Freshwater Biology i\9W) 22, LV32
Growth of macrophytes and ecosystem consequences
in a lowland Danish stream
KAJ SAND-JENSEN, ERIK JEPPESEN.* KURT NIELSEN.t
LILLIAN VAN DER BIJL. LUISE HJERMIND. LISBETH WIGGERS NlELSENt
imd TORBEN MOTH IVERSHN* Freshwater Biological Laboratory,
Hclsingorsgadc 5\. DK-34UO Hillcr0d. Denmark
SUMMARY. I. Ihc River Susa is a small, nulricnt-rich stream situated in
an open landscape with elayish subsoil under intensive cultivation. Discharge was variable daily and seasonally due to low groundwater input.
Above-ground devetoptnent of submerged tnacrophytes was restricted to
late May to Novetnher, when waler velocity and depth were low. Dotninant macrophyles were rooted Potumogeum pectinattis and Spar^imium
emersttm and unrooted Cladoplumt. Biomass development was closely
related to light av;tilability.
2. Growth rates ol m;tcrophytes were linearly related to light availability
when selt-shading was accounted for. Potamogeton pectinatu,s ^TQW Vd^\d\y
in May-June, concentrated the biomass at the water-surface during JulyAugust, and then declined expotientially when the sht>ots became basally
senescent. Spurt^anium emerstnn had linear, flexible leaves that were
continuously replaced from a basal meristem. Sparganium emersum was
less susceptible to high water velocities than Potamogcton pectinatus and
the biomass declined later and at lower rates during autumn. Spar^anhmi
emersum also regrew after cutting that left its meristem intact in the
sediment. Unrooted Cludophora developed a high biomass during sunny
periods and subsequently disappeared at high discharges. The summer
biomass of rooted macrophytes was greater in years with high sumtiier
discharge, whereas the hiomass of Cludophimi and of the epiphytic
microbial community was lower due to scouring.
3. Submerged maerophytes played a key role in structure and functioning of the ecosystem. They reduced water velocities two to four fold during
summer and promoted extensive organic sedimentation. The biomass of
benthic diatoms declined parallel to increased tnacrophyte shiitling and
seditnentation. In addition, submerged macrophytes formed a large
substratum for macroinvertebrates and for a microbia! community.
'Present address: Aarhus Amts Viindvarsen. t.yseng AIIO 1. S27O Hojbjfrg, Dcnnuirk.
•'Present ;idUress: Aarhtis Amts V;indarcscn. l.yseng AIIO 7. S27() Hojhjery. Dennuirk.
C•orre^pnndenee: l>r K;ij Sand-Jensen, !Jnt;iiiic;il instiliite, Nnrdliindsvej W. S24() Kissko\. Dctimiirk. Freshwater t.iibiirat(iry. Aiicncy ol Envirnnnientiil Proteetinn. Lyshrovej .^2. Wi(H) Silkeinirij. Denmark.
15
16
Kaj Sand-Jensen et al.
Introduction
T.ow-gradienl streams are an important feature
of the iipcn hindscapc of nuitiy tL-mpcraie lowland regions. Small lowhind strcatiis are charai:tcrizcJ hy low water velocities and tine-grained
sediments, which provide a suitable environment for submerged macrophyte growth.
Speeies composition and ahundancc of submcrped macrophytes are regulated by management regimes and physico-chemical stream
conditions which are mollified in turn by Ihe
submerged tnacrophytes (Butcher. I'>.V^: Olscn.
195(1; Wcstlake, 1975). In particular, submerged
macrophytes reduce water velocity, increase
water level and promote sedimentation
(Wcsllake. 1975: Kern-flansen i-i al.. 19S(I).
They inhibit growth of microbenthic algae
directly by shading and indirectly by increasing
the water level, and thus Ihe length of the light
path to the sediment surface (Sand-Jensen.
1983). Addiiionally. they provide shelter for fish
and invertebrates, and serve as substratum
for invertebrates, microalgae and bacteria
(Witcomb. 1%8; Hynes. 1970; Mortensen.
1977).
Despite the marked influence submerged
macrophytes have on lowland streams, little is
known about their seasonal dcvclopmetit or the
factors that result in differences iti macrophvte
development both among streams and between
years in a [larticular stream. Most European
lowland streams drain fertile farmland atid
receive ati abundant supply o\ inorganic
nutrients such as nitrogen and phosphorus which
satisfy
the
macrophyles'
requirements
(Westlake. 1975: Kern-Mansen <V
, Dawson.
I97S). Physical factors, such as hydrology and
light availability, should iherefote account for
most of the observed variability in macrophyte
growth and abundance. High dischargeincreases water veloeity and creates a mechanical stress on plants and setlimetiis. High
discharge also reduces lighl availability by
increasing water depth and turbidity. The
impt)rtance of light on stream tnacrophytes has
beeti clearly demonstrated. Dawson tV: Kernllanscn (197S. 1979) showed that shading by
bank vegetation resulted in the reduction of the
maximum sumtner biomass of submerged
macrophytes in shallow Danish streams. In
deeper Etiglish streams. Dawson (|97(i)
observed an exponential decrease of plant
biomass with water depth coincident with exponential light attenuation.
Here we examine the seasonal de\clopment
of submerged macrophytes in a nutrient-rich
Danish lowland stream (River SusJi) to (I)
establish the relationship between seasonal
plant development and water discharge and light
availahility. and (2) study the response of
different macrophyte spceies to harvesting and
occasional dredging of surface sediments. The
ultimate goal of this contribution is to demonstrate the key role of maerophytes in the ecology
ol' Uiwland streams by quantifying their
inflticnce un the light climate, water velocity,
sedimentation regime and population dynamics
of microbenthic algae.
Study sites
1 he Kt\er Susa is the largest river of Zealand.
Denmark. It isS7 km long and drains S15 km-of
glacial deposits dominated hy clay (Berg et al..
194S). The soil is rich in carbonates, resultitig in
high bicarbonate alkalinity (aboul 4 meq I ':
Jacobsen. 19SI). The catchment is mainly farmland ((t9'V ). and the maiority oi this is drained
(Kelstrup.Binzer& Knudsen. 1981). Because of
the poor permeability of the clay soil, the river is
fed mainly by ilrainage water and by water from
the secondary grt>undwater reservoir; less than
UVr is frotii the primary groundwater reservoir
(Dyhr-Nielsen. 1981). The secondary groundwater reservoirs consist of water-lilled pockets
of satid OT gravel in the clay positioned above the
permanenl primary groundwater table.
We selected three, unshaded. 4(H) m reaches
in the river as sampling stations; one at Pindso.
l.^ km from the source and two next to each
other at Vetterslev, 3S km downstream. Flow at
all stations is regulated and the channel is
straight with a utiiform depth. The bottotn sedinienls consist of clay and sand at Pitidso and
clay, sand and gravel at Vettcrslev. The mineral
sediments were covered by several centimetres
of organic detritus during summer. In previt)us
years, macrophytes had been harvested once or
twice during the summer lo reduce waler depth
and the riskof flooding of the fields, in addition,
the herbaceous vegetation on the banks had
been cut during aututnn. At Pindsii. macrophytes wete cut manually or surf:ice sediments
and macrophytes were dredged and placed on
Macrophytes in streams
the banks. At Vetterslev, macrophytes were cut
about 10 ctii above the sediment by a special boat
equipped with rotating knives. During the 3
years of invesligation (1979-81). cutting of
marginal vegetation continued but other
activities were abandoned except for moderate
cutting of submerged macrophytes at Vetterslev
in 1980 and 1981 to avoid flooding.
The reach at Pindso was 2.5-5 m wide, and the
gradient ranged from 0,28 to 0.84 m km"' (Table
1). Input of biologically treated sewage was 0.5 I
s"', about 3% of the median minimum discharge. Mean POj-P concentration was moderately high (0.14 mg I ' ) . and the mean NOi-N
concentration was high (5.6 mg I '). The two
reaches at Vetterslev were similar to each other:
5-9 m wide with a slope of 0.13-0.59 m km '
(Table 1). The high rate of sewage input (501 s"'
or 37% of the median minimutn water discharge) and agricultural activity resulted in
greater nutrient concentrations (0.5 mg POj-P
| - ' , 7 . 7 m g N O r N T') than those at Pindso.
TABLE 1. PhysiL'o-cheniical chiiractcrisUcs (if reaches
at Pindso :mij VetlLTsIfv in Ihu River Stisa. Data for
scwiige input iirid uinccntrations of inorganic N and P
are for iy7K-K0 from .lacobscn (IWI). Mcdiim
minimum tlow is from Bjurnov & l.und;igcr-Jcnson
Variable
Slope (m km ')
Widlli(m)
Median minimum
flow (Is 1)
Sowiige input (1 -^ ')
Inorganic P. mean
(/(g 1 ') viiriiiiion
Inorganic- N, mean
(mg 1 1) variation
Pindso
(I.2S-O.84
2.5-.S
29
U.5
140
5.6
l).9-17
Vetterslev
(l.t.MI..'>'*
134
50
500
SI)-1600
7,7
1.5-17
Methods
17
Light availability in the stream was calculated
from measurements of surface light and light
attenuation by the water and the submerged
macrophytes. Surface photon flux density (/n,
PAR) was measured continuously at the
Hydrotechnical Laboratory. 40 km away. The
attenuationeoefficientof the water(A:,. m"')was
calculated by linear regression of photon flux
(Lambda sensor) versus depth measured at 5 cm
depth intervals in macrophyte-free areas. The
median attenuation coefficient was the same for
all stations (2.4 m ' . /i^24). The attenuation
coefficient attributable to the biomass of the
dominant macrophyte, Potamogeton pectinatus
L. (/c^, m' g"' DW of biomass) was measured
eight times during the season in a laboratory
set-up with the method of Westlake (1964). The
^2 value was calculated by linear regression of
the natural logarithm to the ratio of incident to
transtnittcd photon llux that passed different
plant densities (details in Bijl & Hjermind.
1985). To calculate the mean photon flux density
(/;) at a given depth (2, m) we used a conservative HK/r estimate for surface reflection (Wetzel,
1983) and the mean value {k:) for plant attenuation in the formula:
where B is the mean total plant biomass (g DW
m ' ) above the depth z. When r equals the mean
water deplh and B equals the mean plant
biomass, then L represents the mean photon
flux density reaching the sediment surface. Since
we neglected shading from banks and emergent
vegetation in our calculations, we used it primarily eariy in the season (May-June) before
marginal plants became tall and dense. At dates
between sampling we estimated A, by linear
interpolation and B was interpolated from the
figures showing biomass versus time. A': was
found to be constant over the season at 0.024 ±
0.003 ( S D ) m - g - ' D W .
Waler discharge and light
Continuous hydrologic records of water level
at Pindso and Vetterslev allowed the ealculatit)n
of discharge, using established relationships
between discharge and water level and biomass
of macrophytes (Det Danske Hedeselskab.
1979, 1980, 1981). Mean velocity was measured
by itijecting an acetic solution of rhodamine B in
a transverse section 15 m upstrcatn from each
reach. Passage time of the red concentration
peak was followed visually.
Macrophyte biomass
The macrophyte biomass was measured at
about 10 day intervals from May to November.
Calculation of mean biomass was based on 400
estimates of plant cover and twenty measurements of plant biomass on each sampling day.
Each 400 m reach was divided into five 80 m long
sections and two transects were placed across the
river within eaeh section using random numbers
18
Kaj Sand-Jensen et al.
between 0 and 80. Transects were moved 2 tn
upstream on successive sampling days to avoid
disturbance. Transects were divided into twenty
equal intervals and plant cover was estimated for
each interval along the transect according to a
seven step scale: 0, l=0-10'fr, 2=10-30%,
3=30-50%, 4-50-70%. 5=70-90%. and 6=90100%. Two biomass samples were taken randomly among the twenty intervals along each
transect. Positions chosen for biomass samples
among the twenty were not reused. Therefore.
the twenty samples for the reach were evenly
distributed across the transverse profile, but
could still be treated statistically as randomly
selected (Jtppesen. 1978). Biomass samples
were taken by cutting a cylindrical core (surface
area 195 cm') through the vegetation and harvesting the above-ground parts. Cladophora
occurred intermingled with the above-ground
parts of rooted macrophytes and the amount of
Cladophora attached to stones was negligible.
Plants were rinsed, sorted into species and
weighed after drying at 105°C to constant
weight.
Mean biomass for the stretch [B) was calculated by combining the frequency of coverindices 1-6 (/"i-/h) with the average biotiiass for
eaeh cover category (5,-6^):
/;
(2)
Variance of frequency and biotnass for each
cover category were combined when confidence
limits of the mean biomass for the stretch were
calculated. Confidence limits at 95% were
usually within 40% of the mean total biomass
and 40-60'f of the mean biomass for
common spt-eies like Potamogeton pectinatus.
Sparganium emersum Rehman and Cladophora.
Confidence limits of similar magnitude were
calculated directly from the twenty biomass samples without including the information on cover.
Nevertheless, webelieve that the estitiiate which
included cover is rnore accurate than the confidence litiiits indicate because it showed an even
and reproducible seasonal paUcrn, whereas the
simple direct estimates displayed erratic and
unexpected changes in biomass.
Organic material and tnicroalgae in the sediment
Sedimented organic material and benthic
microalgae were examined by the same frequency and sampling strategy as above. Detailed
treatments of the two variables appear
elsewhere (Sand-Jensen. M0ller & Olsen. 1988;
Jeppesen & Sand-Jensen, unpublished). The
sediment surface consisted of light-coloured
minerals after high winter discharges. With
reduced water velocities during summer, dark
organic detritus accumulated on top. Twenty
cores (surface area 21 cm') were collected on
each sample date and the detritus layer was
extruded. The sediments were dried at IO5°C
overnight and organic content determined from
tnass-loss after ignition at 55(rC,
Measurements of chlorophyll a in surface
sediments were used as an index of the biomass
of benthic microalgae. Twenty cores (surface
area 21 cm-) were collected on each sample date
and the upper 0.5 cm of the surface sediment
(approx. the euphotic zone. Fenche! &
Straarup. 1Q71) was extruded and extracted in
^iWt- ethanol (w/w after addition to sample) in
the dark for 18 h in the refrigerator. Chlorophyll
a was measured spectrophotometrically with
correction for degradation products by the
acidification procedure (S0ndergaard & Riemann. 1979).
Results
Hydrology
Discharge was highly variable both on a seasonal and daily scale (Fig. 1). Maximum monthly
winter discharge at Vetterslev was usually (i.ll7.0 m ' s " ' and summer discharge during July was
0.4-1.8 m"s '. Mean discharge was below 0.4 m^
s ' during July in most years, asin 1979.The two
following years were extraordinarily wet, with
high summer discharges, particularly in I9K1.
Discharge at Pindso was lower because of its
location closer to the source, but showed similar
seasonal patterns. Short periods with heavy precipitation alternating with dry. hoi weather were
also tapidly retlectcd in discharge as indicated by
the large range for each month (Fig, I).
Changes in mean velocity for a particular
reach (Fig. 2) were due mainly to variatiotis in
discharge and macrophyte biomass. Velocity
increases with increasing discharge and
decreases with increasing macrophyte biomass
because of the frictional resistance of the plants.
Mean velocity was therefore markedly reduced
during summer, when discharge was low and
macrophyte biomass was high (Figs. 1-3). Mean
Macrophytes in streams
19
F I G . 1. Mean tnonthly discharge at Veticrslcv from 1979 to 19S1. Vertical bilr^ show ihc monthly range and
uil hiirs iiuirk periods with tlischargc iibovc 1 m ' s ' , C'ylculatoii from daily mciisurcmciits.
velocity during summer increased from 1979 to
1981 because discharge increased (Figs. 1 and2).
Macrophyte biomass during summer increased
from 1979 to 19SI but was not sufficient to offset
the velocity increase caused by greater discharge
(Figs. 1-3). The influence of macrophytes on the
^
velocity was quantified by calculating the
expected velocity in the absence of macrophytes
from einpirical relationships between velocity
and discharge developed for each station during
the macrophyte-free period (Det Danske
Hedeselskab. 1979. 1980). This indicated that
20 -
J
M
M
J
1979
S
N
J
M
M
1980
J
S
N
J
M
M
1981
F K i . 2 . M e a n w a t e r v e l o c i t y a t P i n d s o (^>) a n d V e t t e r s l e v ( • ) d u r i n g
J
S
N
20
Kaj Sand-Jensen et al.
TABLE 2. Effect of macrophytes on moan water velocities during summer at reaches at Pindso and Vetlerslev.
The mean and ranges of measured Wiiter velocities ( H - 4 - 7 ) arc compared with estimated water velocities in the
absence of mucrophytes. For further information see text.
Water velocity (cm s"' )
Biomass
Station
Period
Measured
Estimated
Pindso
7 julv to 1 August I9H0
10-27 August 1981
29 June to 17 August 1981
ti.4 (5.8-7.0)
14.4 (10,5-18.4)
18.9 (13.1-24.7)
21.0(20.0-22.0)
33.5 (31.0-36.0)
28.8 (24.5-33.0)
Vetterslev
macrophytes were responsible fora 1.3-3.5-fold
decrease in velocity (Table 2). The macrophytes
had a greater relative effect on velocity at low
than high discharge.
Seasonal pattern in macrophyte biomass
The biomass of submerged macrophytes was
dominated by two rooted species. Poiamogeton
pectinattis and Sparganium emersum and by
Cladophora sp. that grew intermingled among
the rooted plants. Development of aboveground part^ was usually restricted to late May
through to November, when light availability
120-133
170-207
60-220
was high and discharge and velocity were low
(Figs. 1-3). During the high autumn-winter
discharges of 19X0 and 1981 all above-ground
parts disappeared, reappearing next springs by
sprouting from overwintering tubers {P. pectinattts) or rhizomes [S. emersum) in the sediment. During the winter of 1979-80. when
discharge was lower than usual. .S'. etnersum
maintained a small above-ground biomass at
Vetterslev.
Total above-ground biomass of all species
increased during June, stayed high in JulyAugust and declined during September-November (Fig. 3). The pronounced summer peak in
150
100
50
J
M
M
J
1979
N
J
M
M
J
1980
J
M
M
J
1981
FIG. 3. Mean ;ibove-groiind biumass of ;ill submerged macrophytes ;ii Pindso and Vetterslev during
Arrows mark macruphyte cutting at Vetterslev.
Macrophytes in streams
1979 was due to the rapid accumulation of
Cladophora (see below); in 1980 and I9H1, when
development of Cladophora was low. total
biomass showed a summer plateau. Maximutii
total biomass was about the same in 1979 and
1980 (r. 130 g D W m - a t Pindso. 147gDWm at Vetterslev) and higher in 19SI (192 g D W m at Pindso and 256 g DW m"- at Vetterslev).
Potamogeton pectinatus dominated the total
biomass at Pindso and co-dominated with
Sparganium emersum at Vetterslev (Figs. 4 and
5). The biomass of Potamogeton pectinatits
increased exponentially during June, the summer maximum was maintained for a period of
21
40-50 days, and the biomass declined exponentially during September-November (Fig. 4).
The biomass of Sparganium emersum increased
and declined over long periods and at lower
rates than Potamogeton pectinatus (Fig. 5).
Sparganium emersum grew successfully after
weed cutting, whereas regrowth was small for
Potamogeton pectinatus. The maximum biomass
of both species increased markedly from 1979 to
1981 (Figs. 4 and 5).
Cladophora
development
was erratic.
depending on discharge (Fig. 6). Because
Cladophora was unattached and intermingled
with the rooted macrophytes it may occur occa-
200
100
/•
\-0-04l
V «
50
10
1
\
200-168
\-O-O35
• \
5-
/0-07l
/
\
• /
\
/o-O46
\-0-051
;
•
/
•
/t
O
M
2A S O
1979
M
J
J A
1980
S
J
J
A
1981
S
0
200
100
1
50
E
Q
20
O
10
E
o
•3
5
0-094
J A
A S
1980
1979
FIG. 4. Mean above-ground biomy.ss of P«/um«^'('/(»/(p(-cr//u(fU5 at Pindso (above) and Vetterslev I (below) during
1979-81. The biomass is plotted on a logarithtnic sciile and linear regressions have been fitted to phases of
exponential biom;iss increase ;ind decline. Points were included so as to attain the besl fil ;ind the most significatit
regression coefficient. The values shown are tiican rates of biomass changes in logn, units day '. Confidcntrc
intervals were omitted for clarity, but 95% C.L. were 4l)-6()% of the mean biomass. Arrows indicate macrophyte
cutting.
J
J
22
Kaj Sand-Jensen et al.
lOOi
——
40
-0'046
J
J
f
l
S
O
M
J
J
A
S
O
M
J
J
A
S
O
S 0
FIG, 5. Mean above-ground biomass of .Sparganium emersum al Vetterslev I (above) and Vetterslev II (below)
during 1979-Sl. See legend lo Fig. 4.
sionaily and rapidly disappear (e.g. only two
records at Pindso 1980 Ltnd one al Vettcrslev
lysi). OtJring sunny periods with low stable
discharge (e.g. August 1979. June I9SU). the
biomass increased rapidly due lo the intrinsically
high growth rate of Cladophora. During high
discharge. Cladophora was washed out and the
biomass declined abruptly. With increasing
mean summer discharge the ma.ximum biomass
of Cladophora dropped from ?2-72 g DW m" - at
the three reaches in 1979 to 2-15 g DW m ' in
19S1.
Furthermore, the period of high
Cladophora biomass became shorter from 1979
to 1981 (Fit; 6).
Light and biomass development
Although the seasonal pattern of biomass
de\elopmeni was similar at the two stations
during the three years (Fig. 3). important
differences existed in the timing and rates of
biomass changes during phases of increase and
decline. We tested whether these differences
could be predicted from the light conditions. We
plotted the rate of biomass accumulation
between successive sampling dates up to the
time of maximum biomass versus the average
photon flux density available to the plants in the
same sampling intervals. We defined the rate oi
biomass accumulation {k. day ') as
k=
log,,, S;-l
(3)
where H^ and B^ are the biomass at day fi and />
respectively. Since loss of plant material was
small for Potamogeton pectinatus during the
phase of increasing biomass (Bijl 6L Mjermind.
1985). biomass accumulation was a measure of
the relative growth rate. We calculated the mean
photon \^v\\ density available to the plants (/.) as
Ihal reaching through half of the water column
and half of the mean plant biomass (i.e. /,^().9/,,
exp (-0.5A:|r-0.fiA:B. seeeq. 1). thus accounting for surface photon flux density, light attenua
Macrophytes in streams
23
100
1979
-
10
'
\
t
\
i
t
i
1980
'
/
J
\ ',
s ',
\ \
V "
10
/
/
\
\\
\\^
\
1
_^
*
1981
10
"
'
B_
—
• ^
'
"
/
_
_
^
-o
/
F I G . 6. M e a n biomass of loose Cladophora'^p.
at Pindso ( o ) , Vetterslev 1 ( • ) al Vctlerslev II ( x ) during
Horizonial bars indic;ite periods wiih discharges a b o v e 1 m ' s ' at Veiterslev.
tion in the water column and self-shading by the
plants. The relative growth rate for each sampling interval was linearly correlated with the
available photon flux density with 69-86% of the
variation in growth rates being accounted for by
variations in light availability (Fig. 7). This result
is highly satisfactory, considering the uncertainties associated with the biomass estimates
and our coarse approximation of the light
availability.
Moreover, we found [hat a single Hnear relationship between log biomass and the cumulative light availability from 1 April (initiation ()f
sprouting), could explain most of the biomass
increase (r=().72-(l.y3. Fig. K). as well as the
differences in timing and rates of biomass
increase during the three years of investigation.
Although biomass accumulation and growth
rates were closely related to light availability,
there were differences between the two stations
in the slope of growth rate (or biomass development) versus light availability (Figs. 7 and S),
These differences may be due to differences in
the stratified depth distribution of plant
biomass.or to greater shading from the banks at
the narrow reach at Findso.
24
Kaj Sand-Jensen et al.
Sparganium emersum replaced its leaves continuously (Nielsen. Nielsen & Sand-Jensen.
I98.S) atid kept some leaves during w inters of low
discharge and high light availability (Fig. 3:
1^79). This continuous rejuvenation probably
means that the age-dependent decline is less
important for Sparganium emersum than for
Potamogeton pectinatus. The rate of biomass
decline for Sparganium emersum during autumn
was influenced by high discharge, but remained
lower than for Potamogeton pectinattis (Fig. 9).
Microbenthic algae and organic .'iedimentation
8
12
16
20
Phofon tlui densily (mol
24
28
^ doy"')
FIG. 7. Riite of biomass change (RBC) at Pindso (;i)
and Vctlerslev (b) for each sampling inicrviil during
the biomass iiicrense us a funetion ol mc;iri daily
photon flii.x densily (PFD. 4(X>-71H) nm) aviiilablc nftcr
passing ihrougi hall the water column and half ihe
plain hiomass Unfilled symbols represent Potamogeton pt'ctiratus and filled-otit symbols all submerged macrophyles logelher Data for all three
years are included. Regression equations are (a)
R B C ( ) ( H e : P F D H K O . r' = tlfiy and (h) RBC
Regulation of bioma.Ks decline
Potamogeton pectinatus and Spargantutn
emersum had different growth morphologies
and patterns of biomass decline. Potamogeton
pectinatus had an age-dependent decline in
autumn, following senescence of the main stem.
The decline started every year after flowering
following a period of 4 ( ^ 0 days w hen biomass
was stable (Fig. 4). This paltcrn was observed
despite mean light availahility and discharge
during autumn remained close to conditions
seen during the phase of biomass increase {Fig.
y). The biomass declined exponentially (l-ig. 4)
at rates that were accelerated by mechanical
stresscau.sed by highauiumndiscliarges(Fig. 9).
The rate of biomass decline was calculated by
the same Uigaiithmic function ;is usetl for the
earlier grt>wth leq. ,i).
Submerged macrophytes had an important
influence on the development of microbenthic
algae and the accumulation of organic material
in the sediment. Maximum development of
microbenthie algae was restricted to a short
period from mid-April to late May (Fig. 10). The
increase and decline of biomass were closely
related to the photon flux density at the sediment. Low insolation and high light attenuation
in the water (i.e. high turbidity and water depth)
limited light availability before mid-April,
whereas shading by macrophytes limited the
growth of microbenthic algae after May. Cutting
of macrophytes increased isolation of the sediment in September I9St enablingthe occurrence
of a secondary maximum in microbenthic algae
biomass (Fig. 111).
The accumulation of organic sediment
amounted to several centimetres, and paralleled
Ihe development of macrophytes (Fig. 11).
1 he organic material consisted of fine particles
that probably derived mainly from the
agricultural catchment. Submerged macrophyles with their large surfaces (up to 7 m-'
per m- of bottom), promoted particle sedimentation by reducing the mean water velocity
(Table 2) and by stabilizing the sediment
(Stepbcns(7n/.. 1%.1: Dawson. 1978). The maximum anuiunt of accumulated organic material
(7ll(Mi()(l g organic DW m ') exceeded the maximum macrophyte biomass by a factor of about
10. A sudden increase in discharge, natural
senescence of macrophytes during autumn and
culling oi niacrophytcs led to resuspension and
loss of the accumulated organic material (Fig.
11). In contrast, the progressive increase in
water velocity during summer from li'79to UWl
(Fig. 2) did not decrease the amount of organic
material. Moreover, the higher macrophyte
(o)
(c)
a/
100
y^ A
/ «
10
(d)
(0)
* /
• /
100
•
y%
10
800
1200
800
2000
1600
1200
2000
1600
Cumulaiive photon flux density (mol
FiG. 8. Mean above-ground biomass (B) of Potatnogeton pectinatus and all submerged macrophytes together at
Pindso (a and b) and Vettcrslcv (cand d) during Ihree years as a function ofcumulative photon flux density (CPFD)
availalile after passing through half Ihe water column and half ihe plant biomass. Available lighl was cumulated
from I .April and uniil maximum biomass and accounts lor surface insolation, rellection, waler ailenualioii and
self-shading. Symbols represeni the diflcrcnt years: I97y (o), iys()(«)and IWl (A). Regression equations are (a)
log fl=l).lH)33CPFD-2.2fi7. r-^^l9^, (b) log fl-().(l(l2SCPFD-1.476. /•- = ().42, (c) lop «=O.O()l.H"PFD-1.045.
r'-(l,S(), and (d) log B = n.()()15CPFD-(l,S.'^t<. r'=(1.72.
u
1
(ol
!•
(bl
-0-01
\
^T\ 1
^
f -0-02
01
TI
a
o
°
"o
•
-0-04
-
or
0
200
400
600
800
0
200
400
too
800
Dischorqe ( i s " ' , " 4 at Vettecslevl
FIG. y. Mean rale of biomass decline |RBD) for Potamngeton pectinatus (a) and Spurgimium etnerstitn (b) as a
function of mean discharges ((^) during autumn. Mean rale of decline ( ±9^"^') confidence intervals) wasealculaled
by lineal regression of log 1,1 bicjmass versus imie over periods which included three lo ninc dates oi sampling (Figs. 4
and 5). For comparison between Pindso and Veilerslev ihe discharge al Veuerslev was divided by 4. the ratio ot
mean annual discharges at ihe twosiies. lo bring 1 hem to Ihe sante scale. Symbols represent Pindso (o). Veiterslev
I ( • ) , Vetlerslev II ( A ) . Regression equations are (a) R B D - - 2 . 9 x 1 ( 1 ^(2+('.l'l2. r'-O.^S. and (b) RBD
=2.6x|tl
26
Kaj Sand-Jensen et al.
(o)
50-
40PFD
400
•300
V 20
•200
•100
o
40-
400
300
ZOO
100
FIG. 10. Sciisonal changes in (.lilDruphyll a cnnicni ol micioliciuhic ;ilgac and pholmi flux density al the water
surface and ihe sediment ai VL-UCTSICV durinj; !4Slt (a)and 1981 (h). Allcnuationof light reaching the sediment is
due tt) rcflcclion, ihc water and the macrophytes.
biomass, and perhaps higiier particif export
from the catchment rt-suiting from higher rainfall, led to greater amoiinl^ i»f accumulated
organic sediment in 1981 than the preceding
years.
Discussion
Macrophvtc community structure
The species composition of submerged
macrophytes in the River Susa corresponds to
thai of eutraphic and turbid streams in temperate regioits of Europe. The dominaiil
macrophytes, P. pectinatus. S. ctuersum and
Cladophoru and the other species present.
Potamogeton crispiis L- and Etodea canadensis
Michaux are all common in eutrophic streams
{Haslam. 1978). and have increased in
abundance in polluted Danish streams (KernHansen ef at., 1980). TTie unr<ititL'd Chtdophora
is particularly dependent on abundant supply of
dissolved inorganic nutrients, whereas the
rooted species can meet their nutrient requirements from the sediments as well (Peltier &
Weich. I%y: Westlake. 1975).
The dominant species. /'. pectinatus and S.
emersum. compensate for high tight attenuation
in the water by ninrphological and physiological
adaptations, respectivciy. Polanuygcton pectinatus has a rapid apical growth ;md ctmcentrates the biomass at the water surface (Bijl.
Macrophytes in streams
|Ou»(Diu aiuofijo
27
Sand-Jensen & Hjermind. 1989). Sparganium
emersum has basal growth and is physiologically
adapted to shade condititins by its low light compensation and photosaturation levels (SandJensen, unpublished).
The effect of management practices on the
different species depends on the type of disturbance and the growth strategy of the plants.
Frequent disturbances will keep plant communities in an early successional stage, dominated by a few opportunistic species with
capacity for rapid colonization and growth
(Grime. 1979) represented by unrooted species,
such as Ctadopliora sp., and species with
ephemeral root systems, sueh as Etodea
canadensis and P. pectinatus (Haslam. 1978).
This type of vegetation was well represented in
the River SusS. and species richness was also low
compared to undisturbed streams (Thyssen.
1981).
Another factor controlling the success of a
species is the ability to maintain a growth potential during disturbance (Haslam. 1978). The
reach at Vetterslev in the River Susa is mainly
managed by cutting the plants about U) em
above the sediment. Sparganium emersum was
common at Vetterslev and could tolerate this
cutting regime which left its basal meristem
unaffected in the sediment. Basal parts of old
leaves and new leaves of S. emersum could
rapidly rebuild the biomass. Potamogeton pectinatus was more severely affected by cutting,
despite its higher growth capacity, because apical mcristems were lost and new growth had to
be initiated from the rhizome. Growth could not
be re-established from below-ground tubers
because cold exposure during winter is required
for induction (van Wijk. 198.^). The reach at
Pindso was subject to dredging of surface sediment every few years prior to this investigation.
This management regime is expeeted to reduce
the abundance of robust, slowly growing species
such as S. emersum whose below-ground
rhizomes and meristemsare lost. In contrast. P.
pectinatus should be less affected because their
deep tubers are likely to be left untouched,
allowing regrowth the next year. Potamogeton
pectinatus constituted more than 9(1'"'? of the
total biomass at Pindso and S. emersum was not
present at all.
Ihc vegetation in managed lowland streams is
therefore far from the climax vegetation
expected under the prevailing physico-chemical
28
Kaj Sund-Jensen et id.
Ciodopiiora
Epiphyte community
Substratum
Increase water depth
Shading ond
inhibition of
and gas exchange
Reduce water velocity
Rooted
macrophytes
Subsiroium
Shading
Promote sedimentation
Shading
Vegetation
fauno
Food source
Smothering
Benthic
microolgae
FIG. 12. Conceptual model showing the kcy-rolc of rooted macrophytes fnr regulating the physical conditions and
the biological structure in lowland streams.
conditions (Dawson.Castellano& Ladle. 1978),
Slowly growing species, with more uniform
cover over the year and high competitive potential for light, increase in abundance when disturbance stops iThyssen. 19H1). This includes the
invasion of species that are emerged during a
greater part of the growth season (Haslam.
1978).
Patterns in submerged macrophyte growth
Hydrology is an important determinant of the
development of macrophytes in lowland
streams. In the River Susa. macrophyte
development in most years was restricted to a
period of minimum discharge from late May to
November. Growth rates were high, however,
reaching a maximum for /'. pectinatus at approximately n.Ofi Ingiii day ' (doubling titne ahout 5
days. Fig. 7) higher than those reported for
other species growing in streams with more stable discharge (Dawson. 1976; Thyssen. 1981:
Ham etai. 1982; Wright c/«/.. 19S2). This probably resulted from the growth strategy and high
photosynthetic capacity of /*. pectinatus.
Submerged plants are present all the year
round in Danish and Hnglish streams, where
high groundv^ater input dampens discharge
variations (Dawson. 1976:Thyssen, 1981). Maximum biomass is reached earlier in these svstems
(May-June), since growth starts from a significant winter biomass (Kelly. Thyssen &
Moeslund. 1983). Still, high discharge events
during winter reduce the biomass in these
streams, resulting in a decrease of the following
spring and summer biomass (Wright et at..
1982).
Macrophyte speeies have different growth
patterns which may influence the way they respond to various environmental factors (Butcher.
1933). Sparganium emersum has a basal
meristem and long, linear, flexible leaves that
provide little resistance to flow, and it tolerated
higher velocities than did /'. pectinatus (F ig. 9;
Haslam. 1978). Potamogeton pectinatus lost all
green shoots every autumn, even though
discharge was low. and formed overwintering
tubers. This growth strategy was probably
genetically fixed in this population {van Wijk.
1983). In habitats such as the Susa. where P.
pectinatiLs is a summer annual, it forms primary
and secondary tubers (van Wijk. 1983). In
habitats where P. pectinatus is perennial (i.e.
with leaves the year round) it does not allocate as
much energy to below-ground parts and forms
fewer and only primary tubers (van Wijk. 19S3).
Potamogeton pectinatus therefore appeared well
adjusted to a period of rapid growth during summer in the SusS and a dormant period, with high
water discharge from autumn to spring.
Macrophytes in streams
Biomass increase in the Susa was predictable
from the availability of light (Figs. 7 and 8).
Light attenuation in the water was tnainly regulated by the effect of discharge on depth and turbidity, and this explained much of the variation
in biomass development among years. Thus,
high discharge during spring and early summer
could delay macrophyte development for up to 3
weeks. A similar delay of the spring development of microbenthic algae, prior to macrophytic growth, was also observed (Sand-Jensen et
at.. 198S). We cannot exclude the possibility that
physical effects associated with, high discharge
(e.g. scouring, instability of sediments) may also
play a role in retarding biomass development.
Our examinations of the relationship between
light and photosynthesis, however, are consistent with the key role of light in regulating plant
growth (Bijl etal.. lysy) and is also supported hy
previous investigations in temperate lowland
streams (Owens & Edwards. 1961; Westlake.
1975; Wong. Clark & Pointer. 1976). This investigation, however, is the first quantitative demonstration of such a relationship. The overall
importance of light for macrophyte development is further supported by the hyperbolic relationship between light and total stream
photosynthesis in macrophyte-rich Danish
streams (Kelly el at.. 1983). Light saturation of
photosynthesis was almost never reached.
Maximum summer biomass of rooted species
increased from 1979 to 1981 paralleling an
increase in summer discharge and a decline in
Cladophora hiotiiass. A similar stimulating
effect of discharge on Ranunculus sp. was alst>
noted by Ham et al. (19S2). There are several
possible explanations for this behaviour. First
there might be a progressive increase in belowground tissues and thus in recruitment potential
over the years. Hypothetically. the severe winter
of 1978-79 when the River Susa remained
covered by ice and snow for about 2 months
might also have reduced the viability of belowground tissue. This explanation is discarded.
however, because macrophyte cover reached
UW/r the following summer and a stable
biomass was maintained for more than a month.
Second, increasing mid-summer water levels
and velocities from 1979 to 1981 may have
stimulated macrophyte growth by improving
light perception and the exchange of dissolved
substances (Westlake. 1967). Mean water levels
were typically ()..S-().9 m at Pindso and ().(>-1.2 m
29
at Vetterslev during the summer, with values in
the upper range leading to less dense packing of
macrophyte tissue and perhaps better spatial
arrangement of leaves for light interception
(Wcstlake, 1975). A third factor, resulting in
differences in maximum biomass of rooted
species, is the variable amount of Cladophora
that covers the rooted macrophytes and probably retards their growth hy shading and by forming a thick layer of stagnant water surrounding
the plants (Sand-Jensen. 1977; Sand-Jensen.
Revsbech & Jorgensen. 1985). Considering the
important role of light in controlling growth,
variable shading by Cladophora may be a very
important side effect of variable summer
discharge. Maximum sutnmer biomass of
Cladophora in 1979 was 72 g DW m"- at Pindso
and 4(1 g DW m • at Vetterslev II which, given a
specific light attenuation coefficient for
Cladophora of 0.024 m - g ' DW (Bruskin.
19S4). results in H2''4 and 62^Y of the incident
photon tlux density being absorbed by
Cladophora. The biomass of the epiphytic community on the submerged macrophyte also
decreased from 1979 to 1981 (Sand-Jensen &
Borg. unpublished), and is expected to affect
macrophyte growth in the same way as
Cladophora (e.g. Ham et al.. 1982; Sand-Jensen
el al., 1985). Unfortunately, the relative
influence of the suggested positive effect of high
discharge during summer growth and the
indirect effect due to removal of the epiphytic
and loose-lying Ctadophora communities cannot
be evaluated.
Overall, a nn)re stable seasonal discharge pattern (i.e. abtjve normal during summer and
below normal during winter-early spring)
appears to promote macrophyte development.
This is in accordance with the higher maximum
biomass found in streams with a more stable flow
than in the Susa (e.g. Westlake. 1975; KernHansene/«/..
Ecosystem conseiptences of macrophyte
devetopment
Submerged macrophytes have many direct
and indirect effects on the stream ecosystem
(summarized here in the conceptual diagram in
Fig. 12. see also Marshall & Westlake. 1978).
We have shown that macrophytes reduced
velocity and promoted particle sedimentation.
Accumulation of plant biomass and organic sedi-
31)
Kaj Sand-Jensen et at.
ment trapped nutrients during summer reduce
the loading of downstream lakes and coastal
areas. The lact that indirect nutrient removal via
sedimentation can exceed direct plant uptake
substantially (about ten-fukl here) has been
overlooked in the past. Nutrient trapping was
only temporary because plants and sediments
were washed out during autumn and winter.
Nevertheless, trapping of nutrients during the
period of active growth (summer) may considerably contribute to reduce downstream production. The proportion of nutrients retained in
this form ;imounted to I(I-2()'~Y of the total
downstream transport of N and P during the
summer (Jeppesen et ai, 1984). Nutrient
removal may be even more important in streams
with lower nutrient concentrations.
Extensive organic sedimentation during summer also influeneed the benthic environment,
inducing seasonal changes in the fauna toward
detritivores able to tolerate soft bottoms and low
oxygen concentrations (herson ei ai. 1984).
Processes in the sediment during summer
included high rates of bacterial degradation of
organic compounds (about 6 g O: m - d a y ' ;
Jeppesen ct ai, I9S4). and oxygen production
was close to zero because of low algal hiomass
and low light availability.
Submerged macrophytes regulated the
development of microbenthic and epiphytic
alga! comtninities (Fig. 12). Microbenthic algae
had an extensive spring bkiom and declined
rapidly with progressive macrophyte shading.
Simultaneous accumulation of organic particles,
cmering the microbenthic algae, may rcinft)rce
the decline. In contrast, epiphytic communities
are dependent on macrophytes as substratum.
Epiphytic communities grew thicker at sites with
low water velocities and total biomass was also
higher in summers with low discharge (SandJensen & Borg. unpublished). Early in the season the epiphytic community was predominantly
autotrophic. Later, at higher macrophyte density and greater shading it was predominantly
heterotrophic. The bacterial activity on plant
surfaces was particularly active in nitrification
and degradation of organic compounds (SandJensen & Borg. unpublished).
Cladophora sp. followed a biomass pattern
similar to the epiphytic community (Fig. 12).
and it was more abundant during low discharge
periods. Dredging of streams, which increases
the depth and width and reduces water velocity,
enables Ctadophora to grow better on rooted
macrophytes without being washed out and also
to grow as long filaments loosely attached to the
sediment (Iversen etal.. 1984). Increased abundance and nuisance growth of Ckulophoru is
therefore a likely combination of increased
eutrophieation and poor management of
streams.
There is a dense invertebrate fauna attached
to macrophyte surfaces (Fig. I2;Witcomb. 1968;
Hynes. 1970). Chironomids and simuliids in the
SuSi^ reached densities of (l.5x](r individuals
m -" of bottom, exceeding benthic densities
several fetid (Jeppesen et ui, 1984). Chironomids and simuliids grew rapidly, formed one
to several generations during summer, and were
well suited to the short duration ot macrophyte
cover and the rich food supply provided by suspended particles and organic material in the
epiphytic community.
Fish can benefit directly or indirectly from
^ubtnerged macrophytes (Fig. 12; Mortensen.
1977; Kern-Hanseneffj/., 1980), Territorial fish
such as brown trout (Salmo trutta L.) which is the
most important fish in mosl Danish streams,
attain higher densities in streams with
macrophytes because of the increased physical
heterogeneity associated with macrophyte
development. In addition. macrophytes
encourage a rich food supply of invertebrates.
O[i the other hand, uniform coverage of the
stream bottom with dense macrophyte stands
and organically rich surface sediments may
create problems for the fish populations due to
associated risks of critically low O- concentrations during the night. The described system
changes may cause high respiration rates in sediments and macrophyte-fpiphyte associations
and low ()• reaeration with the atmosphere due
to the reduced velocities and greater depths
associated with dense plant growth. Thus an
intermediate plant density with a tnixture of
plant beds and uncovered sediments is probably
most suitable to the trout populations.
In conclusion, submerged macrophytes affected the physico-chemical environment and the
bioiogicai structure of lowland streams in
many ways. Associated biological processes,
such as oxygen and nitrogen transformations,
were therefore also affected (Jeppesen et al.,
1984). Development of submerged macrophytes
was mainly affected by the interdependent
physical variables, water discharge and light
Macrophytes in streams
availability. Indirect effects of discharge and
light availability were difficult to separate from
direct effects on sedimentation, detachment of
organisms, plant nutrient uptake, etc. Nevertheless, we were able to predict the rate of biomass
increase of macrophytes by light availabiiity and
demonstrate the role of discharge in accelerating
the biomass decline during autumn. The
influence of discharge on so many physical and
biological phenomena and its variability among
or within streams underscores its importance for
the abundance of organisms and the rates ot
biological processes. Changes in hydrology due
to human activity (e.g. groundwater amendment, irrigation, channelization, deforestation,
macrophyte-cutting. etc.) therefore have a
strong impact on the stream ecosystem (Tett ei
ai, 1978).
Acknowledgments
We thank Carlos Duartc. Mahlon Kelly and
Derek Westlake lor comments on the
manuscript.
Funds were provided from the Danish Natural
Science Research Council and the Agency of
Environincntal Protection to the River Susa project. We thank Hanne Moller. Vibeke Kolbe
and Birte Brandt for patience with manuscript
typing.
References
B c r g K . . B o i s c n - B c n n c k c S . A . Jiiniissiin P . M . . KL-ILIing J. (k Nik'lscn A. (144.S) Hiolniiical studies on the
River Susa. Folia iininoloiiiai Sctitiihiitniva, 4.
BijlL. van der & Hjorniind A.L. ( WK.>| Okotysuilogi
hos Potamogeton pcctinutus i Susacn. M.S. thesis,
[-"reshwiik-r Biolojiical Lahonitory. University o(
CcipL-iihagcn.
Bijl L. v;in ilcr. Sand-Jcnscti K. & Hjermind A.L.
(iyS9) Photosynthesis and canopy strucmrc of the
subnii.'rt!ciJ macrophxtc. I'otumoiifttin peclinatua
L.. in ;i Danish lowland \\t\:,in\. Journal of Fcology
(iti press).
BJEirnov S. i^: LurKlager-Jcuscn J. (IW-S) AfstrtMiining
i Susa-omradct. Dansk kninitc lor hydroloei. Rapporl Susa II HI. CopL-nhagcn.
Bruskin A.B. (14S4) V;i'kst ng biomasse af
Chultiphora i Susacn. M.S. thesis. Freshwater
BioloyJL'al Laboratory. University of Copenhagen.
ButL-her R.W. (WS?'] Suuliesoii ihc cailojiy of rivers.
(.)n ihc dislrihution ul macrophyte vi'tciation in
the rivcr^ot Britain.7(»nr/i(i/f)(t(o/(Ji;>. 2I.."iK-yL
31
Oawson F.IL (197(1) Tho annual prtnliiciion nj the
aquatR' macrophyte Rutnincuhvi peniciUutus var.
ciilcareiis (R-W. Butehcr) C.D.K. Cook. Aifuaiic
Botany. 2. 51-7.1.
IJawson P.H. (iy7S) T\K seasonal offeels of aquatic
plant growth on the flow of water in a stream.
I'roceedirigs FWRS 5th .Symposium on Aquatic
Weedi. pp. 14.VI5().
[Jawson F.H . Castellann F- & Ladle M (iy7K) Coneept oi' species succession m relation to river
vegetation
and
management.
International
.'\s\oiiutioii iif Theoretical anil Applied Limnology,
20. M2y-14.14.
Dawson F M. & Kern-IIaiiscn U. (|y7H) Aquaiic
weed management in natural streams The effect o(
shade hy marginal vegetation. Inicrntittonal
Associaiioiutf Theoretical ami Applieil Litnnolog\,
20, !451-l4.Sft.
Dawson F.H. A: Kern-Hanson LI. (1^79) The effect of
natural and artificial shade on the maerophytes of
lowland streams and the use of shade as management leehniqiie. Intcrnationalf
Revue der
Ccsamtcti llyilrohioltigic. 64, 4;i7-45.'>.
DL-I Danske Hedeselskah (1479) Hydrometriskc
malinger i Susacn.
Dct Danske Hedeselskah (19SI)) Hydrometriskc
malinger I Susaen.
Del Danske Hedeselskah (19SI) Hydrometriskc
malinger i Susacn.
Dyhr-Niclscn
M.
(I9S1)
Dei
hydrologiske
forskningsprojekt i Susaens opUnd. Dansk komiic
for hydrologi. Rapport Susa H 1. C opeiihagen.
Fenchel \- ic Straarup B.J. (1971) Verlica! distribution ol" photosynlhetie piiimenis and ihe penetration ol hght ill marine setltinents. Oikos. 11, 172183.
Grime J.P. (1979) Plani Struttxn-'^ nnd \'ef;etatitm Proce.sscs. John Wiley anJ Sons. Chiehester.
Ham S.F.. Cooling D A . . Hilcy P.D.. MeLeish P.R..
ScorgielLR.A. A; BerrieA.n.(19S2) Growth and
reeession ol aquatic macrtiphytes on a shaded setlion ot the KiverLamhoiirn. England, from 1971 lo
t9Rtl Frfshwater Biology. 12, \-\^.
Haslam S.M. (197S) Rivtr Plants. Cambridge University Press.
\\\n<:s\\.\i.^.(\^l())The Ecology of RunningWaiers.
Liverpool University Press.
Iversen r.M.. Jeppesen E.Sand-Jcnscn K.&Thorup
J, (!9S4) Okologiske konsekvenser af reduceret
vandlt>ringi Susaen. Bind 1: Den biologiske struktur. Miljostyrelsen. Copenhagen.
Jacobsen J. (19S1) Vantllobskemi og iransport al
oplost stof i Susaen. Dansk koniiie for hydrologi.
Rapport Susa H Uv Copenhagen
Jeppfsen E. (I97S) Orgamsk stof j Holme a. MS
thesis. Freshwater Biological Laborator>. University ol Copenhagen.
Jeppesen E.. hcrscn I .M . Sand-Jcnscn K. &
Jdrgensen C P . (19S4) Okologiske konsekvcnser
af redueeret vandforing i Susaen, Bind 2: Biologiske processer og vandkvalitets-torht»Id. Miljostryrelsen. Copenhatien.
Kelly M',(i..rhyssenN. A Moeslutui B. (l9.s.MUghi
and the annual varialion ol oxygen- anil earbon
baseil measurements of prtHluttivit\ in a
32
Kaj Sand-Jensen et al.
macrophvic Jomin;ilcti river. Ltmnologv and
Oceanography. 28, .'S.13-.'=i45.
Kclsirup N.. Binzor K. & Knudsen J. (IWI) Hydro(lettlouiskc f<irhiild i Susn nmradci. DiiriNk kcimite
for hytlrnloi;]. Rapporl Siisa H 7, C"n|iL'nh;igon.
Kcrn-Hanscn U. A: Dawson F.H. (iy7K) The sUindinL:
crop of iii^itiatif plants o! lovvkind sircams m DcniTiiirk and the inter rchitionship ul nuiricnts in
plan!, secimeni and w;tlcr. Proceedings EWRS5th
Symposium on Aquatic Weeds, pp. I4.^-1.SO.
Kcrii-Hansen V.. Holm 1 ,F.. Madscn B.L.. Thyssen
N. & Mikkelsen 1. (IWO) Vedlij-ehokielsc .-if
vandlnb. Miljoprojekter 311. MiljOsiyrclsen.
Copenhagen.
Marshall E..I.P. & Westkikc D.F (197S) Reccni
studies on tlic role of aquatic macrophyles in iheir
ecosystem. Proceedings f-WRS5th .Svmpo.\ium on
Aquatic Weeds, pp, IS.VISS.
Mnrtcnsen U. (1^77) Density dependent moruilitv o|
irout Iry [Siihno trutta) anil its rekiiionship to the
mana^enU'nt of small streams. Jmtrnal of Eish
Bioliigy. 11,613-617.
Nielsen L.W . Nidscn K. & Sund-.lensen K. (IMS?)
High r;ilcs of prodiielion and mnrtaliiy of submerged .Spargunium cmersum Rehntan during its
short grow ih season in a eiilrophic Danish stream.
Aquatic Hotanw 22. 32.^-3.14.
Olscn S. (I'J.'SO) Aquiilie pianEs and hydrospheric I'ac\ciXi..Svensk Botanisk Tidsskrift, 44, 1-34 and 332373.
Owens M & Edwards R.W, (1%!) Ihe effect of
plants on tiver conditions. II. Furihcr crop studies
and eslimatesofnct productivity of maerophytes in
a chalk stream. Journal of Ecology. 49. I I'J-12'>.
Peltier W.H. & Welch F.B. (1969) Factors affecting
i;row Ih of rooied aqualics in a river. Weed Science.
17,412-416.
Sand-Jensen K. (1977) Effect of epiphytes on ecigrass
photosynthesis. Aquatic Botany. 3, 55-f)3.
Sand-Jensen K. (I9S.3) Physicai and chemical parameters regtilating growih of periphylic communities. Periphvton of Freshwater Ecttsvsiems
(Ed. R. ti Wetzel), pp. 6.V72. Dr W Junk Publishers. Ihe Hague. The Netherlands,
Sand-Jensen K.. MollcrJ. & Olsen B. (lysx) Regulation ot biomass of microlienthic algae in Danish
lowland sireams. Oikos, 53. .132-3411.
Sand-Jensen K.. Revsbech N.P. & Jorgensen B.B.
(19S5) Mieroprofiles of oxygen in epiphyle communilies on stihmerged niaerophytes. .Marine Biology. 89. 5;w(>2.
Stephens J.C . Bbckhurn R D.. Seaman P.E. &
Weldon L W, (1963) Flow retardance by channel
weeds anil their control. Journal of Irrigation and
Drainage Diversum- Proceedings American Society
of Civd Engineering. S9, .'1-47.
.Sondergaard M. & Riemann B. (1979) Fersk\andshiologiske analysemelodcr. Akademisk Forlag. Copenhagen.
Teti P . C . Kelly M.G.. Hornberger (i.M. & Cosby
B.J. (t97S) Relalitinships among suhstrute. flow
and benthic microalgal pigment ciensily in the
Mechums River. Virginia. Eimnologv and
Oceanogrtiphy, 23. lV>^l^il.
Thyssen N. (I9SI) Genluftning og fotosyniese i
vandl«h. Miljoprojekter 40. MiljBstyrelsen.
Copenhagen,
Wesllake D.F. (1964) l.ighl extinction. sUmdinp crop
and photosynthesis wilhin weed heds. International .-1 \sociaiion of Iheoreiical and Applted Limnology. 15,415-425.
Westlake D.F. (1967) Some effects of low-vclocity
currents on the metabolism of
aquatic
macrophytes. Journal of Experimental fiottiny. 18.
IS7-2tl5.
Westlake D.F (1975) Maerophytes. Rnrr Ecologv
(Ed B. A. Whitton). pp. I(«S^12S Blackwell Scientific Publications. Oxford.
Weizel R.Ci. (19,S3) Limnology. 2nd edn. W. B. Saunders. Philadelphia.
Wijk R.J. van (19S3) Life-cycles and reproductive
strategies of Potumogeton pectinatus I. in the
Netherlands and the C amarque (France). In: Proceeding\ Intcrnutiomil Symposium im .-\qualic
Macrophyies. Nijmegen, IS-23 September IWJ.
Nijmegen. The Netherlands.
Wiieomh I) (I96S) I h e fauna of aquatic plants. Proceedings ^th British Weed i ontrol C onferencc, pp.
382-385.
Wong S.L.. Clark B. <S; Pamler D.S. (1976) Application of underwater lighl nK-asuremeiils in nutrient
and prodtiction studies in shallow rivers. Ereslnvater Biology. 6, 543-551).
Wrighl J . F . { ameron A C . Hiley P,D. & Berne
A.D. (I9,S2) Seasonal changes in biomass of
macrophytes on shaded and unshaded sections of
Ihe River I.ambourn. England. Eri-\hwatcr Biology, 12, 271-2X3.
(Manttscript accepted 5 Jantiary