Stream size [17], mean annual discharge [16],
C D O C concentration (measured with a Dorhman
54 organic carbon analyzer) and proportion of the
< 1000 A M W fraction (obtained by ultrafiltration
[8]) are given in Table 1, while a detailed description of the sampling sites and information on the
vegetation and geology of the region has been
presented by N a i m a n [16]. This area was studied
during the ice-free seasons of 1983 and 1984.
3.2. Epilithon development and measurement of
metabohc heat output
In each experiment, epilithon was developed on
1.5-mm glass beads threaded onto nylon monofilament line [12,13]. Methods of attachment of these
bead strings in the rivers and streams are described in each section. Metabolic heat output
from the epilithon on the glass beads was determined using the flow microcalorimeter described by Lock and Ford [13]. The microcalorimetry cells (volume 6.7 cm 3) hold a combined
surface area of glass beads of 42 cm 2. The minim u m detection lirmt of the calorimeter is 3 /~W
flow cell-1 (0.07 /~W cm 2 epilithon) and heat
output from a well-developed epilithon (e.g., North
Wales: 107 bacteria cm 2, Ford and Lock, 1985)
can be up to 284 /~W flow cell -1 (6.8 /~W cm -2
epilithon) [13].
3. 3. Short-term perfuston studies
Relative levels of metabolic heat output and
Table 1
S~x b o r e a l rivers a n d s t r e a m s stud~ed m Q u e b e c , C a n a d a
River
Stream Mean
CDOC
order
annual
(mgC.1
discharge
( m 3. s - 1)
First Choice Creek
Beaver Creek
Cran Carre Creek
Muskrat River
Matamek River
Mousse R i v e r
1
2
3
5
6
9
0.013
0.033
0.2
8.4
13.7
466.1
% < 1000
1) A M W "
2
15
18
11
8
5
A v e r a g e of 4 m e a s u r e m e n t s f r o m s u m m e r , 1983.
b A v e r a g e of 2 m e a s u r e m e n s f r o m s u m m e r , 1984
30
11
5 b
11
19
19
91
short term response to manipulations in the
organic energy supply were compared in epilithon
communities from four different stream and river
sites.
Bead strings were attached to Plexiglass plates
which were inserted into 3-m-long, 15-cm-diameter tubes which were either submerged in the
rivers by attachment to a floating pontoon (Moisie
River, Matamek River and Beaver Creek) or fed
directly with stream water via a small dam (First
Choice Creek). At each site there was one opaque
tube (for heterotrophic growth) and one clear tube
(for both phototrophic and heterotrophic growth).
In addition, Z-bend light traps [12] were fitted to
both the dark and light growth tubes to maintain
the same hydraulic conditions between the two
incubation chambers.
Epilithon-coated beads were collected simultaneously from the 'light' and 'dark' tubes at each
sampling date and returned to the laboratory in
river water. Beads with heterotrophic epilithon
were maintained under dark conditions and no
chlorophyll a was detected confirming the absence of algae. Epilithon-colonised beads were
then loaded into flow microcalorimeters and the
heat output measured as described by Lock and
Ford [13]. In each experiment (summarised in
Table 2a) the epilithon was first perfused to heat
output equilibrium at ambient river temperature
using river water filtered through a pre-combusted
W h a t m a n ( G F / F ) glass fiber filter. Subsequently,
the perfusion medium was switched to river water
with compounds > 1000 A M W removed by ultrafiltration. Preparation of the ultrafilters to miniraise contamination and the filtration technique
were performed as described by Ford and Lock
[8]. After 1 - 2 h, the medium was switched to an
organic matter-free (OMF) medium with an inorganic composition similar to that of the river
water [12]. After a further 1 - 2 h, the epilithon was
acid-killed in order to obtain the total heat output
[13]. Heat outputs in the presence of the > 1000
A M W organic matter, < 1000 A M W organic
matter, and the O M F medium were obtained by
difference.
3.4. Long-term perfusion studies
The results of the short-term experiments (see
Table 2
S u m m a r y of perfusion experiments
Experi- Epllithon
ment
(a) 1
Colonised beads
from each river
(dark + light)
Perfuslon M e d m m
Respective
river water
{
< 1000 A M W
$
OMF
+
Perfuslon
time
> 4h
1-2 h
1-2
Kill
(b) 2
Colonised beads
from each river
(dark except
Matamek)
(1) Respecnve
raver water
J,
Kill
(II) < 1000 A M W
4 days
4 days
Kill
(c) 3
Colomsedbeads
River water from
with 'enriched'
specific raver
epihthon from
{
Matamek River
Kill
(dark + light)
(repeated for each river)
4 days
RESULTS AND DISCUSSION) and previous work in
the Clywedog River, North Wales [12] indicated a
lag in the utilisation of organic matter by the
epilithon. We performed a series of experiments
involving long-term perfusion (4 days) of epilithon
with either unfractionated river water or river
water with the > 1000 A M W fraction removed.
Heterotrophic epilithon was developed on beads
in the Matamek River, Muskrat River, Cran Carre
Creek and First Choice Creek in 1984. Colonised
beads were perfused in the laboratory for 4 days,
with either river water or river water with material
> 1000 A M W removed (Table 2b). After perfusion, the epilithic heat output responses were measured as above. Because of the large volumes of
ultrafiltrate required for this work, a Millipore
Tangential Flow Cell with a 1000 nominal molecular weight ultrafiltration cassette was used to remove the > 1000 A M W fraction from the river
water. Before filtration, a 5% NaC1 solution, followed by several liters of ultrapure water (Milli-Q
system), were passed through the ultrafilter to
minimise leachate contamination.
92
3.5. Comparative microbial labzhty of riuer and
stream waters
A bioassay was set up to determine the metabolic response of a 'standard' epilithon to river
and stream waters known to have widely differing
A M W spectra of CDOC. This was set up to test
the hypothesis that water, as it moves through the
drainage system, becomes progressively depleted
in microbially labile organic matter. If this is true,
one would expect the largest epilithic heat responses from water from the smallest streams, and
the smallest responses from the largest rivers.
'Standard epilithon' was grown on bead strings
placed in a flume (R.J. Gibson, Ph.D. thesis, University of Waterloo, Canada, 1973) built into the
Matamek River, the second largest river in the
study area [18]. Because this site was located towards the lowest point in the drainage network of
the river [18], it was assumed that an epilithon
growing there would have been naturally exposed
to the more recalcitrant of organic compounds
and thus be potentially capable of some metabolic
response to them. Based upon the results of the
short-term perfusion studies, which showed low
levels of epilithic activity, the 'standard epilithon'
was enriched with a mixture of xylose, mannose,
galactose and dextrose (1 : 1 : 1 : 2) to a final concentration of 9/~g C- 1-1. Thus, the epilithon was
exposed to both high-molecular-weight 'recalcitrant' compounds, and (albeit at extremely
low concentrations) labile low-molecular-weight
compounds.
Experiments were run in September, 1984, using
both the dark (heterotrophic) and the light (phototrophic/heterotrophic) epilithon. Colonised beads
were perfused in the laboratory for 4 days in
individual microcalorimeter cells with water from
the 6 rivers listed in Table 1 (method summarized
in Table 2c), while maintaining the ambient temperature of the Matamek River. After 4 days, the
heat outputs in response to the 6 different waters
were determined.
4. R E S U L T S
proportional contribution of the C D O C A M W
fractions to epilithic metabolism are presented in
Table 3. All the incubation systems were affected
to some extent by silting or foaming inside the
tube. This meant that the epilithon at each site
occasionally had to be disturbed in its development period to maintain a clear flow of water
through the incubation chambers. For the first
sampling period, a surface area of epilithon equivalent to that colonising the glass beads was scraped
off the rocks, for each river, and filtered onto a
G F / F glass fiber filter. The filter was then rolled
up and placed in the calorimeter cell. This substitution was made due to insufficient colonisation
of the glass beads.
Six heat output measurements (0.05-0.67 ~tW •
cm -2) were obtained for heterotrophic (dark
grown) epilithon, two of which were relatively
substantial: 0.35/~W. cm -2 in First Choice Creek
(1 September ) and 0.67 ~tW. cm -2 in Muskrat
River (16 September). First Choice Creek epilithon did not respond to short-term manipulation
of the perfusion medium (with all the heat output
representing heat in the presence of the O M F
medium). However, the Muskrat River epilithon
did respond with 19% of the heat output attributable to organic matter > 1000 AMW, 15% attributable to material < 1000 AMW and 65% of the
heat output occurring in the presence of the O M F
medium.
Ten heat-output measurements of sufficient
magnitude to construct a heat budget (0.27-1.05
/~W. cm -2) were obtained from the phototrophic/heterotrophic epilithon. Of these ten, 5
responded to manipulation of the organic matter
content of the perfusion medium spread over all 4
rivers. Heat outputs attributable to organic matter
> 1000 A M W occurred in the Muskrat River and
First Choice Creek (18% and 32% of total heat
outputs, respectively). Heat outputs attributable to
organic matter < 1000 AMW were found in all
four rivers, ranging from 9-31% of the total heat
output, while the heat outputs obtained by perfusing with the O M F medium ranged from 48-100%
of the total heat output.
4.1. Short-term perfusion studies
Data on total heat output (metabolism) and the
4.2. Long-term perfuslon
The results of the long-term perfusion study are
93
Table 3
The epdithic m e t a b o l i c heat output a n d p r o p o r t t o n of heat output attributed to the > 1 000 A M W fraction, the < 1000 A M W
fraction and the organic matter free medium ( O M F )
Sample
Heat output
Heat output (%)
period a
(~tW. c m - 2)
> 1000
Heterotrophic epilithon ( d a r k - g r o w n )
First Choice C r e e k
2
Beaver Creek
Muskrat Rwer
< 1 000
OMF
0.05
-
-
_
3
0.35
0
0
100
2
0.05
-
-
_
2
0.13
-
-
_
4
0.67
19
16
65
4
0.12
-
P h o t o t r o p h l c + heterotrophic epihthon (light-grown)
First C h o i c e C r e e k
1
0 55
Molsle R i v e r
Beaver Creek
Muskrat River
Molsle R i v e r
_
-
32
20
48
2
3
1.03
0.52
0
0
0
0
100
100
1
2
0.80
0.13
0
-
19
-
81
_
3
4
0.27
0 22
0
-
0
100
_
1
0.23
18
9
73
2
013
-
3
0.60
0
0
100
4
1.05
0
14
86
1
0.35
0
31
69
3
0.98
0
0
100
d 1, 1 - 8 June; 2, 2 0 - 2 6 June; 3, 31 A u g - 5 Sept; 4, 1 6 - 1 9 Sept.
shown in Table 4. Heat output measurements
from long-term perfusion with whole river water
are similar to those obtained in the previous
short-term experiment, ranging from 0.22-0.87
ttW • cm 2. However, long term perfusion with the
< 1000 A M W fraction resulted in a greater range;
0.15-1.72 # W - c m -2. In fact, in 12 out of 15
(80%) of the experiments, perfusion with the <
1000 A M W fraction resulted in epilithic heat outputs greatly in excess of those produced through
perfusion with whole fiver water.
4.3. Comparative microbial labifity of river and
stream water
The results of this experiment are shown in Fig.
1. Metabolic heat output was plotted against total
C D O C concentration for each stream and river to
show the strong negative correlation between these
"•
20
LIGHT (autotrophs +
::k
I--F0
~
I
10
"~....~.~.R
K (heterotrophs)
o
en
~ o.5
0
I
0
2
I
I
6
I
I
10
CDOC mg ~-I
[
I
14
[
I
18
Fig. 1. R e l a t i v e macroblal lability of C D O C from 6 boreal
rivers ( T a b l e 1) as i n d i c a t e d b y the metabolic heat output from
a ' s t a n d a r d ' epdithon after a 4 - d a y p e r f u s i o n period.
94
Table 4
Metabohc heat output from eplhthon from four boreal ravers perfused with unfractionated river water and river water with organic
matter > 1000 AMW removed
River
Heat output (/~W.cm 2) from
Sample
month
Algae
free
(A) Water
(B) < 1000 AMW
fraction
(B) as % of (A)
First Choice
Creek
June
August
Yes
Yes
0.28
*
0.50
0.82
178
Cran Carre
Creek
June
August
September
Yes
Yes
Yes
0.82
0 22
0.87
1.18
0.40
1.30
144
182
149
Muskrat
River
August
September
September
Yes
Yes
Yes
0 40
0.50
*
*
0.87
1.08
174
Matamek
River
August
September
September
September
September
September
September
Yes
Yes
Yes
Yes
No
No
No
0.28
0.28
0.57
0.32
0.22
0.35
*
0.15
I 12
0.93
0.15
1.72
1.65
1.22
53
400
163
46
781
471
* Below detection
two factors, for b o t h the light- a n d the d a r k - g r o w n
epilithon (r=-0.962
a n d - 0 . 8 1 4 respectively,
N = 6). M e t a b o l i c heat o u t p u t r a n g e d f r o m 0 . 7 - 2 . 0
/~W c m -2 in the light a n d from 0 . 1 - 1 . 0 # W - c m -2
in the dark, generally higher t h a n those o b t a i n e d
in the previous experiments. T h e effect of the low
level e n r i c h m e n t to o b t a i n a ' s t a n d a r d e p i l i t h o n '
resulted in a 1 . 5 - 8 - f o l d increase in m e t a b o l i c heat
o u t p u t over an u n e n r i c h e d c o n t r o l epilithon ( d a t a
n o t shown).
5. D I S C U S S I O N
Overall, the level of m e t a b o l i c activity in epil i t h o n f r o m these b o r e a l rivers was low ( < 1.05
/~W. c m 2) in c o m p a r i s o n with e p i l i t h o n f r o m a
t e m p e r a t e fiver in N o r t h Wales, U . K . (0.8-6.8
~ W . c m -2 [13]). Differences in m e t a b o l i c heat
o u t p u t c o u l d be due to differences in m e t a b o l i c
activity a n d / o r b i o m a s s . In this s t u d y we d i d n o t
a t t e m p t to distinguish b e t w e e n activity or growth,
b e c a u s e o u r p r i m a r y interest was in the relative
lability or r e f r a c t o r y n a t u r e of the organic m a t t e r
in transport.
S h o r t - t e r m p e r f u s i o n studies in three s e p a r a t e
rivers i n d i c a t e d that organic m a t t e r > 1000 A M W
m a d e a s u b s t a n t i a l c o n t r i b u t i o n to the total
m e t a b o l i c heat o u t p u t (18-32%). This was in contrast to the N o r t h W a l e s w o r k [12], where > 1000
A M W m a t e r i a l d i d not c o n t r i b u t e to total
m e t a b o l i s m d u r i n g s h o r t - t e r m perfusions, suggesting that b o r e a l river epilithon has the c a p a c i t y to
m e t a b o l i s e larger a n d p o s s i b l y m o r e recalcitrant
o r g a n i c c o m p o u n d s . This m a y b e an a d a p t a t i o n to
the e x t r e m e l y high c o n c e n t r a t i o n s of h u m i c
m a t e r i a l f o u n d in the b o r e a l rivers. T h e heat o u t p u t due to m a t e r i a l < 1000 A M W r e p r e s e n t e d
9 - 3 2 % of the total heat o u t p u t for the b o r e a l
rivers while the equivalent p r o p o r t i o n s in the
W e l s h rivers were 0 - 1 5 % . However, in b o t h river
systems, the heat o u t p u t in the presence of an
O M F m e d i u m was substantial (48-100% of total
m e t a b o l i s m ) i n d i c a t i n g utilisation of stored p r o d ucts in the e p i l i t h o n m a t r i x [12]. T h e lack of
s h o r t - t e r m r e s p o n s e to changes in the organic
energy in t r a n s p o r t has been a t t r i b u t e d to a lag in
95
organic matter utilisation [12]. We consider that
this lag may be due to exoenzymatic processing of
the stored products prior to the change in the
organic matter supply [19]. In order to overcome
this lag, a long perfusion period was chosen to
allow sufficient time for the epilithon to adapt to
the new river water, through mechanisms such as
enzyme induction and bacterial selection.
In the long-term perfusion experiment, substantial amounts of organic matter (70-95%, Table 1)
were removed in the ultrafiltration process. Thus,
we anticipated that the heat output from epilithon
perfused with this fraction would be less than the
heat output from whole river water. However, this
was not the case, and our results suggest that the
presence of high-molecular-weight ( > 1000 AMW)
compounds somehow slows down epilithic metabolism. Alternatively, a limiting nutrient might
have leached from the membrane during ultrafiltration (i.e., N O 3 has been found to leach from
cellulosic membranes; S. Horrigan, personal communication). However, our washing procedure was
extensive, and the bioassay experiment where no
ultrafiltration occurred supports an inhibitory role
for high-molecular-weight material.
In the bioassay experiment, we expected a
negative correlation between the heat output responses and increasing stream size, because a decrease in metabolic heat output could be expected
as organic material becomes more recalcitrant
down the river continuum [20]. However, we found
no correlation with stream size, but rather a strong
negative correlation with the concentration of
C D O C in the fiver water. This relationship could
be explained by previous studies that had shown
that high C D O C concentrations in the study area
were due primarily to an increase in high-molecular-weight organic carbon (Table 1) [21]. Again,
the data can be explained by a hypothesis that
high-molecular-weight organic matter in some way
inhibits or slows down microbial metabolism.
Several mechanisms can be proposed for the
effect of high-molecular-weight material on epilithic metabolism.
This material may: (1) immobilise micronutrients
necessary for metabolism [22]; (2) contain toxic
compounds (e.g., phenolic groups associated with
humic material) that act as antibiotics [23]; or (3)
saturate 'adsorption sites' on the epilithon matrix
[24].
If binding of necessary micronutrients was occurring, these would be removed in the ultrafiltration process, and no stimulation of metabolism
would be expected on the removal of high-molecular-weight material. The presence of toxic compounds associated with humic material may inhibit metabolism on the surface of the epilithon,
but the polysaccharide matrix would not readily
allow their diffusion [15]. Furthermore, the polysaccharide matrix of a biofilm has been shown to
provide bacteria with extremely efficient protection from antibiotic activity [25]. The very large
increases in metabolic heat output associated with
removal of this material are unlikely to be associated solely with the loosely attached microorganisms on the surface of the epilithon. These
loosely attached microorganisms may be transient
from the water column, exhibiting lower metabolic
activity than those in the epilithon matrix [8].
Recent work on the uptake of C D O C by epilithon [24] presents strong evidence that the initial
step for organic matter uptake by the epilithon is
an abiotic adsorption of the organic molecules to
the surface of the film. We therefore favour the
third mechanism for the inhibition phenomenon.
We suggest that there may be competitive access
problems for molecules diffusing into or adsorbing to the epilithon matrix. The removal of the
more microbially recalcitrant high-molecularweight organic matter would then allow greater
access for the low-molecular-weight molecules. The
high-molecular-weight material may be utilised
only at a very slow rate, requiring an initial
breakdown step by extracellular enzymes [19],
whereas smaller molecules could potentially enter
microbial cells directly, thus eliciting an immediate and substantial metabolic response. This
model is particularly appropriate if we consider
that the epilithon acts as an ion-exchange matrix
[25]. As such, the matrix would be expected to
have a finite number of active sites which become
loaded with high-molecular-weight material.
In conclusion, the use of short-term perfusion
studies indicates that high-molecular-weight material may be utilised as an organic energy source in
these systems. However, the long-term perfusions
96
indicate that its presence prevents utilisation of
more rapidly metabolised low-molecular-weight
material, which supports a far more active epilithon. Studies on activated sludge microorganisms [26] and on pure cultures of bacteria
[27] have also indicated metabolic inhibition by
high-molecular-weight material, suggesting that
this effect may be a general phenomenon. If this is
so, then it would appear that there is an additional
source of inefficiency (other than respiration and
maintenance costs) in the conversion of organic
matter into microbial biomass.
ACKNOWLEDGEMENTS
We are most grateful to the Woods Hole Oceanographic Institution, U.S.A., the Natural Environment Research Council, U.K., and the Royal
Society, U.K. for funding this work. We also wish
to express our thanks to Dr. Robert J. Naiman
and Mr. R. Morin for creating a superb working
environment at the Matamek Research Station.
We thank J. Maki, J. Manning and S. Horrigan of
Harvard University for critical review of this
manuscript.
LITERATURE CITED
[1] Mlnshall, G.W., Peterson, R.C., Cummins, K.W., Bott,
T.L., Sedell, J.R., Cushing, C.E. and Vannote, R.L. (1983)
lnterbiome comparison of stream ecosystem dynamics.
Ecol. Monogr. 53, 1-25.
[2] Dahm, C.N. (1981) Pathways and mechanisms for removal of dissolved organic carbon from leaf leachate in
streams. Can. J. Fish. Aquat. Sci. 38, 68-76.
[3] Kaplan, L.A. and Bott, T.L. (1983) Microbial heterotrophic utilization of dissolved organic matter in a piedmont stream. Freshwater Biol. 13, 363-377.
[4] Kaplan, L.A. and Bott, T.L. (1985) Acchmation of
stream-bed mlcroflora: metabolic responses to dissolved
organic matter. Freshwater Biol. 15, 479-492
[5] Lock, M.A. and Hynes, H.B.N. (1975) The disappearance
of four leaf leachates in a hard and soft water stream in
South Western Ontario, Canada. Int. Rev. Ges. Hydrobiol. 60, 847-855.
[6] McDowell, W.H. and Fisher, S.G. (1976) Autumnal
processing of dissolved organic matter in a small woodland stream ecosystem. Ecology 57, 561-569.
[7] Thurman, E.M. (1985) Organic Geochemistry of Natural
Waters. Nijhoff/Junk, Dordrecht.
[8] Ford, T.E. and Lock, M.A. (1985) A temporal study of
colloidal and dissolved organic carbon in rivers: apparent
molecular weight spectra and their relationship to bactenal
activity. Olkos 45, 71-78.
[9] Gloor, R., Leldner, H., Wuhrman, K. and Fleischmann,
T.L. (1981) Exclusion chromatography with carbon detection: a tool for further characterization of dissolved organic
carbon Water Res. 15, 457-462.
[10] Kaplan, L.A, Larson, R.A. and Bott, T.L. (1980) Patterns
of dissolved organic carbon in transport. Linmol. Oceanogr. 25, 1034-1043.
[11] Meyer, J.L and Tate, C.M. (1983) The effect of watershed
disturbance on dissolved organic carbon dynamics of a
stream. Ecology 64, 33-44.
[121 Lock, M.A. and Ford, T.E. (1985) Mlcrocalonmetric approach to deterrmne relationships between energy supply
and metabohsm in river epilithon. Appl Environ. MIcroblol. 49, 408-412.
[13] Lock, M.A. and Ford, T E. (1983) An mexpenslve flow
microcalonmeter for measuring the heat production of
attached and sedimentary aquatic microorganisms. Appl.
Environ. Mlcrobiol. 46, 463-467.
[14] Peterson, B.J., Hobbie, J.E., Hershey, A.E., Lock, M.A.,
Ford, T.E., Vestal, J.R., McKinley, V L., Hullar, M.A.J.,
Miller, M.C, Ventullo, R.M. and Volk, G.S. (1985)
Transformation of a tundra river from heterotrophy to
autotrophy by addition of phosphorus. Science 229,
1383-1386.
[15] Lock, M.A., Wallace, R.R., Costerton, J.W., Ventullo,
R.M. and Charlton, S.E. (1984) River epllithon' toward a
structural-functional model. Oikos 42, 10-22.
[16] Nalman, R.J. (1982) Characteristics of sediment and
organic carbon export from pristine boreal forest
watersheds. Can. J. Fish. Aquat. Sci 39, 1699-1718.
[17] Strahler, A.N. (1957) Quantitative analysis of watershed
geomorphology. Trans. Am. Geophys. Union 38, 913-920.
[18] Naiman, R J. (1983) The annual pattern and spatial distribution of aquatic oxygen metabolism m boreal forest
watersheds. Ecol. Monogr. 53, 73-94.
[19] Lock, M.A. (1981) River eplllthon--a light and organic
energy transducer, m Perspectives in Running Water Ecology (Lock, M.A and Williams, D.D., Eds.), pp. 3-40.
Plenum Press, New York.
[20] Vannote, R.L., Minshall, G.W., Cumrmns, K.W., Sedell,
J.R. and Cushmg, C.E. (1980) The river continuum concept. Can. J. Aquat. Scl. 37, 130-137.
[211 Lock, M.A and Ford, T.E. (1986) Colloidal and dissolved
organic carbon dynamics m undisturbed boreal forest
catchments: a seasonal study of apparent molecular weight
spectra. Freshwater Biol. 16, 187-195.
[22] Chnstman, R.F. and Mlnear, R.A. (1971) Orgamcs in
Lakes, in Organic Compounds in Aquatic Environments
(Faust, S.J. and Hunter, J.V., Eds.), Marcel Dekker, New
York
[23] Jackson, T.A. (1975) Humic matter in natural waters and
sediments. Soil Sci. 119, 56-64.
[24] McDowell, W.H. (1982) Mechanisms controlling the
97
organic chemistry of Bear Brook, New Hampshire. Ph.D.
thesis, Cornell University.
[25] Costerton, J.W., Marrie, T.J and Cheng, K.-J. (1985)
Phenomena of bacterial adhesion, in Bacterial Adhesion
(Savage, D.C. and Fletcher, M., Eds.), Plenum Press, New
York and London.
[26] Chudoba, J. (1985) Inhibitory effect of refractory organic
compounds produced by activated sludge microorganisms
on microbial activaty and flocculation. Water Res. 19,
197-200.
[27] Schmidt, S.K. and Alexander, M. (1985) Effects of dLssolved orgamc carbon and second substrates on the biodegradation of organic compounds at low concentrations.
Appl. Environ. Mlcrobiol. 49, 822-827