Post on 05-May-2023
Macrophyte decline in Danish lakes and streams over
the past 100 years
KAJ SAND-JENSEN*, TENNA RIIS*{{ , OLE VESTERGAARD* and
SéREN ERIK LARSEN{*Freshwater Biological Laboratory, University of Copenhagen, Helsingùrsgade 51, DK-3400 Hillerùd, Denmark;
and {National Environmental Research Institute, Vejlsùvej 25, DK-8600, Silkeborg, Denmark
Summary
1 Freshwater habitats in cultivated and densely populated lowland regions of
Europe have experienced profound changes during the last 100 years. We take
advantage of the long interest in aquatic plants in Denmark to compare the sub-
merged ¯ora in lakes and streams in 1896 and 1996.
2 Most of the lakes which contained a diverse submerged vegetation 100 years ago
now have the high phytoplankton biomasses and summer transparencies below 2.0
m characteristic of eutrophication. The majority of 17 lakes included in both old
and recent studies have lost all or most of their submerged species. Species richness
for those lakes that were vegetated did not, however, di�er signi®cantly between
old and recent studies.
3 Species richness declined markedly in the 13 streams included in both studies.
Over all sites, there was also a signi®cant decline of species richness per locality.
Potamogeton species declined from 16 to 9, despite an 8-fold increase in the number
of sites surveyed.
4 Similar compositions and rank-abundances of Potamogeton species in lakes and
streams studied 100 years ago re¯ect suitable growth conditions and mutual
exchange of propagules. Today, low habitat diversity and frequent disturbance in
streams and low recruitment from lakes favours only robust, fast-growing species
capable of regrowth following weed cutting and dredging.
5 A positive interspeci®c relationship observed in the contemporary stream vegeta-
tion between mean local abundance and number of occupied sites was probably
promoted by redistribution of plants as a result of disturbance and e�cient disper-
sal in the interconnected stream network.
6 The freshwater macrophyte ¯ora in north-west Europe presently includes a high
proportion of rare species which are threatened by extinction. Both species typical
for oligotrophic conditions (e.g. P. ®liformis and P. polygonifolius) and another
group of large, slow-growing species (e.g. P. alpinus, P. lucens, P. praelongus and P.
zosterifolius), were once common but are now infrequent, while other transient spe-
cies have remained rare (e.g. P. acutifolius, P. colouratus, P. densus and P. rutilus).
The presence of many species that barely survive in small and distant populations
will make re-assembly of submerged aquatic communities di�cult.
Key-words: disturbance, eutrophication, freshwater macrophytes, historical changes
Journal of Ecology (2000) 88, 1030±1040
Correspondence: K. Sand-Jensen, Freshwater Biological Laboratory, University of Copenhagen, Helsingùrsgade 51, DK-
3400 Hillerùd, Denmark (fax� 45 48241474) or T. Riis (e-mail t.riis@niwa.cri.nz).{Present address: National Institute of
Water and Atmospheric Research Ltd (NIWA), PO Box 8602, Christchurch, New Zealand.
Journal of
Ecology 2000,
88, 1030±1040
# 2000 British
Ecological Society
Introduction
At the end of the 1800s there was rich and wide-
spread submerged vegetation in the small lakes and
streams of lowland Denmark (Baagùe & Ravn
1895±96) and a diverse aquatic ¯ora in other Danish
and English localities (Nielsen 1872±74; Raunkiñr
1895±99; Andersen 1910; Ostenfeld 1913;
Mountford 1994; work cited in Preston 1995).
Palaeolimnological studies of plant macrofossils in
sediments from 10 Danish lakes con®rm the pre-
sence of a well-developed submerged ¯ora 100±200
years ago, which is now absent following heavy
eutrophication (Klein 1993; Anderson & Odgaard
1994; Odgaard et al. 1997).
Stream sediments, however, are mobile and can-
not provide a cumulative, chronological series of
plant remains such as is possible in lakes (Anderson
1993). There are, however, many historical publica-
tions, excursion reports and herbarium collections
that record the past occurrence of submerged species
in Denmark. Moreover, recent extensive surveys of
Danish lakes (Vestergaard 1998) and streams (Riis,
Sand-Jensen & Vestergaard 2000) enable testing of
how species richness and relative abundance of sub-
merged plants have changed.
The past 100 years have brought profound envir-
onmental changes to freshwater environments (e.g.
Wetzel 1983; Sand-Jensen & Pedersen 1997; Moss
1998). As in other parts of Europe and North
America industrial production and population den-
sity have increased so that most waste products are
now removed through freshwaters (Sand-Jensen &
Lindegaard 1996; Kamp-Nielsen 1997) albeit with
reduced input of organic pollutants recently
(Kristensen & Hansen 1994). There is extensive
application of fertilizers and drainage of wet mea-
dows has also increased enormously (Madsen 1995)
as has management of streams (98% cited by
Brookes 1984), for instance by regular weed cutting
and dredging.
Such changes have presumably led to the substan-
tial changes observed in species composition, species
richness and cover of the aquatic vegetation (Olsen
1964; Chambers & Kal� 1985; Rùrslett 1991;
Barrat-Segretain 1996). The number of suitable
habitats has diminished over the past 100 years, but
if habitat destruction has taken place randomly, the
rank species abundance may not have changed with
the rarest species (transients) disappearing by chance
(Grime 1998). Relative abundance may, however,
have changed as the progressive environmental
changes have a�ected individual species di�erently
(Roelofs 1983; Bobbink et al. 1998). In eutrophic
lakes, for example, small and slow-growing species
have often disappeared, while robust, fast-growing
species, capable of forming a canopy just below the
water surface, have survived (Adams & McCracken
1974; Phillips et al. 1978; Sand-Jensen 1997). A few
fast-growing species with a high capacity for disper-
sal and colonization may also be favoured by the
disturbed conditions in managed streams (Henry &
Amoros 1996; Henry et al. 1996; Barrat-Segretain et
al. 1998). The many plant species which are now
restricted to a few localities may be prone to extinc-
tion and may well be unable to expand into newly
favourable sites (Thompson & Hodgson 1996;
Gaston & Curnutt 1998).
The availability of historical data from Danish
lakes and streams allows us to evaluate changes in
the patterns of submerged vascular plants over the
past century. Special emphasis is placed on the spe-
cies-rich genus Potamogeton, for which comprehen-
sive and accurate information is available from past
studies. Although strong environmental changes
have taken place world-wide, such broad-scale
changes of the richness of the aquatic ¯ora have not
been studied before. We also test whether directional
changes of abundance and relative rank of
Potamogeton species are as expected from their life
history strategies, the overall nutrient enrichment of
freshwaters, and the intense disturbance of streams.
The relationship between local abundance and geo-
graphical range size in the contemporary stream
¯ora was also evaluated to complement our other
analyses of stream vegetation (detailed in Riis et al.
2000; Riis & Sand-Jensen. 2000).
Materials and methods
Information on the presence of submerged vascular
plants in lakes and streams distributed throughout
Denmark was compiled from literature published
between 1870 and 1929. We refer to this as data
from 1896 (corresponding to the most comprehen-
sive study, Baagùe & Ravn 1895±96) although data
from ®eld studies between 1870 and 1927 are
included. Most of these were published in Danish in
Botanisk Tidsskrift (Journal de Botanique,
Copenhague), either as normal articles or as reports
from excursions organized by the Danish Botanical
Society. We examined all specimens and written
records from the period kept in the main herbarium
in the Botanical Museum, Copenhagen, and used
this information to check and augment the pub-
lished data. We omitted studies in which the aquatic
vegetation of lakes had only been studied close to
the shore or in surface waters and data from ponds
(<1ha) and acid (pH<5.5), saline (>3 g NaCl
Lÿ1) or arti®cial lakes, whose vegetation is unrepre-
sentative of the majority of natural freshwater lakes.
Danish streams are usually small and shallow,
allowing comprehensive description of their vegeta-
tion.
Species identi®cation in the old studies is likely to
be accurate, because the excellent Danish ¯ora avail-
1031K. Sand-Jensen
et al.
# 2000 British
Ecological Society
Journal of Ecology,
88, 1030±1040
able (Raunkiñr 1890) had keys for identi®cation of
most aquatic plants and its species delimitation clo-
sely resembles that used today (Moeslund et al.
1990). Potamogeton species were the subject of
much interest and information on the group is likely
to be particularly accurate and comprehensive
(Baagùe & Ravn 1895±96; Raunkiñr 1895±99).
Species of Callitriche were excluded because ¯owers
or seeds are required for identi®cation, and plants
collected in the ®eld are usually sterile. We compiled
historical data from 40 lakes and 27 streams.
Data from studies of 66 lakes between 1983 and
1993 were compiled by Vestergaard (1998) and
include careful recordings of aquatic plants along
several depth transects. Studies at 208 stream local-
ities from 1996 and 1997 include careful recording
of the presence of aquatic plants in 150±250 quad-
rats (20� 20 cm) along 10±20 transects across 100
m-long stretches at each stream locality (Riis et al.
2000). These recent studies are collectively referred
to as 1996 data (100 years after the historical data
with which they are compared).
The current mean local abundance and the geo-
graphical range size of each species was calculated
based on the abundance and presence at the 208
stream localities. The local abundance of a species
was calculated as the proportion of the 150±250
quadrats at a particular locality, and the mean local
abundance as the average abundance at those local-
ities where it occurs. The geographical range of a
species was calculated as the proportion of the 208
localities at which it was found.
For the larger lakes the contemporary surveys are
more thorough than in the past, with two persons
spending between 1 and 3 days (c. 8±24 h), including
Scuba diving, at each site. In recent surveys two
people spent about 5 h at each stream. Two pairs of
botanists conducted the classical study of 42 fresh-
water localities during a 14-day excursion in 1895
(Baagùe & Ravn 1895±96). Allowing for longer
working days but slower transport, we estimate they
spent a total of 224 h at the sites, suggesting a simi-
lar mean e�ort per locality (and thus equal likeli-
hood of ®nding a species if it was present) to the
contemporary surveys.
We used a randomization procedure to facilitate
comparison between old and recent studies and
between lake and stream studies, irrespective of dif-
ferences in number of localities. Mean number and
2.5±97.5% percentiles in the relative abundance of
Potamogeton species were calculated to form a 95%
con®dence interval for the contemporary frequency
had the same number of sites been used, by ran-
domly drawing 27 lake or 40 stream localities 1000
times from the greater number surveyed. Similar
procedures were used to compare lake and stream
data within each survey period.
The ability of the past and present vegetation to
withstand physical disturbance was compared based
on the ability of Potamogeton species to recolonize
an open stream. Two numerical indices (scores: 1±
3), describing the capability of each species to dis-
perse between sites (by shoot fragments, specialized
short shoots and seeds) or locally (via the root-rhi-
zome system) were derived from Riis & Sand-Jensen
2000, their table 4). Evaluation of the response to
eutrophication was based on their predicted perfor-
mance in the turbid waters expected when phyto-
plankton growth is enhanced. Two morphological
indices (scores: 1±3) of height and growth form of
the species (Riis & Sand-Jensen 2000, their table 4),
describe the ability to grow up through the water
column and spread their canopy close to or on the
water surface.
We acknowledge that these morphological indices,
although based on all the available literature, pro-
vide only a crude description of ecological strategies
and the performance of Potamogeton species under
disturbed and turbid conditions, as they do not
account for di�erences in growth capacity or the
ability to tolerate weed cutting, ¯ow peaks, dred-
ging, reducing/anoxic sediments and overgrowth of
epiphytic communities.
Results
DECLINE OF SPECIES RICHNESS IN THE
SAME LOCALITIES
Species richness of all submerged vascular plants,
and of Potamogeton species in particular, has
declined signi®cantly in nearly all of the 17 lakes
that were surveyed on both occasions (Fig. 1). All of
these had submerged vegetation in 1896, but seven
have since lost all submerged vegetation and a
further three now contain only 1±3 species.
The prevalence of lakes with high phytoplankton
biomass and unclear water is demonstrated by med-
ian summer chlorophyll concentration of 55mg mÿ3
and a median summer Secchi-transparency of 0.85m
for 182 Danish lakes sampled in 1999 (J. P. Jensen
1999, personal communication). Only 13% of these
lakes had transparencies exceeding the 2.0m
required for development of a diverse submerged
vegetation (Vestergaard 1998). The 66 lakes included
in our contemporary study, however, had clearer
than average water with 33% having a Secchi-trans-
parency greater than 2.0m (and 75%>0.85m;
Vestergaard 1998).
Mean species richness of submerged vascular
plants has also declined signi®cantly in streams (13.6
vs. 7.0, Fig. 1). The mean number of Potamogeton
species declined even more markedly (6.7 vs. 2.6,
Fig. 1) with the mean ratio of Potamogeton species
to all submerged species falling from 0.59 (�0.09,
SE) to 0.38 (�0.08).
1032Macrophyte
decline in Danish
lakes
# 2000 British
Ecological Society
Journal of Ecology,
88, 1030±1040
SPECIES RICHNESS AT ALL VEGETATED
LOCALITIES
Figure 2 shows frequency distribution for species
number in sites at which submerged plants were pre-
sent. In lakes, neither total species number nor that
of Potamogeton species alone, was signi®cantly dif-
ferent between surveys (Mann±Whitney, P>0.05;
Fig. 2). Species richness in streams was much higher
100 years ago than it is today (Mann±Whitney, P<
0.0001; Fig. 2).
RELATIVE OCCURRENCE OF POTAMOGETON
SPECIES
The relative occurrence of each Potamogeton spe-
cies, calculated as the proportion of vegetated lakes
Fig. 1 Number of submerged species and Potamogeton species alone in the lakes and streams that were studied in both 1896
and 1996. The diagonal line marks the 1±1 relationship between past and present species number. Species number has
declined signi®cantly over the 100 years in all four instances (signed rank test, P<0.05).
Fig. 2 Frequency distribution of number of submerged species and of Potamogeton spp. in a large number of vegetated
lakes and streams studied in either or both 1896 (*) and 1996 (W).
1033K. Sand-Jensen
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Ecological Society
Journal of Ecology,
88, 1030±1040
and streams in which it occurred is given in Table 1
together with 95% con®dence intervals for reduced
data sets derived from randomization procedures
(see methods).
Comparing only species present in at least one of
the appropriate data sets, 10 Potamogeton species
had a signi®cantly higher occurrence in 1896 in
streams than in lakes, while the remaining six spe-
cies did not di�er signi®cantly (Table 1).
Potamogeton species observed in streams declined
from 16 to nine despite an eight-fold increase in sur-
vey sites. All but one declined signi®cantly over
time. Three Potamogeton species in lakes occur sig-
ni®cantly more frequently now than in the past and
only seven of the 14 species declined signi®cantly,
leading to a reversal of the 1896 situation.
Consequently all except one species now occur
markedly less often in streams than in lakes (Table
1).
Among the individual species, Potamogeton zos-
terifolius was very common in lakes and streams stu-
died 100 years ago, while it is rare today (Table 1),
and P. alpinus, P. ®liformis, P. frisii, P. natans, P.
lucens and P. polygonifolius have also declined signif-
icantly in both habitats (P<0.05). Species typical
for eutrophic sites such as Potamogeton crispus, P.
natans, P. pectinatus and P. perfoliatus, however,
remain relatively abundant.
RANK-ABUNDANCE AND DIRECTIONAL
CHANGES AMONG POTAMOGETON SPECIES
We found signi®cant correlations between the rela-
tive abundance of individual Potamogeton species in
lakes and streams in old and recent studies (Table
2). Correlations in species abundance were stronger
between lakes and streams studied 100 years ago
than recently, mainly due to profound decline of the
stream vegetation.
Highly signi®cant linear relationships were
observed between log relative abundance and falling
rank of Potamogeton species in lakes and streams in
the two study periods (Fig. 3). The negative slope of
Table 1 Mean frequency of occurrence of Potamogeton species in streams and lakes in 1896 and 1996. For comparison
between di�erent studies, a randomization procedure (see text) was used to calculate 2.5% and 97.5% con®dence limits of
occurrence. n � number of original localities, N � number of localities sampled in the randomization procedure. Roman
numbers indicate columns used in comparisons: I, lakes vs. streams in 1896 (using 27 lakes selected at random from 40
sampled); II, lakes 1896 vs. 1996; III, streams 1896 vs. 1996; IV, lakes vs. streams 1996
Original mean frequency C.L. of occurrence in randomization procedure
Species
I
Stream
1896
n� 27
II
Lake
1896
n� 40
III
Stream
1996
n� 208
IV
Lake
1996
n� 66
I
Lake
1896
N� 27
II
Lake
1996
N� 40
III
Stream
1996
N� 27
IV
Stream
1996
N� 66
P. acutifolius 0.11 0.00 0.00 0.03 ± 0.00±0.05 ± ±
P. alpinus 0.41 0.15 0.00 0.08 0.07±0.22 0.03±0.13 ± ±
P. crispus 0.41 0.45 0.30 0.47 0.33±0.55 0.38±0.55 0.15±0.48 0.21±0.39
P. densus 0.07 0.00 0.00 0.00 ± ± ± ±
P. ®liformis 0.15 0.20 0.00 0.12 0.11±0.30 0.05±0.18 ± ±
P. friesii 0.33 0.23 0.02 0.09 0.11±0.30 0.03±0.15 0.00±0.07 0.00±0.05
P. gramineus 0.15 0.15 0.00 0.17 0.07±0.22 0.10±0.25 ± ±
P. lucens 0.52 0.30 0.03 0.14 0.19±0.41 0.08±0.20 0.00±0.17 0.00±0.06
P. natans 0.74 0.73 0.18 0.52 0.62±0.81 0.43±0.63 0.04±0.33 0.11±0.26
P. obtusifolius 0.26 0.13 0.00 0.46 0.04±0.19 0.35±0.55 ± ±
P. pectinatus 0.48 0.30 0.18 0.62 0.19±0.41 0.53±0.73 0.07±0.33 0.11±0.26
P. perfoliatus 0.59 0.50 0.15 0.49 0.41±0.59 0.38±0.58 0.04±0.30 0.08±0.23
P. polygonifolius 0.04 0.08 0.00 0.00 0.00±0.11 ± ± ±
P. praelongus 0.41 0.13 0.08 0.12 0.04±0.19 0.05±0.18 0.00±0.19 0.03±0.14
P. pusillus 0.33 0.23 0.01 0.49 0.15±0.30 0.40±0.58 0.00±0.07 0.00±0.05
P. rutilus 0.00 0.00 0.00 0.03 ± 0.00±0.05 ± ±
P. zosterifolius 0.56 0.25 0.00 0.08 0.15±0.33 0.03±0.13 0.00±0.04 0.00±0.02
Table 2 Correlations between the frequency of occurrence
of Potamogeton species in lakes or streams in 1896 and
1996. Spearman rank correlation coe�cients (r) are given
with the level of signi®cance in parenthesis
Lakes 1996
Lakes 1896 vs. streams 1896 0.87 (0.001)
Lakes 1896 vs. lakes 1996 0.78 (0.01)
Streams 1896 vs. streams 1996 0.69 (0.01)
Lakes 1996 vs. streams 1996 0.73 (0.01)
1034Macrophyte
decline in Danish
lakes
# 2000 British
Ecological Society
Journal of Ecology,
88, 1030±1040
the abundance-rank relationship is a measure of
evenness in the distribution, with low slopes
observed at high evenness (Smith & Wilson 1996).
The contemporary stream vegetation has a signi®-
cantly lower evenness than that of either contempor-
ary lakes (t-test for di�erences in slope, t� 4.6, P<
0.05) or old streams (t� 16.3, P<0.01). Evenness
of lake vegetation has also declined signi®cantly
over time (t� 4.4, P<0.05). Evenness was not sig-
ni®cantly di�erent in lakes and streams studied 100
years ago (t� 1.6, P>0.05).
We evaluated whether environmental changes
over time had led to directional changes in the rela-
tive abundance of Potamogeton species by determin-
ing the correlations between their relative
occurrence and morphological indices re¯ecting
eutrophication tolerance and dispersal capacity
(Table 3). We found no systematic changes in the
correlations in lakes; although correlations between
species abundance and local or regional dispersal
capacity had increased, the di�erences were not sig-
ni®cant.
ABUNDANCE-RANGE SIZE RELATIONSHIP
In the contemporary stream ¯ora, mean local abun-
dance was signi®cantly and positively related to geo-
graphical range size of the 57 species found at one
or more of the 208 localities (Fig. 4). Thus, wide-
spread species were also locally abundant, while
some species of a restricted range size were also
locally rare, while others were relatively abundant at
the few localities at which they occurred. There were
no widespread species of a low local abundance
(Fig. 4).
Fig. 3 Rank-abundance diagrams for Potamogeton species in lakes and streams in 1896 (*) and 1996 (W). All regression
lines were highly signi®cant (r2 : 0.87±0.95; P<0.01).
Table 3 Correlations between frequency of occurrence and indices of dispersal and morphology (see text) of Potamogeton
species. Frequency of occurrence is based on studies in 40 lakes and 27 streams in 1896 and 66 lakes and 208 streams in
1996. Spearman rank correlation coe�cients (r) are given with the level of signi®cance in parenthesis. There were no signi®-
cant di�erences between the two surveys in any of the parameters (P>0.05)
Species frequency 1896 Species frequency 1996
Lakes
Dispersal index 0.540 (0.037) 0.510 (0.048)
Regional dispersal 0.550 (0.033) 0.576 (0.026)
Local dispersal 0.379 (0.142) 0.312 (0.227)
Morphology index 0.525 (0.042) 0.451 (0.081)
Floating leaves/rami®cation 0.154 (0.551) 0.229 (0.374)
Plant height 0.628 (0.015) 0.484 (0.061)
Streams
Dispersal index 0.145 (0.574) 0.374 (0.148)
Regional dispersal 0.098 (0.704) 0.380 (0.141)
Local dispersal 0.132 (0.609) 0.229 (0.374)
Morphology index 0.576 (0.026) 0.488 (0.059)
Floating leaves/rami®cation 0.125 (0.629) 0.073 (0.777)
Plant height 0.716 (0.006) 0.663 (0.010)
1035K. Sand-Jensen
et al.
# 2000 British
Ecological Society
Journal of Ecology,
88, 1030±1040
Discussion
CHANGES OF LAKE FLORA
The relationship between the trophic state of lakes
and the species richness of vascular plants is uni-
modal with the maximum number being observed
under mesotrophic conditions. The coarse nutrient-
poor sediments can support small slow-growing spe-
cies either alone or as an understorey below taller,
more nutrient-demanding species (Chambers 1987;
Rùrslett 1991; Vestergaard & Sand-Jensen 2000).
Development of lake vegetation therefore depends
both on the initial nutrient status, and on the extent
of nutrient enrichment. Thus, in Finland the predo-
minantly oligotrophic lakes have become richer in
submerged species following 40 years of eutrophica-
tion (Rintanen 1996), whereas in fertile lowland
regions of Denmark and other parts of Europe
many lakes were already mesotrophic or eutrophic
100 years ago and have su�ered a reduced distribu-
tion and richness of submerged species associated
with greater phytoplankton blooms, and denser
stands of emergent and ¯oating-leaved plants
(Wesenberg-Lund et al. 1917; Christensen &
Andersen 1958; Klein 1993; Odgaard et al. 1997).
Although some Danish lakes have been polluted by
municipal sewage and have thus had little sub-
merged vegetation since medieval times (Klein
1993), most have experienced deterioration of sub-
merged macrophyte populations only over the past
100 years (Fig. 1; Olsen 1964; Odgaard et al. 1997).
Changes of lake vegetation upon eutrophication
are characterized by an early loss of small, slow-
growing vascular rosette species (i.e. isoetids),
mosses and characeans (Roelofs 1983; Arts et al.
1990; Blindow 1992; Sand-Jensen 1997). Six lakes
lost their isoetid communities entirely between the
two surveys (Baagùe & Ravn 1895ÿ96; Iversen
1929; Sand-Jensen 1997; Odgaard 1998; Vestergaard
1998), four isoetid species are now found in fewer
than ®ve sites (Vestergaard 1998) and only Litorella
uni¯ora remains relatively common. Characeans
used to be common in deep-water communities in
transparent, alkaline lakes (Klein 1993; Odgaard
et al. 1997), but 13 species are now restricted to just
a few lakes (Vestergaard 1998).
Potamogeton species have a rather tall growth
form (elodeid form), and this is generally associated
with less dramatic decline (Odgaard et al. 1997;
Sand-Jensen 1997). Species typical of oligotrophic
habitats (e.g. P. alpinus, P. ®liformis and P. polygo-
nifolius) have tended to be replaced by those of
eutrophic habitats (e.g. P. crispus, P. pectinatus and
P. perfoliatus, Newbold & Palmer 1979; Holmes &
Newbold 1984; Preston 1995) in Danish lakes
(Table 1), and evenness has thus declined signi®-
cantly. P. pectinatus has become the most common
submerged species and is often associated with
Ceratophyllum demersum, Elodea canadensis,
Myriophyllum spicatum and P. crispus (Vestergaard
1998), which are similarly fast-growing, nutrient
demanding species capable of forming a dense
canopy below the water surface (Adams &
McCracken 1974; Nichols & Shaw 1986; van der
Bijl et al. 1989).
The morphological indices do not, however, sug-
gest systematic alterations towards greater abun-
dance of Potamogeton species better able to tolerate
eutrophic and turbid waters (Table 3). Insu�cient
knowledge of the autecology of Potamogeton spe-
cies, the crude nature of morphological indices and
the presence of di�erent lakes in the old and con-
temporary studies, however, makes the trend from
eutrophic to oligotrophic species di�cult to evalu-
ate. For example, two small species (P. obtusifolius
and P. pusillus) apparently have a relatively greater
distribution today than 100 years ago, while the
large P. zosterifolius has declined steeply and pre-
sently only holds marginal populations (Table 1,
Vestergaard 1998). Comprehensive recent studies of
lakes, often aided by Scuba diving, make it very
likely that any species declines observed are real,
and may even be underestimated, while species
increases could be artefacts of the production of
more careful inventories, particularly for small,
deep-growing species. The nutrient and light require-
ments of these three species are not known, and the
morphological indices make no allowance for di�er-
ences in resource requirements, intrinsic growth rate
or ability to withstand environmental stresses linked
to eutrophication (e.g. high pH, reduced sediments
and alternating oxygen conditions).
The general pattern is, however, the same as
recorded for the distribution of many individual
Potamogeton species in lakes in England (Preston
1995), Finland (Kurimo 1979; Rintanen 1996) and
The Netherlands (Mesters 1995) with P. alpinus, P.
®liformis, P. lucens, P. polygonifolius, P. praelongus
and P. zosterifolius declining and P. crispus and P.
pectinatus becoming relatively more abundant.
Fig. 4 Mean local abundance as a function of range size of
57 species in 206 stream localities in Denmark. The regres-
sion line was signi®cant (r2� 0.222; P<0.001).
1036Macrophyte
decline in Danish
lakes
# 2000 British
Ecological Society
Journal of Ecology,
88, 1030±1040
Palaeolimnological studies (Klein 1993) suggest that
eutrophication had initiated the decline of P. zosteri-
folius in the 1700s and 1800s. Detailed studies in
Lake Fure during its shift from a transparent meso-
trophic state in the early 1900s to a turbid,
eutrophic state in the 1950s also document a com-
plete loss of isoetids, characeans and mosses, a
decline of small elodeid species and the persistence
of most tall canopy-forming elodeids (Petersen &
Raunkiñr 1917; Christensen & Andersen 1958;
Sand-Jensen 1997). While P. ®liformis and P. zosteri-
folius disappeared early during the eutrophication of
Lake Fure, P. crispus and P. pectinatus have
increased from a low rank among the original 36
submerged species to a high rank among the surviv-
ing 12 species (Sand-Jensen 1997).
CHANGES OF STREAM FLORA AND
SIMILARITIES WITH LAKES
Richness and evenness of Potamogeton species have
declined very markedly in streams. Firstly, physical
disturbance due to channelling, weed cutting and
dredging of sediments has generated a more stan-
dard environment which imposes greater demands
on robustness, growth rate and dispersal capacity of
the plants (Barrat-Segretain 1996; Henry et al. 1996;
Barrat-Segretain et al. 1998). In the past the pre-
sence of ri�es with high ¯ow and coarse sediments
and meanders or backwaters with low ¯ow and ®ne
sediments has facilitated the existence of a diverse
¯ora, even including some species typical of the shel-
tered habitats also found within lakes (Ostenfeld
1913). Secondly, many streams have become pol-
luted by organic wastes and dissolved nutrients lead-
ing to the point where susceptible species su�er
acute toxicity, and die-back due to anoxia and over-
growth by blanketing macroalgae and microbial
communities (Sand-Jensen et al. 1989a,b). Thirdly,
eutrophic, turbid lakes located along the course of
streams have lost their submerged vegetation and
therefore act as a sink for plant dispersal (Riis &
Sand-Jensen 2000) rather than a source for the
export of propagules (Nielsen 1872±76; Baagùe &
Ravn 1895±96; Andersen 1910).
Such environmental changes can account for the
overall decline of species richness in streams, the
loss of many Potamogeton species and the possible
trend towards a relatively greater abundance of spe-
cies with a high dispersal capacity (Table 3; Riis &
Sand-Jensen 2000). Only P. crispus, P. natans, P.
pectinatus and P. perfoliatus have remained relatively
common in streams, and all four are fast-growing
species typical of eutrophic, disturbed environments
(Newbold & Palmer 1979; Grime et al. 1988; Sand-
Jensen et al. 1989b; Nùrgaard 1992). It has been
shown that P. crispus (Nichols & Shaw 1986;
Sabbatini & Murphy 1996), P. pectinatus (Sand-
Jensen et al. 1989b) and P. perfoliatus (Nùrgaard
1992) all respond to cutting through enhanced
regrowth. Oligotrophic species (e.g. P. alpinus and
P. polygonifolius; Newbold & Palmer 1979) and
large, broad-leaved species (e.g. P. lucens, P. prae-
longus and P. zosterifolius) have, in contrast,
declined dramatically in abundance (Table 1). The
importance of management is reinforced by the
lower abundance of large Potamogeton species 100
years ago at sites where manual weed cutting had
just commenced (Baagùe & Ravn 1895ÿ96), and the
richer and spatially more heterogeneous vegetation
today in uncut reaches, compared with neighbour-
ing, regularly cut reaches (Baattrup-Pedersen et al.
1998).
The submerged vegetation in Danish streams
today is typically dominated by Elodea canadensis
and Sparganium emersum and a few robust species of
Ranunculus and Callitriche (Riis et al. 2000). These
species show a high growth capacity and e�cient
dispersal of detached shoots, which are transported
by the current and rapidly develop adventitious
roots but form new monospeci®c stands when
caught on the stream bed (Moeslund et al. 1990;
Kronvang et al. 1994; Sand-Jensen et al. 1999).
It is di�cult to test to what extent aspects of
resource use, resource availability and metapopula-
tion dynamics of dispersal and colonization (e.g.
Gaston et al. 1997) can account for the positive
interspeci®c relationship between local abundance
and range size observed in the contemporary stream
vegetation. Several factors suggest, however, that
metapopulation dynamics are particularly important
for the stream vegetation. Streams are naturally
very perturbed due to the frequent changes in velo-
city which lead to the loss of plant stands and sedi-
ment erosion and thus result in unvegetated stream
beds, and disturbance has been further enhanced by
human impact (particularly weed cutting and dred-
ging; Madsen 1995). In addition, many stream
plants are e�ectively dispersed via turions and
detached shoots which are capable of rooting and
then forming new stands through fast clonal growth
(Barrat-Segretain et al. 1998; Sand-Jensen et al.
1999). Dispersal is enhanced by the longitudinal
connectance within stream systems and the proxi-
mity to permanent semi-aquatic populations close to
the shore (Henry & Amoros 1996).
This frequent decline, redistribution and recoloni-
zation of the plant stands would be expected to pro-
mote a strong positive relationship between local
abundance and range size in the stream commu-
nities, similar to that observed in terrestrial commu-
nities of herbaceous plants in wheat ®elds, Brassica
®elds and fallow arable land (Thompson et al.
1998). There are larger dispersal barriers between
separate stream systems than between individual
sites within a system. The positive interspeci®c rela-
tionship is therefore even stronger among such sites
1037K. Sand-Jensen
et al.
# 2000 British
Ecological Society
Journal of Ecology,
88, 1030±1040
(data not shown) than that presented for a range of
systems with a number of di�erent dominant spe-
cies.
FUTURE DEVELOPMENT OF THE AQUATIC
FLORA
The aquatic ¯ora presently includes a large propor-
tion of rare species. In streams such species vary in
abundance at the few localities where they occur
(Fig. 4). Rare aquatic plants in Denmark, include
®ve species of Potamogeton (P. acutifolius, P. colour-
atus, P. densus, P. rutilus and P. zosterifolius), four
isoetids and 10±15 other vascular species (Moeslund
et al. 1990; Vestergaard 1998; Riis & Sand-Jensen
2000) and many of these are also threatened in
other European countries, appearing in national and
international Red-lists (Preston & Croft 1997).
Some species with a solely European distribution
(e.g. Luronium natans, Oenanthe ¯uviatilis and
Potamogeton acutifolius) face the risk of going
extinct, while many others hold only marginal popu-
lations and will therefore have di�culty colonizing
newly suitable habitats generated through improved
water puri®cation, restoration of lakes and re-estab-
lishment of meandering streams (Iversen et al.
1993). The physico-chemical quality of streams, and
in some cases lakes, can be restored, but the possibi-
lity and time scale of recovery of the former diver-
sity of submerged species is unknown. Robust
species have monopolized the environment and
these may resist the spread and regrowth of rare
species, hampering recolonization from marginal
populations.
The landscape of European lowland countries is
increasingly fragmented and subject to continuous
disturbance by natural and anthropogenic events
(Bobbink et al. 1998; Bradshaw & Holmqvist 1999).
The extent to which communities can be reconsti-
tuted under suitable conditions must be related to
species traits, population size and the proximity of
potential colonisers (Grime 1998). Freshwater habi-
tats suitable for submerged plants have been parti-
cularly eroded, and coupled with the decline in
population sizes will make re-assembly of aquatic
communities even more di�cult.
Acknowledgements
We thank the Danish Natural Science Research
Council (grant 11±7795) and the Danish Research
Academy for ®nancial support. We thank Bent
Odgaard and Anne Garde for helpful comments on
the manuscript.
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