J. Great Lakes Res. 25(1):187–197
Internat. Assoc. Great Lakes Res., 1999
Changes in the Dreissenid Community in the Lower Great Lakes with
Emphasis on Southern Lake Ontario
Edward L. Mills1,*, Jana R. Chrisman1, Brad Baldwin2,
Randall W. Owens3, Robert O’Gorman3, Todd Howell4, Edward F. Roseman5, and Melinda K. Raths5
1Department
of Natural Resources
Cornell Biological Field Station
900 Shackelton Point Road
Bridgeport, New York 13030
2Biology
Department
Bewkes Hall
St. Lawrence University
Canton, New York 13617
3U.S.
Geological Survey
Biological Resources Division
Great Lakes Science Center
Lake Ontario Biological Station
17 Lake Street
Oswego, New York 13126
4Ontario
Ministry of Environment and Energy
125 Resources Road
Etobicoke, Ontario M9P 3V6
5Department
of Fisheries and Wildlife
Michigan State University
13 Natural Resources Building
East Lansing, Michigan 48824
ABSTRACT. A field study was conducted in the lower Great Lakes to assess changes in spatial distribution and population structure of dreissenid mussel populations. More specifically, the westward range
expansion of quagga mussel into western Lake Erie and toward Lake Huron was investigated and the
shell size, density, and biomass of zebra and quagga mussel with depth in southern Lake Ontario in
1992 and 1995 were compared. In Lake Erie, quagga mussel dominated the dreissenid community in the
eastern basin and zebra mussel dominated in the western basin. In southern Lake Ontario, an east to
west gradient was observed with the quagga mussel dominant at western sites and zebra mussel dominant at eastern locations. Mean shell size of quagga mussel was generally larger than that of zebra mussel except in western Lake Erie and one site in eastern Lake Erie. Although mean shell size and our
index of numbers and biomass of both dreissenid species increased sharply in southern Lake Ontario
between 1992 and 1995, the increase in density and biomass was much greater for quagga mussels over
the 3-year period. In 1995, zebra mussels were most abundant at 15 to 25 m whereas the highest numbers and biomass of quagga mussel were at 35 to 45 m. The quagga mussel is now the most abundant
dreissenid in areas of southern Lake Ontario where the zebra mussel was once the most abundant dreis-
*Corresponding
author. E-mail: [email protected]
187
188
Mills et al.
senid; this trend parallels that observed for dreissenid populations in the Dneiper River basin in the
Ukraine.
INDEX WORDS: Quagga mussel, Dreissena, zebra mussel, Lake Erie, Lake Ontario.
INTRODUCTION
Two species of dreissenid mussels were introduced into the Great Lakes in the late 1980s (Griffiths et al. 1991, Mills et al. 1993), one was
Dreissena polymorpha Andrusov, 1897 (herein referred to as zebra mussel) and the other was given
the working name of “quagga” later identified as
Dreissena bugensis (Rosenberg and Ludyanskiy
1994, Spidle et al. 1994). The quagga mussel was
first sighted at Port Colborne in Lake Erie in 1989
(Mills et al. 1993). By 1993, the distribution of
quagga mussel extended from Lake St. Clair eastward to Quebec City on the St. Lawrence River.
The quagga most likely originated from a population in the Black Sea and Dneiper River drainage in
the former Soviet Union (Spidle et al. 1994, Mills
et al. 1996) and its release into Great Lakes waters
is linked to discharge of ship ballast water (Mills et
al. 1994). North American populations of quagga
mussel were once thought to have a lower thermal
maximum than zebra mussel (Mills et al. 1993, Spidle et al. 1995); however, Mitchell et al. (1996)
demonstrated that these populations, similar to
Ukrainian populations, were not thermally separated. In fact, Ukrainian populations of quagga
mussel have a higher thermal maximum than zebra
mussel and areas once dominated by zebra mussel
are now dominated by the quagga mussel (Mills et
al. 1996).
In Lake Erie’s eastern basin, dense colonies of
quagga mussel have infested profundal areas up to
depths of 55 m (Roe and MacIsaac 1997). Although
some meiofaunal species have benefitted from the
presence of D. bugensis in the profundal zone, burrowing amphipod Diporeia hoyi numbers have declined sharply (Dermott and Kerec 1997). Water
temperatures in eastern Lake Erie’s profundal zone
rarely exceed 5°C yet recent evidence indicates that
gonadal development and spawning by quagga
mussel occurs at these low tempertures (Roe and
MacIsaac 1997).
In this paper, differences in spatial distribution
and population structure of dreissenid populations
in the lower Great Lakes are examined. More
specifically, the range expansion of quagga mussel
westward from eastern Lake Erie toward
Lake Huron is examined and the shell size, density,
biomass, and depth distribution of zebra mussel and
quagga mussel in southern Lake Ontario between
1992 (Mills et al. 1993) and 1995 are compared.
METHODS
Sampling Locations and Gear
Dreissenid mussels were sampled in spring, summer, and fall (1992 to 1995) at locations in Lakes
Huron, St. Clair, Erie, and Ontario and the St. Clair,
Detroit, and St. Lawrence rivers (Fig. 1) using bottom trawls, a ponar dredge, a benthic sled, and a
benthic egg pump. A summary of waterbodies sampled, months of collection, number of locations,
depths sampled, and gear used to collect dreissenids
is shown in Table 1. At sites where a ponar dredge
(523 cm 2 ) was used to collect dreissenids, five
replicate samples of benthic invertebrates were collected, washed through a 0.6-mm mesh sieve, and
preserved in 10% formalin. Mussels were identified
to species and enumerated in the laboratory. Samples of mussels obtained by the benthic sled, egg
pump, and 5.5-m, 10-m, and 12-m headrope bottom
trawls were qualitative and included a few hundred
to several thousand individual dreissenids from
each location. Mussels collected with these devices
were frozen until processing, then thawed in the
laboratory where they were sorted by species,
counted, and shell length measured to the nearest
millimeter.
Density and Biomass of Zebra and
Quagga Mussel in Southern Lake Ontario
Besides determining relative densities, wet
weight (kg) of zebra and quagga mussel was determined at four sites in southern Lake Ontario (Olcott, Thirty Mile Point, Rochester, Smoky Point) in
April and October, 1992 and 1995. LORAN was
used to locate trawl sites and samples were collected with a 12-m headrope bottom trawl at 10-m
depth intervals from 12 to 85 m. On occasion, trawl
collections were made between 95 and 130 m. All
trawl hauls were made on soft substrate and tow
times were usually 10 min. Catches in shorter tows
were adjusted by simple proportion to weight per
10 min trawl. If the wet weight of Dreissena in the
trawl was < 0.2 kg, the entire sample was saved;
Quagga Mussel in Lower Great Lakes
189
FIG. 1. Sampling locations of zebra (Z) and quagga (Q) mussels in the Great Lakes and
the St. Lawrence River in 1993 (◆
◆ ), 1994 (■
■ ), and 1995 (●
● ). Solid symbol = zebra and
quagga mussels absent; open symbol = zebra and/or quagga mussels present (asterisk represents the first sighting of the quagga mussel).
otherwise, only a subsample grab of the total catch
was retained. Wet weight of individual dreissenids
was estimated from shell length by converting
length into dry weight (g) and then converting dry
weight to wet weight by use of the following equations (Mills et al., unpublished).
Zebra mussel: dry weight =
e^(2.7622 ⋅ lnL – 9.2349)
wet weight =
2.5692 . dry weight – 0.0477
Quagga mussel: dry weight =
e^(2.9538 . lnL – 10.068)
wet weight =
2.5692 . dry weight – 0.0477
(1)
(2)
(3)
(4)
where L = shell length in mm.
Total numbers and wet weight biomass of each
dreissenid species caught was determined using the
following equation: (# or kg in subsample/wet
weight of subsample) . (wet weight of total catch).
Data Analysis
Shell Length
To determine which species, quagga or zebra
mussel, was the largest among five sites in Lake
Erie, eight sites in Lake Ontario, and one site in the
St. Lawrence River, the length-frequency distributions were first tested for normality using the Pro
Univariate statement (SAS Institute, Inc. 1989) to
determine the appropriate test. As 22 of the 28 distributions tested were not considered normal, a nonparametric test was employed. The Mann-Whitney
U test (in which the data are ranked) was used to
determine which species was the largest at the various sites. Tukey’s Studentized Range test was used
to assess multiple comparisons in mean shell length
of the two dreissenid species between years for
Lake Ontario (1995 quagga vs. 1995 zebra, 1995
quagga vs. 1992 quagga, 1995 zebra vs. 1992 zebra,
and 1992 quagga vs. 1992 zebra). Regression
analysis (Zar 1984) was used to determine if shell
length of the two dreissenid species differed at each
depth zone in southern Lake Ontario.
Lake Ontario
Olcott to Mexico Bay
Eastern basin
Waterbody
St. Clair River
Lake St. Clair
Detroit River
Lake Erie
Niagara River
Lake Ontario
Six Mile Creek to Hamilton
Oakville to Toronto
Presquile to Amherst Island
Bay of Quinte
St. Lawrence River
Lake Huron
Lake St. Clair
Lake Erie
Western basin
April–June, September–October
8
5
1
October
October
4
3
3
2
3
5
3
Number of
of locations
1
2
2
7
1
April–May, August
April–May, August
April–May, August
May, August
April–May, August
August
October
Months of collection
April, July, September
April, July, September
April, July, September
April, July, September–October
May, July, October
15, 25, 35, 45, 55, 65
75, 85, 95, 110, 130
15, 19, 20, 20, 21
22, 25, 26, 29
2, 3, 5, 7, 10
18, 19, 23, 23
9, 15, 21
3, 21, 25
4, 5
4, 5, 11
6, 9, 11, 17, 20
1, 3, 7
Depths (m)a
2
5, 5
1, 7
9, 10, 10, 11, 11, 11, 12
3
Bottom trawle
Ponar, Bottom trawlb
Egg pumpc
Bottom trawld
Ponar
Ponar
Ponar
Ponar
Ponar
Ponar
Ponar
Collection gear
Ponar
Ponar
Ponar
Ponar
Ponar
St. Lawrence River
Cape Vincent
November
1
9
Benthic sled
Total number of sites
51
aaverage over surveys to nearest meter
b5.5-m foot rope, 8-mm, stretch measure, cod end (E.F. Roseman, Department of Fisheries and Wildlife, Michigan State University, 13 Natural Resources Building, East Lansing, MI 48824, personal communication)
c(Stauffer 1981)
d10-m head rope, 9-mm, stretch measure, cod end (Culligan et al. 1992)
e12-m head rope, 9-mm, stretch measure, cod end (O’Gorman et al. 1991)
1995
1994
Year
1993
TABLE 1. Waterbody, month of collection, number of locations, number of depths, and collection gear used to collect zebera and
quagga mussels, 1993–1995.
190
Mills et al.
Quagga Mussel in Lower Great Lakes
Dreissenid Community Structure
Differences in quagga mussel density (number .
10 min trawl) and biomass (kg ⋅ 10 min trawl)
across three depth ranges (15–25 m, 35–55 m, and
65–85 m) were tested in southern Lake Ontario.
These three depth ranges were selected based on
thermal profiles of Lake Ontario observed during
1972 (Almazan and Pickett 1980) and were chosen
to reflect different temperature regimes bottom
dwelling mussels could be exposed to from July
through October. In general, the depths 15–25 m
were above the thermocline from the end of July
through fall turnover, 35–55 m were above the thermocline only during fall turnover, and 65–85 m
were never above the thermocline. Water temperatures in the 65–85 m depth range reached 7°C
briefly during fall turnover (Almazan and Pickett
1980; USGS, unpublished data). Density and biomass for each dreissenid species was transformed
by log(x+1) and an ANOVA was performed to test
for differences between dreissenid species at increasing depth within and between years. Tukey’s
Studentized Range test was used to assess which
depth ranges were different. Both ANOVA and
Tukey’s Studentized Range test were adapted from
SAS (SAS Institute, Inc. 1989). Comparisons were
considered statistically significant at p < 0.05.
RESULTS
Geographic Distribution
Quagga mussels were not observed in the Detroit
nor the St. Clair rivers. A single quagga mussel was
detected in fall samples from a location in eastern
Lake St. Clair in 1993. However, further sampling
at the Lake St. Clair site in 1995 did not produce
any quagga mussels, nor were any quagga mussels
found northward at sites in Lake Huron. The pattern
of colonization of zebra and quagga mussels differed along an east to west gradient in Lakes Erie
and Ontario. In 1995, quagga mussels were present
at all locations sampled in both the western and
eastern basins of Lake Erie (Fig. 1). Quagga mussels comprised 13% of the dreissenid population by
number in the western basin whereas eastern basin
dreissenids were 78 to 100% quagga mussels (Table
2). In Lake Ontario, quaggas were not observed
along the northwest shore (9 to 21 m depth) between Oakville and Toronto but were observed
along the northeast shore (3 to 25 m depth) between
Presqu`ile Bay and Wolfe Island. Along the south
shore of Lake Ontario, the most abundant dreis-
191
senid shifted from quagga to zebra mussels progressively from west to east. Dreissenids at locations
from Smoky Point westward (Olcott, Thirty Mile
Point, Hamlin, Rochester, and Smoky Point) were
mostly quagga mussels whereas dreissenids at locations east of Smoky Point (Fair Haven, Nine Mile
Point, and Mexico Bay) were mostly zebra mussels
(Table 2).
Shell Length
To determine which of the two dreissenid species
might attain the largest body size in the Great
Lakes, mean shell lengths of quagga and zebra
mussels at each of 14 sites in Lakes Erie and Ontario and the St. Lawrence River in 1995 were compared. In Lake Erie, there was no clear pattern
among the four sites tested where both dreissenid
species were present. Mean shell length of quagga
mussels was longer than that of zebra mussels at
two sites (p < 0.01), shorter than that of zebra mussels at two sites (p < 0.01), and not statistically different (p = 0.37) at the Sturgeon Point site (Table
2). However, in Lake Ontario, mean shell length of
quagga mussels was significantly greater (p < 0.01)
than that of zebra mussels at seven of eight sites
tested, and in the St. Lawrence River the quagga
was significantly the longer (p < 0.01) of the two
species. Not only were quagga mussels larger on
average than zebra mussels at 10 of 14 sites in the
lower lakes, the largest individuals in the random
samples taken for length-frequency measurements
were quagga mussels at all locations. These results
suggest that quagga mussels have the potential to
be the larger of the two forms in the Great Lakes
(Table 2).
The relationship between mean shell length and
depth for zebra and quagga mussels was examined
from collections made in Lake Ontario at 10-m intervals from 15 to 85 m in 1992 and 1995. Samples
were pooled across four sites (Olcott, Thirty Mile
Point, Rochester, and Smoky Point) and two seasons (spring and fall) but samples from each year
were analyzed separately. No relationship was
found between mean shell length and depth for either species in either year—slopes of the shell
length-depth linear regressions were significantly
different from zero (ANOVA, p > 0.05). The same
data sets were used, but pooled across depths, to
examine changes in mean shell length between
1992 and 1995 and found that zebra mussels and
quagga mussels were significantly larger in 1995
(p < 0.01).
192
Mills et al.
TABLE 2. Mean shell lengths of zebra (Z) and quagga (Q) mussels collected April to November from
Lake Erie, Lake Ontario, and St. Lawrence River in 1995. All shell lengths for each dreissenid species
were pooled for the range of depths indicated (asterisk indicates species with significantly larger shell
length, p < 0.01; N = number of zebra and quagga mussels in subsample of trawl; SE = standard error).
Location
Lake Erie
Western Basin
Locust Point
Eastern Basin
Seneca Shoals
Sturgeon Point
Silver Creek
Dunkirk
Barcelona
Lake Ontario
Olcott
Thirty Mile Point
Hamlin
Rochester
Smoky Point
Fair Haven
Nine Mile Point
Mexico Bay
%Q
()b
Depth range (m)
Length (mm)
SE
Species
N
Z*
Q
2,582
388
13
2–10
2–10
10.5
8.0
0.10
0.30
2–30
2–31
Z
Q*
Z
Q
Z*
Q
Z
Q*
Za
Q
48
173
13
482
14
962
12
378
0
276
78
15
15
19–20
19–20
20–25
20–25
21–29
21–29
26
26
8.9
14.2
11.8
12.9
16.1
12.3
9.4
13.5
—
8.7
0.46
0.31
0.72
0.19
1.27
0.17
0.54
0.21
—
0.28
4–17
4–26
8–18
5–25
10–26
2–31
7–13
3–27
—
2–27
Z
Q*
Z
Q*
Z
Q*
Z
Q*
Z
Q*
Z
Q*
Z
Q
Z
Q*
143
990
359
1,159
301
537
498
418
653
716
47
20
231
30
300
23
25–110
25–110
15–130
15–130
25–75
25–75
15–110
15–110
35–95
35–85
45–110
45–110
35–65
35–65
25–45
25–45
12.7 (10.8)
17.3 (12.6)
11.8 (9.9)
17.3 (14.0)
14.5
18.5
14.9 (9.9)
19.0 (12.8)
14.3 (4.0)
19.9 (12.8)
14.9
23.1
13.4 (10.3)
15.2 (12.0)
13.2
21.8
0.40
0.16
0.21
0.16
0.23
0.20
0.20
0.18
0.15
0.17
0.72
1.72
0.37
1.31
0.39
0.75
2–23
3–32
3–26
2–31
5–25
3–33
3–24
8–32
2–23
6–33
4–22
11–35
3–24
4–28
3–28
14–30
10.0 (19.2)
25.4 (29.2)
0.17
1.74
4–31
19–32
97
99
97
100
87 (36)
76 (39)
64
46 (23)
52 (33)
30
11 (2)
7
St. Lawrence River
Cape Vincent
Z
300
3 (10)
8–10
Q*
8
8–10
ano statistical test done, N = 0 for zebra mussel
b% quagga mussel in 1992 (after Mills et al. 1993)
cmean length of quagga and zebra mussels in 1992 (after Mills et al. 1993)
Dreissenid Density and Biomass in
Southern Lake Ontario
Dreissenid mussels were collected at 10 different
depths in southern Lake Ontario in April and October. The mean density and biomass of quagga and
zebra mussels per 10 min trawl tow was determined
at each depth at four locations (Olcott, Thirty Mile
Mean
()c
Min–Max
Point, Rochester, and Smoky Point) in 1992 and
1995 (Fig. 2). In 1992, quagga mussels were observed at all depths < 85 m and average densities
ranged from 1 to over 2,700 individuals per 10 min
trawl. Mean densities of quagga mussels increased
from 1992 to 1995 at all depths except at 75 and 85
m. However, the biomass of both dreissenids in-
Quagga Mussel in Lower Great Lakes
FIG. 2. Percent biomass (A) and number (B) of
quagga mussels based on a 10-min tow with bottom trawls (about 0.73 ha area swept with each
tow) along 10-m depth contours in Lake Ontario
in 1992 (Mills et al. 1993) and 1995. For each
depth and above each bar are mean biomass
(panel A) and mean number (panel B) determined
from April and October samples pooled at four
sites (Olcott, Thirty Mile Point, Rochester, and
Smoky Point).
creased at all depths between 15 and 85 m. The average biomass of all Dreissena per 10 min tow
summed across depths 15 to 85 m increased dramatically from 0.8 kg/10 min trawl in 1992 to 145
kg/10 min trawl in 1995 (Fig. 2).
In 1992 and 1995, the index of total dreissenid
density differed significantly among three depth
ranges (15 to 25 m, 35 to 55 m, and 65 to 85 m)
(ANOVA, 1992, p = 0.0350; 1995, p = 0.0001). In
1992, quagga mussel numbers did not differ among
three depth ranges (ANOVA, p = 0.4546) whereas
zebra mussel numbers did differ significantly
between the two deepest depth ranges (Tukey,
p < 0.05). By 1995, the relative density of both
193
dreissenid species did not significantly differ between the two shallowest depth ranges, but 15–25
m differed with 65–85 m and 35–55 m differed with
65–85 m (Tukey, p < 0.05). Density per 10 min
trawl tow of zebra and quagga mussels significantly
increased from 1992 to 1995 (ANOVA, p =
0.0001). While total dreissenid mean biomass differed with depth in both years (ANOVA, 1992, p =
0.0179; 1995, p = 0.0001), the biomass of each
species examined separately did not differ with
depth in 1992 (ANOVA, zebra mussels, p = 0.1040;
quagga mussels, p = 0.1256). By 1995, biomass of
zebra mussels differed between the 15–25 m and
65–85 m depth ranges and biomass of quagga mussels differed between the 15–25 m and 65–85 m and
between 35–55 m and 65–85 m depth ranges
(Tukey, p < 0.05). The relative change in biomass
for both dreissenid species at each depth range significantly increased from 1992 to 1995 (zebra mussels, p = 0.0005; quagga mussels, p = 0.0001).
The index of dreissenid mussel biomass in Lake
Ontario indicates that zebra mussel and quagga
mussel biomass increased over the study period and
that areas of the lake bottom dominated by zebra
mussel biomass in 1992 were dominated by quagga
mussel biomass in 1995. For example, quagga mussel biomass in 1992 averaged by depth for sites at
Olcott, Thirty Mile Point, Rochester, and Smoky
Point was lower than zebra mussel biomass at all
depths from 15 to 75 m. By 1995, zebra mussel biomass exceeded quagga mussel biomass only at 15
m, while at all other depths, 51 to 88% of the dreissenid biomass was quagga mussel (Fig. 2). The
mean biomass of quagga mussel increased dramatically with depth from 15 to 45 m in 1995 compared
to 1992, reaching a maximum of 55.3 kg/10 min
tow at 35 m, whereas zebra mussel biomass was 2
to 17 times lower over the same depth range.
DISCUSSION
Geographic Distribution of Quagga Mussel
These findings indicate that the demographics of
quagga mussel colonization differs in Lakes Erie
and Ontario. In Lake Erie, quagga mussels dominate the dreissenid community in the eastern basin
and have recently colonized the western basin. Few
or no quagga mussels have been observed upstream
of western Lake Erie indicating that northward expansion of quagga mussels is progressing slowly. In
contrast to Lake Erie, the highest proportion of
quagga mussels in southern Lake Ontario occurred
in the west and declined eastward; the percentage of
194
Mills et al.
quagga mussels caught in bottom trawls declined
precipitously east of Smoky Point (Table 2). In
Lake Ontario, the proportion of quagga mussels has
increased since 1992 along the heavily colonized
southwest shore (Fig. 2) and in the eastern basin
(Nine Mile Point) where quagga have increased
from 2% of the dreissenid population in 1992 (Mills
et al. 1993) to 11% in 1995 (Table 2).
Changes in the Size of Dreissenid Mussels
In Lake Erie, shell lengths of quagga mussels in
1995 were larger than zebra mussels at all sites except those in the western basin and at one site in the
eastern basin (Mills et al. 1993; Table 2). Quagga
shell length increased 30% in the eastern basin of
Lake Erie despite documented reductions in phytoplankton biomass (MacIsaac et al. 1992, Holland
1993, Leach 1993, Nicholls and Hopkins 1993,
Madenjian 1995). In the eastern basin of Lake Erie
(off Van Buren and Brockton), the mean shell
length of quagga mussels was 8.6 mm in 1992
(Mills et al. 1993) and 12.3 mm in 1995 (based on
mean of five locations, Table 2). In contrast, the
mean shell length of zebra mussels in Lake Erie’s
eastern basin decreased (13.3 to 11.6 mm) over the
same time period. These changes are consistent
with MacIsaac (1994); shell length of small zebra
mussels transplanted from the western basin to the
eastern basin of Lake Erie decreased by 7%,
whereas shell length of small transplanted quaggas
increased by 1%. Similarly, Nalepa et al. (1996)
found that mean shell length of zebra mussels declined along with dramatic declines in chlorophyll
between 1990 and 1994 in Lake St. Clair. In southwestern Lake Ontario, where dreissenid infestation
was high and food resources low (Mills et al.
1995), mean shell length of quagga mussels exceeded that of zebra mussels by as much as 15%
even though zebra mussel shell length increased between 1992 and 1995.
A Species Shift toward Quagga Mussel
In this study, bottom trawls towed along depth
contours were used to integrate dreissenid densities
over large areas. This approach did not give true estimates of dreissenid density but it did provide relative changes in density and biomass of quagga and
zebra mussels over the study period. Consequently,
it is reasonable to conclude that the quagga mussel
is increasing in importance in southern Lake Ontario. Catches of quagga mussels in 1995 were
higher in areas of southwestern Lake Ontario that
were dominated by zebra mussels in 1992. Such a
species shift is supported by evidence that the trawl
index of quagga mussel densities exceeded zebra
mussel densities at all depths > 15 m in 1995
whereas this was not true in 1992 (Fig. 2). Similarly, dreissenid biomass has also shifted toward the
quagga mussel. In 1992, zebra mussel biomass exceeded quagga mussel biomass at depths of 15 to
65 m whereas in 1995 the biomass of zebra mussels
exceeded that of quagga mussels only at 15 m. The
apparent species shift toward quagga mussels in
Lake Ontario is consistent with species shifts that
occurred among Ukrainian dreissenid populations
in the Dneiper River basin. In shallow Ukrainian
reservoirs, D. polymorpha has been largely replaced
by D. bugensis (Mills et al. 1996).
This study supports earlier evidence
(Stanczykowska and Lewandowski 1993, Mitchell
et al. 1996, Mills et al. 1993) that quagga mussels
have broad thermal tolerance and can inhabit a wide
range of depths. Quagga mussel in Lake Ontario
were observed to depths of 85 m and at bottom
water temperatures ranging from 4 to 15°C. At
depths of 25 to 55 m, however, the quagga mussel
encountered 12 to 15°C water for only a short time
period during fall turnover (based on bottom water
temperatures collected by USGS, Oswego, NY). In
the deep waters of eastern Lake Erie where quagga
mussels dominate, near-bottom temperatures rarely
exceed 7 to 8°C during the summer and only approach 10 to 11°C during fall turnover (Schertz et
al. 1987). In contrast, Lake Ontario D. polymorpha,
which dominated in shallower water (15 m), were
exposed to maximum bottom water temperatures
ranging from 12 to 18°C from late July to fall
turnover.
The dreissenid species shift toward the quagga
mussel in both Lakes Erie and Ontario suggest that
quagga mussels can “outcompete” zebra mussels
when seston is reduced; this should be reflected in a
higher “net energy balance” (Hilbish et al. 1994) or
“scope for growth” (Warren and Davis 1967) for
quagga mussels under low food levels. When seston
is limited, quagga mussels may maintain higher ingestion and/or assimilation rates, and/or lower
metabolic rates and thus grow faster, reach larger
sizes, and maintain higher fecundity than zebra
mussels. According to Walz (1978a, c), metabolic
costs increases with size of zebra mussels and a decline in available food affects the growth and survival of large zebra mussels more than small ones.
This pattern, however, may not be true for quagga
Quagga Mussel in Lower Great Lakes
mussels and may be the reason they do better under
low food.
Ecological Considerations
Insight on the species shift to Dreissena bugensis
in the lower Great Lakes can be gained from the extensive work on sympatric species of marine mussels, some of which have been invasive species. For
example, along the western coast of Europe, where
Mytilus edulis and the larger M. galloprovincialis
co-occur, Gardner (1994) has argued that there is
“strong directional selection in favor of the M. galloprovincialis genotype” based on its greater
growth, survivorship, fecundity, strength of attachment, and resistance to parasitic infestations. Consistent with this argument, Hilbish et al. (1994)
found greater growth and feeding rates of M. galloprovincialis over M. edulis in the field. In South
Africa, M. galloprovincialis invaded and colonized
extensive intertidal regions and within 20 years became the dominant mussel, comprising about 74%
of the intertidal mussel biomass (van Erkom
Schurink and Griffiths 1990, Hockey and van
Erkom Schurink 1992). The success of this invading species, and its ability to displace native mussels, has been attributed to several factors including
rapid growth, ability to grow in dense mussel beds,
and high fecundity.
The fact that the quagga mussel attains larger
shell size than the zebra mussel suggests that the
quagga mussel possesses bioenergetic and reproductive advantages over the zebra mussel. Because
fecundity is usually directly related to body size in
iteroparous marine bivalves (Calow 1983) and in
zebra mussels (Walz 1978b, Sprung 1991, Sprung
1995), it is likely that the larger-sized quagga mussel produces more gametes per individual than do
zebra mussels. Moreover, since physiologicallystressed marine mussels produce fewer and smaller
eggs (Bayne et al. 1978), it is possible that zebra
mussel fecundity is further limited under low seston
conditions. Finally, reproduction by quagga mussels
does not appear to be constrained in the colder hypolimnetic waters of the eastern basin of Lake Erie.
Gonadal development and spawning of quagga
mussels can occur in waters 4.8°C but it is unclear
whether such activity contributes to local recruitment (Roe and MacIsaac 1997).
This study provides the first evidence of a
species shift within the dreissenid community to
quagga mussels in the lower Great Lakes. In the
eastern basin of Lake Erie, D. polymorpha is now
195
rare and, in Lake Ontario, quagga mussels dominate
in areas of lake bottom that once were mostly zebra
mussels. If these trends continue, the quagga mussel might be expected to eventually colonize the
upper Great Lakes and other waters as well. At present, it is possible only to speculate on what factors
underlie the shift to quagga mussels in Lakes Erie
and Ontario but it is clear that further work on exploitative competition, recruitment, and growth is
required to better understand the population dynamics of these two sympatric dreissenids.
ACKNOWLEDGMENTS
This study was supported by funds from New
York Sea Grant and Cornell University. We especially wish to thank Christine Mayer, Veronica
Ludyanskiy, Robert Haas (Michigan DNR), Shawn
Sitar, Kristi Sitar, vessel crews of the New York
State Department of Environmental Conservation
for Lakes Erie and Ontario, crew of the Kaho, crew
of the R/V Channel Cat, Don Einhouse, and Cliff
Schneider for their assistance in collecting mussels.
Dr. Charles McCulloch provided valuable advice
for statistical analyses. We also wish to thank two
anonymous reviewers who made valuable contributions to improving the manuscript. Contribution
177 of the Cornell Biological Field Station and contribution 1044 of the USGS, Great Lakes Science
Center.
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Submitted: 24 April 1998
Accepted: 29 November 1998
Editorial handling: Thomas F. Nalepa