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Primary determinants of macrophyte community
structure in 62 marshes across the Great Lakes
basin: latitude, land use, and water quality effects
Vanessa L. Lougheed, Barb Crosbie and Patricia Chow-Fraser
Abstract: We collected water quality, land use, and aquatic macrophyte information from 62 coastal and inland
wetlands in the Great Lakes basin and found that species richness and community structure of macrophytes were a
function of geographic location and water quality. For inland wetlands, the primary source of water quality degradation
was inputs of nutrients and sediment associated with altered land use, whereas for coastal wetlands, water quality was
also influenced by exposure and mixing with the respective Great Lakes. Wetlands within the subbasins of the less de
-
veloped, more exposed upper Great Lakes had unique physical and ecological characteristics compared with the more
developed, less sheltered wetlands of the lower Great Lakes and those located inland. Turbid, nutrient-rich wetlands
were characterized by a fringe of emergent vegetation, with a few sparsely distributed submergent plant species. High-
quality wetlands had clearer water and lower nutrient levels and contained a mix of emergent and floating-leaf taxa
with a diverse and dense submergent plant community. Certain macrophyte taxa were identified as intolerant of turbid,
nutrient-rich conditions (e.g., Pontederia cordata, Najas flaxilis), while others were tolerant of a wide range of condi
-
tions (e.g., Typha spp., Potamogeton pectinatus) occurring in both degraded and pristine wetlands.
Résumé : Des données recueillies sur la qualité de l’eau, l’utilisation des terres et les macrophytes aquatiques dans
62 terres humides des régions côtières et intérieures du bassin des Grands Lacs indiquent que la richesse spécifique et
la structure des communautés de macrophytes dépendent de la situation géographique et de la qualité de l’eau. Dans
les terres humides intérieures, la source principale de dégradation de la qualité de l’eau est l’apport de nutriments et de
sédiments causé par les changements dans l’utilisation des terres, alors que, dans les terres humides côtières, la qualité
de l’eau est aussi influencée par le contact avec le Grand Lac adjacent et les mélanges d’eau qui s’y produisent. Les
terres humides des sous-bassins des Grands Lacs d’amont, qui ont subi moins de développement et qui sont plus expo-
sés, possèdent des caractéristiques physiques et écologiques tout à fait particulières, par comparaison avec les terres
humides des Grands Lacs d’aval qui sont plus développés et moins protégés, et les terres humides intérieures. Les
terres humides turbides et riches en nutriments sont caractérisées par le développement d’une ceinture de végétation
émergente et la présence sporadique de quelques plantes submergées. Les terres humides de grande qualité possèdent
une eau plus claire, des concentrations plus faibles de nutriments et une combinaison de taxons de plantes émergentes
et de plantes à feuilles flottantes, d’une part, et d’une communauté diversifiée et dense de plantes submergées, d’autre
part. Certains taxons de macrophytes se sont révélés intolérants aux conditions de turbidité et de richesse en éléments
nutritifs élevées (e.g., Pontederia cordata, Najas flexilis), alors que d’autres tolèrent une gamme étendue de conditions
(e.g., Typha spp., Potamogeton pectinatus) et se retrouvent dans les terres humides tant dégradées qu’intactes.
[Traduit par la Rédaction] Lougheed et al. 1612
Introduction
Long-term changes in the macrophyte communities of indi
-
vidual Great Lakes coastal marshes, especially those located in
settled areas of Lakes Erie and Ontario, have been well docu
-
mented over the past two decades (e.g., Crowder and Bristow
1986; Klarer and Millie 1992; Chow-Fraser et al. 1998); how
-
ever, the underlying factors causing these changes have rarely
been investigated. These preliminary studies have indicated
that year-to-year changes in areal cover of emergent vegetation
are probably controlled by fluctuating water levels (Keddy and
Reznicek 1986; Chow-Fraser et al. 1998), whereas growth and
diversity of submergent vegetation are more likely controlled
by water clarity (Lougheed et al. 1998).
The combined effects of hydrology, local bedrock geology,
and wetland morphology, referred to jointly as hydro
-
geomorphic factors, tend to be the primary regional determi
-
nants of plant community structure in wetlands and littoral
systems (e.g., Minc 1997; Thiébaut and Muller 1998; Keough
et al. 1999). Where the Great Lakes basin is concerned,
hydrogeomorphic variation and climatic differences associ
-
ated with a total shoreline length of 12 017 km contribute to a
great diversity of physical environments (Smith et al. 1991;
Keough et al. 1999). These regional differences have impor
-
Can. J. Fish. Aquat. Sci. 58: 1603–1612 (2001) © 2001 NRC Canada
1603
DOI: 10.1139/cjfas-58-8-1603
Received November 17, 2000. Accepted May 18, 2001.
Published on the NRC Research Press Web site at
http://cjfas.nrc.ca on July 18, 2001.
J16072
V.L. Lougheed,
1,2
B. Crosbie, and P. Chow-Fraser.
Department of Biology, McMaster University, Hamilton,
ON L8S 4K1, Canada.
1
Corresponding author (e-mail: loughee2@msu.edu).
2
Present address: Department of Zoology, Michigan State
University, 203 Natural Science Building, East Lansing,
MI 48824-1115, U.S.A.
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tant consequences for human settlement and land use patterns,
with the most productive agricultural land and largest urban
centers occurring south of the 46th parallel (Environment
Canada and U.S. Environmental Protection Agency 1995).
Consequently, although wetlands throughout the entire Great
Lakes basin are at risk from non-point-source pollution
(Crosbie and Chow-Fraser 1999), water level regulation
(Keddy and Reznicek 1986), disturbance by nuisance exotic
species such as common carp (Cyprinus carpio) (Lougheed et
al. 1998), and internal loading of phosphorus (Chow-Fraser
1998; Mayer et al. 1999), these disturbance factors tend to
predominate in the lower lakes. In particular, agricultural and
urban land use in wetland catchments of the lower lakes has
been shown to affect nutrient enrichment, water clarity and
sediment quality (Crosbie and Chow-Fraser 1999) and will
therefore likely have profound effects on submersed macro
-
phyte growth (e.g., Phillips et al. 1978; Barko and Smart
1983; Magee et al. 1999) and distribution (Lougheed et al.
1998; Crosbie and Chow-Fraser 1999).
Macrophyte growth is limited by both water quality and
sediment quality (Day et al. 1988; Barko et al. 1991). Light
availability is a primary factor determining photosynthetic
potential and can be reduced by nonalgal and algal turbidity,
including periphytic growth (Phillips et al. 1978). As such,
details on the nutrient status, sediment quality, and particu-
late content of the water are required to make conclusions
regarding the causative effects of macrophyte community
changes. Although taxonomic surveys of macrophyte com-
munities in individual Great Lakes coastal wetlands exist
(e.g., Crowder and Bristow 1986; Klarer and Millie 1992;
Chow-Fraser et al. 1998), there have been no published stud-
ies that account for all of these variables and their roles in
causing species replacement in the macrophyte community
on a basin-wide scale. The most comprehensive study that
we have found was a report by Minc (1997), who analyzed
macrophyte data from 110 coastal wetlands in the U.S. wa
-
ters of the Great Lakes and listed the primary determinants
of macrophyte distribution as latitude, soil pH, water tem
-
perature, and turbidity; however, several important factors
were excluded from this study, including trophic state and
sediment fertility. In Canadian waters, Smith et al. (1991)
published a study that described the differences in the physi
-
cal attributes of 160 coastal wetlands in the three southern
-
most Canadian Great Lakes. Their study provided evidence
that geology, exposure, and disturbance vary statistically
among the three lakes but provided no information on water
quality and taxonomic composition of submerged macro
-
phytes that could be affected by changes in sediment and nu
-
trient load.
The challenge facing managers is to determine the degree
to which each of these hydrogeomorphic, climatic, and
disturbance factors individually or in combination alters wet
-
land plant communities. In this paper, we compare the macro
-
phyte community composition of 62 wetlands in the Cana
-
dian part of the Great Lakes basin to examine how water
quality and sediment quality affect the taxonomic composi
-
tion and community structure of the macrophyte community
and relate these to land use in their watershed. In addition to
the 22 marshes sampled by Crosbie and Chow-Fraser
(1999), located primarily along the shoreline of the lower
lakes, our data set includes 40 additional coastal and inland
wetlands covering all four Canadian Great Lakes. This study
will contribute much needed information to help wetland
managers determine the relative importance of these various
factors in structuring the aquatic macrophyte community on
a basin-wide scale.
Methods
Sixty-two marshes in the Great Lakes basin were visited be
-
tween 1995 and 1999 and sampled for water quality and macro
-
phyte community information. These wetlands were selected based
on the amount of agricultural, urban, and forested land in their wa
-
tersheds to ensure a sufficient gradient of disturbance (Crosbie and
Chow-Fraser 1999). In addition, wetlands were chosen from a
broad geographic range: from the St. Lawrence River just east of
Cornwall, down to the Windsor–Detroit area and Lake St. Clair,
and up to Lake Superior and the Ontario–Minnesota border
(Fig. 1). Forty-six of these were coastal marshes (within 2 km of
the Great Lakes shoreline but not separated hydrologically from
the lake due to dams or waterfalls) of the upper (Huron, Superior)
and lower (Ontario, Erie) lakes, while the remaining 16 wetlands
were located inland within the Great Lakes – St. Lawrence River
basin. All wetlands were visited once in midsummer (June 18 to
July 30).
To choose the sampling location, the general character of each
marsh was first assessed with a brief inspection of accessible near
-
shore areas. Subsequently, we selected a sheltered location (if
available) containing relatively dense submergent plants (if pres-
ent). All water and sediment samples were collected during day-
light hours from the middle of the water column at an open-water
site 3 m from the edge of macrophyte beds. Because of the large
site-to-site, year-to-year, and seasonal variation in water levels,
which are characteristic of Great Lakes coastal marshes (Chow-
Fraser 1999), water depths in this study ranged from 5 to 260 cm,
depending on wetland, time of year, and the sampling year in ques-
tion; however, there were no observed effects of depth on any other
parameter measured in this study.
The protocol for sampling and analysis of water samples has
been documented elsewhere (V.L. Lougheed and P. Chow-Fraser,
unpublished data). For each wetland, we analyzed samples for total
phosphorus (TP), soluble reactive phosphorus (SRP), total nitrogen
(TN) (sum of total Kjeldahl nitrogen (TKN) and total nitrate nitrogen
(TNN)), total suspended solids (TSS), total inorganic suspended sol
-
ids (TISS), and planktonic chlorophyll a corrected for phaeopigments
(CHLa). Temperature, pH, dissolved oxygen, and conductivity were
measured with an H2O
®
Hydrolab. Turbidity readings were taken us
-
ing a portable Hach turbidimeter (model 2100P).
Sediment samples were collected using either an Ekman grab
sampler or a 5-cm Plexiglas tube with plunger and analyzed to de
-
termine the proportion of inorganic matter (INORG
SED
) and total
phosphorus (TP
SED
) in the sediment (Crosbie and Chow-Fraser
1999). The maximum and dominant grain size in each sediment
sample (20–70 g dry sediment) was classified according to the
Wentworth scale (Wotton 1990); wetlands visited by Crosbie and
Chow-Fraser (1999) were not analyzed for sediment size (n = 22).
We counted every species of submergent, emergent, and floating-
leaf vegetation encountered within a 3-m radius of the selected
vegetated site. In wetlands where submergent plant distribution
was sparse (estimated as <5 plants·m
–2
), we conducted an ex
-
panded survey along approximately 100 m of shoreline. Keys by
Fassett (1940) and Newmaster et al. (1997) were used to identify
the macrophyte specimens to species where possible. Because
many wetlands were visited only once, and because certain species
are difficult to identify accurately without flowering parts, many
taxa were identified to genus only. Most submergent taxa were
identified to genus, except for several common species that were
keyed to species. All emergent taxa encountered were identified at
© 2001 NRC Canada
1604 Can. J. Fish. Aquat. Sci. Vol. 58, 2001
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© 2001 NRC Canada
Lougheed et al. 1605
least to genus, and the dominant taxa were noted. The objective of
this plant survey was not meant to be exhaustive but rather to ob
-
tain floristic information on common taxa in a structurally diverse
plant community that would include emergent, submergent, and
floating-leaf components. Consequently, we did not sample along
parallel transects of a systematic grid, an approach employed in
studies designed to produce comprehensive species lists (e.g.,
Minc 1997; Magee et al. 1999). Our approach therefore likely ex
-
cluded rare species and many grasses, rushes, and sedges that
would have been more common in wet meadow environments
(e.g., Minc 1997).
Proportion of agricultural, urban, and forested land was deter
-
mined as described in Crosbie and Chow-Fraser (1999); however,
for the three largest watersheds (>1500 km
2
), land use was esti
-
mated from Detenbeck et al. (1999) (Goulais and Spanish River)
and the Grand River Conservation Authority (Cambridge, Ont.;
www.grandriver.on.ca). Wetlands were classified into land use cat
-
egories as follows: (i) wetlands were assigned to the dominant land
use category if the majority (>50%) of their watershed was of that
land use and if <30% of the remaining catchment area was of an
-
other land use, (ii) wetlands were assigned to a combined land use
category (e.g., agriculture/forested) if no land use type accounted
for >50% of the total catchment area and the difference between
any two types was <20%, and (iii) wetlands that occurred as
fringes along the Great Lakes shoreline and did not have any obvi
-
ous inflows were not classified to any land use (n = 11).
All statistical analyses were performed using SAS.Jmp software
(SAS Institute Inc., Cary, N.C.), except for canonical
correspondence analysis (CCA), which was performed using
CANOCO version 4.0 (ter Braak and Smilauer 1998). Principal
components analysis (PCA) was used to create linear combinations
of the environmental data to describe the underlying environmental
gradients in the data. Environmental data were log
10
transformed
(excluding pH) to approximate normal distributions and standard
-
ized to zero mean and unit variance. CCA was used to determine
the best environmental factors to describe aquatic macrophyte dis
-
tribution and has been used in similar studies relating macrophytes
to their environment (e.g., Toivonen and Huttunen 1995; Bini et al.
1999; Magee et al. 1999). CCA maximizes the separation of spe
-
cies optima along synthetic axes, which represent linear combina
-
tions of environmental variables. CCA was appropriate in this
study because the gradient lengths obtained from detrended corre
-
spondence analysis (CANOCO 4.0) indicated that the species data
were moderately unimodal (ter Braak and Smilauer 1998).
Variables entered into the CCA included the presence of macro
-
phyte taxa encountered in >10% of wetlands as well as accompa
-
nying environmental variables (i.e., TP, TN, TSS, CHLa,pH,
conductivity (COND), TP
SED
, INORG
SED
, latitude (LATITUDE)
(in decimal degrees)). We excluded turbidity, SRP, TISS, TKN, and
TNN from the CCA because they showed a high degree of
collinearity with chosen variables (r > 0.90) and contributed less to
explaining the variation in the data set. All included variables had
variance inflation factors <20, indicating that they contributed
uniquely to the analysis (ter Braak and Smilauer 1998). The statis
-
Fig. 1. Map of the Great Lakes of North America showing the location of the 62 wetlands sampled between 1995 and 1999.
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tical significance of the relationship between the species data and
environmental gradients was determined using Monte Carlo permu
-
tations (199 random permutations) (ter Braak and Smilauer 1998).
Results
Water and sediment quality
Wetlands in this study correspond to a wide range of envi
-
ronmental conditions, ranging from very clear and nutrient
poor (e.g., TP = 16.3
g
·L
–1
,TN=920
g
·L
–1
,
CHLa 0
g
·L
–1
, TSS = 3.4 mg·L
–1
) to turbid and
hypereutrophic (e.g., TP = 670
g
·L
–1
, TN = 9164
g
·L
–1
,
CHLa = 239
g
·L
–1
, TSS = 209 mg·L
–1
) (Table 1). PCA was
initially used to determine which environmental variables
explained the greatest amount of variation in the data set
(Table 2). The first three axes explained 72% of the variation
in the data (Table 2), with nearly 50% being explained by
PC axis 1. This axis was highly and positively correlated
with variables associated with nutrient status (TN, TP, TNN)
and particulate content (TSS, TISS, turbidity, CHLa)ofthe
water. Conductivity, an indication of the ionic strength of the
water, was also highly correlated with this axis. The second
axis explained 16% of the variance and was positively corre
-
lated with INORG
SED
and negatively correlated with the
phosphorus in the sediment. Wetlands with high PC axis 2
scores also had relatively high pH. PC axis 3, which ex-
plained only 10% of the variation in the data, was positively
correlated with SRP and temperature.
Since PC axis 1 is associated with a nutrient–turbidity
gradient, wetlands to the extreme right in Fig. 2 are turbid,
hypereutrophic marshes, whereas those to the left are clear,
nutrient-poor marshes. The majority of turbid, nutrient-rich
wetlands in our data set are coastal wetlands of the lower
Great Lakes, while clear, nutrient-poor wetlands correspond
to inland wetlands and coastal wetlands of the upper Great
Lakes. The second PC axis, which separates wetlands with
largely organic, fertile sediment and low pH from those with
inorganic, infertile sediment and high pH, is a good
discriminator for inland and upper coastal wetlands. Coastal
wetlands in the upper lakes (Huron, Superior) tend to have a
higher pH and a greater proportion of INORG
SED
(analysis
of variance (ANOVA), Tukey–Kramer, p < 0.05), while in
-
land wetlands tend to have more organic sediment and
higher TP
SED
(ANOVA, Tukey–Kramer, p < 0.05).
Land use
Given development patterns in the Great Lakes basin over
the past century, it was not surprising that the proportion of
agricultural (r
2
= 0.50, p < 0.0001) and urban (r
2
= 0.08,
p < 0.05) land in wetland watersheds decreased significantly
with latitude, while the proportion of forested land increased
(r
2
= 0.72, p < 0.001). To determine the effect of land use on
water and sediment quality, we regressed PC axis 1 scores
against the proportion of the three different land use catego
-
ries. PC axis 1 scores were positively related to percent agri
-
cultural land (r
2
= 0.39) (Fig. 3a) and negatively correlated
with percent forested land (r
2
= 0.40) (Fig. 3b). Although
we found no significant relationship between PC axis 1
scores and the proportion of urban land use, likely due to the
small number of wetlands with >10% urban land in their wa
-
tershed (n = 6), we found a significant regression when both
percent agricultural and percent urban land were combined
(r
2
= 0.48) (Fig. 3c).
The wetlands were further divided into three groups ac
-
cording to their location in the Great Lakes basin: inland
wetlands, coastal wetlands of the lower Great Lakes, and
coastal wetlands of the upper Great Lakes. For inland
wetlands, land use explained a large amount of variation in
PC axis 1 (indicative of water quality) and PC axis 2 (indic-
ative of sediment quality). By contrast, land use was not as
good a predictor for water quality in lower coastal wetlands
and was not a significant predictor for upper coastal
wetlands (Table 3).
Sediment grain size and organic content of the sediment
also can be affected by land use as well as by geological fac-
tors and exposure. Because the PCA showed that sediment
characteristics tended to vary by location, wetlands were
again divided into the three groups according to their loca
-
tion. For inland wetlands, agricultural watersheds had a sig
-
nificantly higher proportion of INORG
SED
relative to
forested watersheds (Wilcoxon rank sum, p < 0.05), while
forested wetlands had a somewhat larger maximum sediment
grain size (Wilcoxon rank sum, p < 0.10) (Table 4). There
were no significant trends observed for the more exposed
lower and upper coastal wetlands, where all wetlands con
-
tained significantly more inorganic soils relative to inland
wetlands, especially in the upper lakes (ANOVA, Tukey–
Kramer, p < 0.05).
Submergent plant species richness
The number of submergent plant species in the wetlands
varied inversely with both PC axes 1 and 2 scores, although
there was a great deal of scatter about the best-fit line (r
2
=
0.32 and 0.15, respectively) (Figs. 4a and 4b). A greater
amount of the variation in the species richness data was ex
-
plained by turbidity alone (r
2
= 0.45, p < 0.0001) (Fig. 4c).
When data were sorted according to location, the percent
variance explained by turbidity increased for inland (r
2
=
0.67, p < 0.0001) and lower (r
2
= 0.54, p < 0.0001) coastal
wetlands but was not significant for upper lakes (r
2
= 0.27,
p = 0.0554). Similarly, PC axis 2 explained 51% of the vari
-
ance in species richness for inland wetlands but was not sig
-
nificant for either lower or upper wetlands. Figure 4 also
© 2001 NRC Canada
1606 Can. J. Fish. Aquat. Sci. Vol. 58, 2001
Variable Mean Range
TP (
g
·L
–1
) 101 16.3–670
TN (
g
·L
–1
) 2351 920–9164
TSS (mg·L
–1
) 23.3 3.4–209
Turbidity (NTU) 17.4 1.3–260
CHLa (
g
·L
–1
) 20.2 0–239
Temperature (°C) 23.5 16.5–28.9
Dissolved oxygen (mg·L
–1
) 8.46 1.55–15
pH 7.54 6.0–8.7
COND (
S
·cm
–1
) 388 39–1387
% INORD
SED
79.2 22.3–99.44
TP
SED
(mg·g
–1
) 0.78 0.12–2.17
No. of submergent plant taxa 6 0–15
Note: CHLa determined from up to 500 mL of water filtered through
GF/C filters. Zero values likely reflect insufficient sample volume.
Table 1. Range of environmental variables observed at 62
wetlands in the Great Lakes basin.
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shows that there were generally fewer submergent species in
upper compared with lower and inland wetlands with similar
turbidity PC axis 1 or PC axis 2 scores.
Macrophyte community structure
Submergent taxa that occurred in >50% of the wetlands in-
cluded Ceratophyllum, Elodea, Myriophyllum, Potamogeton
pectinatus, Potamogeton richardsonii, and Vallisneria. Typha,
Scirpus, and Lythrum salicaria were the most prevalent
emergents; Nymphaea and Nuphar also occurred in at least
half of the wetlands. As explained in the methods, these
emergent species are only representative of the dominant
forms encountered in these wetlands.
We performed a CCA to determine the association be-
tween environmental variables and the distribution of macro-
phytes (Fig. 5). Sixty-two percent of the variation in
macrophyte distribution could be explained by the first two
synthetic environmental gradients. The most important pre
-
dictors of macrophyte distribution, as indicated by their cor
-
relation with CCA axis 1, were TSS (r = 0.79), TP (r =
0.73), CHLa (r = 0.70), TN (r = 0.65), and COND (r = 0.61)
(Fig. 5a). The second CCA axis was correlated with vari
-
ables indicative of sediment quality (TP
SED
(r = –0.54),
INORG
SED
(r = 0.34), pH (r = 0.41)), although the strength
of these correlations was weaker than that for CCA axis 1
scores. Not surprisingly, these variables were also the pri
-
mary descriptors of the PC axes and those describing most
of the variation in the submerged macrophyte species rich
-
ness data. LATITUDE was moderately correlated with both
CCA axes 1 (r = –0.38) and 2 (r = 0.38) and therefore con
-
firms the distinction between higher latitude sites (clearer
water, less nutrient- and organic-rich sediment) and lower
latitude marshes (more turbid water, nutrient-rich, higher or
-
ganic content sediment).
Location of plant taxa to the right of the origin in the
biplot suggests that these plants tolerate a turbid, nutrient-
rich water column (Fig. 5a). These included the emergent
taxa Typha, Sagittaria, and Lythrum, submergent taxa Pota
-
mogeton pectinatus and Potamogeton crispus, and the float
-
ing taxa Nuphar variegatum and Nymphaea odorata. Those
taxa that were least tolerant of turbidity and eutrophication
included the emergent taxa Pontederia cordata and
Sparganium sp., the majority of the submergent taxa (e.g.,
Potamogeton richardsonii, Najas flexilis, Utricularia sp.),
and the floating-leaf species Potamogeton natans. Submer-
gent taxa that were moderately tolerant of turbid water in-
cluded Elodea canadensis and Ceratophyllum demersum.
Taxa more influenced by CCA axis 2 and common in more
inorganic, infertile soils at higher latitudes included Scirpus
sp. and Potamogeton richardsonii. Several taxa were ex-
cluded from the CCA because of their rare occurrence, and
some of these taxa displayed obvious trends in their distribu-
tion. Most notably, Potamogeton gramineus, Eleocharis
smallii, Isoetes sp., and Zizania palustris were found north
of 44° latitude in wetlands with relatively low water turbid
-
ity and nutrient concentrations.
Figure 5b displays the location of the site scores for the
CCA analysis. In general, CCA axis 1 separates the turbid,
nutrient-rich wetlands in highly agricultural and urban water
-
sheds from the clearer, more oligotrophic wetlands located
in moderately to largely forested watersheds. CCA axis 2
separates higher latitude forested sites from all other sites.
Three main macrophyte community types are apparent. In
the upper left quadrant of the CCA biplot are higher latitude
sites in forested watersheds where the emergent vegetation is
often dominated by Scirpus sp., Eleocharis smallii,
Potamogeton gramineus, and Potamogeton richardsonii. The
mean number of submergent taxa found in these wetlands
was 6.5, and one quarter of the wetlands in this quadrant had
a low stem density of submergents (<5 stems·m
–2
). By con
-
trast, Typha was the dominant emergent in the lower latitude
sites, which are plotted in the lower half of the biplot. Wet
-
lands located in the lower left quadrant correspond to rela
-
tively high-quality wetlands in forested watersheds with
relatively high species richness of submergent taxa (mean =
10.3 species) where the submergents formed dense mats (all
wetlands >5 stems·m
–2
). Wetlands located on the right-hand
side of the biplot, however, correspond to turbid, nutrient-
rich wetlands with highly developed watersheds. These con
-
tain relatively few submergent taxa (mean = 3.6 taxa), with
© 2001 NRC Canada
Lougheed et al. 1607
Variance
explained (%)
Environmental
variable r
PC axis 1 45.7 TSS 0.92
Turbidity 0.89
TN 0.88
TP 0.87
TISS 0.87
CHLa 0.83
COND 0.76
TNN 0.71
PC axis 2 15.8 TP
SED
–0.85
% INORG
SED
0.78
pH 0.68
PC axis 3 10.4 Temperature 0.68
SRP 0.52
Table 2. Correlation coefficients between PC axes 1, 2, and 3
scores and environmental variables.
Fig. 2. Plot of PC axis 1 versus PC axis 2 scores including in
-
land (ⵧ), lower coastal (+), and upper coastal (䊊) wetlands.
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45% of the wetlands containing a low stem density of
submergents (<5 stems·m
–2
) and only a fringe of Typha.
Discussion
The species richness and community structure of aquatic
macrophytes in wetlands of the Great Lakes basin appeared
to be a function of the geographic location of the wetland
(i.e., whether they are associated with the upper or lower
Great Lakes or located inland) and the degree of water qual
-
ity degradation. For inland wetlands, the primary source of
water quality degradation was excess inputs of nutrients and
sediment associated with agricultural development in the
watershed. For coastal wetlands, however, water quality may
have also been influenced by mixing with water in the lake
proper. In particular, wetlands in the less developed and
more exposed upper Great Lakes had unique physical and
ecological characteristics when compared with more devel
-
oped and less exposed wetlands of the lower Great Lakes
and inland locations.
The proportion of agricultural and urban land in wetland
watersheds was a highly significant predictor of water qual
-
ity (PC axis 1), explaining almost half of the variation in the
dependent variables. These results are consistent with previ
-
ous studies where water quality was related to land use in
the watershed (Johnson et al. 1997; Crosbie and Chow-
Fraser 1999). There was, however, a great deal of variability
observed due to climatic and geological differences associ
-
ated with different latitudes (upper versus lower lakes) and
exposure (inland versus coastal). Water quality in inland
wetlands was substantially degraded by urban and agricul
-
tural land use. Inland wetlands that received a disproportion
-
ate amount of agricultural runoff tended to have a greater
proportion of fine, inorganic silts and clays in their sediment
compared with those in mainly forested watersheds that
have a deep layer of organic muck and larger gravels in their
substrate (Minc 1997). The presence of high silt and clay
content in the sediment is undesirable, since plants grow
better in organic than in inorganic substrates (Day et al.
1988), and small inorganic particles in the sediment can be-
come easily resuspended and stay in suspension (Hamilton
and Mitchell 1997), thus keeping the water column turbid
and light limited for macrophytes. Conversely, forested land
appeared to attenuate the delivery of sediment from the water-
shed to inland wetlands and can apparently contribute to a
greater level of organic matter in these wetlands.
It is noteworthy that land use effects were greatest on wa-
ter and sediment quality in inland systems that do not have
direct hydrological links with the Great Lakes. In coastal
wetlands, the flow of water between the marsh and the Great
Lake in question can be reversed depending on watershed in
-
puts, wind direction, and water level (Chow-Fraser 1999;
Botts 1999); hence, mixing with lake water may ameliorate
the effects of upstream pollution, while wind and wave ac
-
tion in exposed coastal marshes may lead to export of or
-
ganic matter from the wetland to the lake (Day et al. 1988).
The geomorphological characteristics of the Canadian shore
-
lines of the Great Lakes have produced largely exposed
shorelines and lacustrine embayments along the upper lakes
as compared with a large number of riverine-type wetlands
and a smaller proportion of protected lacustrine embayments
in Lakes Erie and Ontario (Smith et al. 1991; Chow-Fraser
and Albert 1999). In our study, 70% of our upper lake wet
-
lands and only 33% of the lower lake wetlands were classi
-
fied as lacustrine. Consequently, intrusion by lake water and
exposure to wave action may play an even larger role in af
-
fecting wetland water quality in the upper lakes than in the
more protected wetlands of the lower lakes.
Many studies have shown that nutrient enrichment can
cause substantial changes in the species richness, composi
-
tion, and density of aquatic vegetation in lakes (e.g.,
Toivonen and Huttunen 1995; Bini et al. 1999; Magee et al.
1999). Our results confirm that submergent macrophyte bio
-
diversity in Great Lakes wetlands declines with deterioration
© 2001 NRC Canada
1608 Can. J. Fish. Aquat. Sci. Vol. 58, 2001
Fig. 3. PC axis 1 scores against arcsine-transformed percent (a) agricultural land, (b) forested land, and (c) agricultural and urban land.
PCA axis 1 PCA axis 2
r
2
pr
2
p
% agricultural Inland 0.67 0.0003 0.34 0.0284
Lower 0.16 0.0489 — —
Upper — — — —
% agricultural and urban Inland 0.61 0.0009 0.29 0.0457
Lower 0.22 0.0181 — —
Upper — — — —
% forested Inland 0.60 0.0012 0.23 0.08
Lower 0.16 0.0511 — —
Upper — — — —
Table 3. Characteristics of the regression relating land use (agri
-
cultural, agricultural and urban, forested) in each region (inland,
lower coastal, upper coastal) to PCA axes 1 and 2.
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Lougheed et al. 1609
in water quality. This is consistent with a study by Findlay
and Houlahan (1997), who also found reduced species rich
-
ness of both aquatic and terrestrial organisms with increas
-
ing development in wetland watersheds of southeastern
Ontario. In particular, we found that 45% of the variation in
submergent plant species richness could be explained by tur
-
bidity alone. The tolerance of aquatic macrophytes to re
-
duced light availability is tightly linked to their structural
characteristics. While emergent and floating-leaf macro
-
phytes are not generally affected by light availability (Day et
al. 1988; Toivonen and Huttunen 1995; Bini et al. 1999),
submergents tend to be greatly influenced by reduced water
clarity. Our results confirm that submerged macrophytes that
are canopy-formers such as Potamogeton pectinatus, Myrio
-
phyllum sp., and Ceratophyllum are able to survive in situa
-
tions of light limitation, as opposed to species such as Chara
and other Potamogeton spp., whose leaves grow below the
surface (Chambers and Kalff 1987; Minc 1997).
The synergistic effects of many factors including eutro
-
phication and algal and nonalgal turbidity (Phillips et al.
1978) and sediment characteristics (Carignan and Kalff
1980; Barko and Smart 1983; Day et al. 1988) can all affect
macrophyte growth. Because of their sensitivity to this large
suite of physical and chemical variables, many macrophyte
taxa have been used as indicators of trophic state in other
systems (e.g., Grasmück et al. 1995; Thiébaut and Muller
1998). In this study, there appears to be certain taxa that are
intolerant of turbid, nutrient-rich conditions (e.g., Pontederia
cordata, Potamogeton richardsonii, Najas flexilis, Utricu
-
laria sp.) but none that occur exclusively in degraded sites.
Although some taxa appear to be more tolerant of water and
sediment quality degradation (e.g., Typha, Sagittaria, Pota-
mogeton pectinatus, Nuphar, Nymphaea), including exotic
species such as purple loosestrife (Lythrum salicaria) and
curly-leaf pondweed (Potamogeton crispus), even these taxa
can be found in high-quality wetlands that are not turbid and
eutrophic. What appears to be a better indicator of wetland
quality is the type of community present rather than the
presence of certain indicator species. The presence of a fringe
emergent community with only a few sparsely distributed
submergent taxa is highly indicative of a turbid, nutrient-rich
wetland, whereas a mix of emergent and floating-leaf taxa
with a diverse and dense submergent community is highly
indicative of a high-quality wetland.
Species richness and plant density tend to be low in oligo
-
trophic systems. As nutrient concentrations increase in these
impoverished systems, both the stem density and the diver
-
sity of macrophytes increase until such time as light be
-
comes limiting and plant abundance starts to level off
(Lachavanne 1985; Toivonen and Huttunen 1995). The most
oligotrophic sites in this study tended to be associated with
the upper lakes, where species richness and density were
substantially lower than in the lower lakes; however, inland
wetlands of similar trophic state showed greater species rich
-
ness and plant density. In the Great Lakes basin, the relation
-
ship between water quality and the macrophyte community
is definitely confounded by the strong latitudinal gradient
that exists from the southern- to northernmost points (Smith
et al. 1991). The macrophyte communities in the upper and
lower lakes differed substantially in their dominant species.
In lower latitudes, emergent marshes tended to be dominated
by Typha sp., while in the upper lakes the dominant emer
-
gent tended to be Scirpus sp. or Eleocharis smallii. There
were fewer submergent taxa overall in the upper lakes, with
the most frequently observed being relatively sparse beds of
Potamogeton gramineus, Potamogeton richardsonii, and
n % organic
Maximum
sediment size ( m)
Inland Agricultural 4 16.5 (4.3)* 0.156 (0.094)*
Forest 8 49.8 (6.8)* 16 (0)*
Agricultural/forested 2 76.1 (1.7) 0.0039 (0)
Lower Agricultural 17 17.2 (4.3) 0.094 (1.98)
Forest 2 23.9 (14.8) 8.03 (7.97)
Agricultural/forested 2 21.8 (0.6) 0.25 (0)
Urban 4 29.1 (11.4) 0.125 (0)
Upper Agricultural 1 6.62 (0) 0.0625 (0)
Forest 8 3.0 (0.8) 9 (2.91)
Agricultural/forested 2 8.73 (7.58) 8.03 (7.97)
Note: Asterisks indicate significant differences among inland sites
(Wilcoxon rank sum, p < 0.05).
Table 4. Comparisons of means (and standard errors) of the per
-
cent organic composition of the sediment and the maximum sedi
-
ment size observed in wetlands characterized by different land
use types (agricultural, forested, agricultural/forested, urban) in
inland, lower coastal, and upper coastal systems.
Fig. 4. Number of submersed plant taxa plotted against (a) PC axis 1 scores, (b) PC axis 2 scores, and (c) log turbidity (nephelometric
turbidity units) including inland (ⵧ), lower coastal (+), and upper coastal (䊊) wetlands.
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Isoetes sp. Conversely, there was a more diverse, dense, and
variable submergent plant community in the high-quality
wetlands of the lower lakes and fewer submergent species
present in more turbid, sparsely vegetated wetlands.
The importance of latitude in structuring the macrophyte
community in the Great Lakes basin is due to several fac
-
tors, including climate and geology (Smith et al. 1991; Minc
1997). A strong latitudinal gradient exists between the upper
and lower lakes, which affects the length of the growing sea
-
son and the annual input of solar radiation. Furthermore, the
soft sedimentary rock underlying the lower lakes provides
expansive areas of shallow water and fine-textured sub
-
© 2001 NRC Canada
1610 Can. J. Fish. Aquat. Sci. Vol. 58, 2001
Fig. 5. Biplot of the CCA. (a) Environmental vectors and common macrophyte species (occurred in >10% of the wetlands sampled):
submergent ( ), emergent (䉭), and floating (䊊); (b) location of site scores classified into land use categories: agricultural (ⵧ),
forested (䉫), agricultural/forested (䉬), urban (+), and unknown (䊏). Submergent species: Ceratophyllum demersum (DE), Chara sp.
(CH), Elodea Canadensis (EL), Myriophyllum sp. (MY), Najas flexilis (NA), Potamogeton sp. (PS), Potamogeton crispus (PC),
Potamogeton pectinatus (PP), Potamogeton richarsonii (PR), Ranunculus sp. (RA), Utricularia sp. (UT), Vallisneria americana (VA).
Emergent species: Lythrum salicaria (LY), Pontederia cordata (PO), Sagittaria latifolia (SA), Scirpus sp. (SC), Typha sp. (TY).
Floating-leaf species: Nuphar variegatum (NU), Nymphaea odorata (NY), Potamogeton natans (PN).
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strates favorable for marsh development, while the older ig
-
neous and metamorphic bedrock in Lake Superior and
Georgian Bay (Lake Huron) results in a deeper and more ex
-
posed shoreline with large-textured inorganic substrates.
Plants grow best in organically rich and fertile substrates
(Barko and Smart 1983), which are generally lacking in the
upper lakes. Consequently, we saw sparse vegetative com
-
munities consisting of species better adapted to the shorter
growing season and lower substrate fertility of the northern
lakes such as Scirpus, Eleocharis, Equisetum, Potamogeton
gramineus, and Isoetes (Day et al. 1988; Minc 1997). These
taxa were largely absent from the lower lakes and were re
-
placed by taxa more indicative of a southern community such
as Ceratophyllum (Minc 1997).
In conclusion, as has been observed in several other large-
scale studies (Smith et al. 1991; Minc 1997), physical fac
-
tors largely related to geology and latitude such as exposure,
sediment composition, and length of growing season may be
important determinants of macrophyte community composi
-
tion in coastal wetlands of the Great Lakes. In lower coastal
and inland wetlands, plants are distributed according to a nu
-
trient and clarity gradient, which was inextricably linked to
land use in the watershed. In coastal lacustrine wetlands, es
-
pecially in the upper lakes, land use plays less of a role in
affecting water and sediment quality than mixing with the
lake proper. Because aquatic plants are tightly linked to the
functional capacity of wetlands, providing habitat and a sink
for sediment and nutrients, the cumulative impacts of altered
land use may reduce these important wetland values. We
echo the management challenge issued to all levels of gov-
ernment at the State of the Lake Ecosystem Conference
(e.g., Bertram and Statler-Salt 1999) to promote land use
that is both efficient and protective of high-value nearshore
habitat in the Great Lakes.
Acknowledgements
We are grateful to many people for their help in locating
and sampling these wetlands, especially R. Haas, C.
MacIsaac, S. McNair, and C. Moulder. This research was
funded by the McMaster Eco-research program for Hamilton
Harbour, an Ontario Graduate Scholarship to V.L.L., the J.F.
Harvey and H.S. Harvey Travel Scholarship to V.L.L., and a
private donation from B.D.L. Bennett.
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