Changes in the Biodiversity of Freshwater Mussels in the Canadian Waters of the Lower Great Lakes Drainage Basin Over the Past 140 Years

Home , Grand River (Ontario), Rideau River, Thames River (Ontario)

J. Great Lakes Res. 24(4):845-858 Internat. Assoc. Great Lakes Res., 1998

Janice L. Metcalfe-Smith1*, Shawn K. Staton2, Gerald L. Mackie3, and Nancy M. Lane4

1 Aquatic Ecosystem Protection Branch National Water Research Institute, P.O. Box 5050, 867 Lakeshore Road Burlington, Ontario L7R 4A6

2R.R. #1, Rosedene Road St. Ann's, Ontario LOR 1Y0

3Department of Zoology, University of Guelph Guelph, Ontario N1G 2W1

42099 Grand Ravine Drive Oakville, Ontario L6H 6B4

Abstract. Severe declines in the diversity and abundance of freshwater mussels have been documented over the past century in the United States. Although similar trends might be expected in Canada, mussels have received little attention to date. The Committee On the Status of Endangered Wildlife In Canada (COSEWIC) expanded its mandate in 1994 to include invertebrates, thus providing the impetus for assessing the health of Canada's freshwater mussel fauna. The purpose of this study was to determine if there have been changes over time in the richness and composition of freshwater mussel communities in the lower Great Lakes drainage basin, which historically supported the most diverse and unique mussel fauna in Canada. Over 4,100 occurrence records for 40 species of mussels collected from approximately 1,500 sites between 1860 and 1996 were compiled and examined together for the first time. Comparisons of historical and recent data revealed a pattern of species losses and changing community composition throughout the basin, particularly in the species-rich Lake Erie and Lake St. Clair drainages. River systems that once supported numerous species characteristic of a wide variety of habitats are now dominated by fewer siltation- and pollution-tolerant species of the Subfamily Anodontinae. A detailed examination of the data for the Grand, Thames, and Moira rivers confirmed that the same trends are occurring in widely- separated systems throughout the basin. The results of this study provide compelling evidence that the steady decline in freshwater mussel diversity that has been documented for the United States is also occurring in Canada.

INDEX WORDS: Freshwater mussels, Unionidae, biodiversity, Great Lakes, historical.

*Corresponding author. Email: [email protected]

Introduction Freshwater mussels are among the most endangered groups of animals in North America (Biggins et al. 1995). The decline of mussels was apparent by the turn of this century, and there is evidence that the process is accelerating at an alarming rate (Neves 1997). The construction of dams and creation of impoundments during the first part of the century destroyed riverine habitat for indigenous species and led to their genetic isolation. Water pollution and the relentless destruction of riparian buffer zones adjacent to watercourses are also im- plicated in the collapse of the mussel fauna. The introduction of the zebra mussel (Dreissena polymorpha) to the Great Lakes in the late 1980s (Hebert et al. 1989), and its eventual spre- ad to the major river systems of 19 states and two provinces (U.S. Geological Survey 1997) has led to catastrophic declines of native mussels in infested areas. Freshwater mussels have been protected under endangered species legislation in the United States since 1973 (Neves 1997). However, it wasn't until 1994 that the Committee On the Status of Endangered Wildlife In Canada (COSEWIC) expanded its mandate to include invertebrates. The Mollusc Working Group of the Lepidoptera and Mollusca Subcommittee of COSEWIC was formed in 1995 to develop a national list of Canadian mollusc species at risk and prepare status reports on them. Although Canada does not have federal endangered species legislation at this time, the provinces of Ontario, New Brunswick, Quebec and Manitoba have standalone Endangered Species Acts (Aniskowicz 1997) that protect endangered flora and fauna from willful destruction. The Committee On the Status of Species At Risk in Ontario (COSSARO) recently began to list aquatic species, including freshwater mussels. The Natural Heritage Information Centre (NHIC), an affiliate of The Nature Conser- vancy, was formed in 1993 to compile, maintain, and provide data on rare, threatened, and endangered species and spaces in Ontario (NHIC 1994). The NHIC currently ranks 50% of Ontario's native mussel species as either very rare, extremely rare, or known only from historical records. Now that the plight of mussels has been officially recognized by these organi- zations, it is timely to conduct a detailed assessment of the health of Canada's freshwater mussel fauna. The purpose of this study was to determine if there have been changes over time in the diversity and/or composition of freshwater mussel communities in the Canadian waters of the lower Great Lakes drainage basin. This area was chosen for study for two reasons. First, the region historically supported the most diverse and unique mussel fauna in Canada, with 40 of the 53 Canadian species occurring here. The Sydenham River, for example, is considered to be "... the richest system for Unionidae in Canada and one of the richest small river systems in North America" (Clarke 1992). Secondly, zebra mussels have virtually eliminated native mussels from Lake St. Clair (Nalepa et al. 1996), western Lake Erie (Schloesser and Nalepa 1994), and the upper St. Lawrence River (Ricciardi et al. 1996), leaving the rivers and stre- ams of the drainage basin as the last refuge for many species. This study relied upon historical occurrence records for mussel species, which were gathered over a 140-year period by nearly 200 different investigators, as its information base. As might be expected, the data were very inconsistent with respect to sampling method, sampling effort, and spatial and temporal coverage. In most cases, only the presence of a species was recorded; i.e., measures of abundance were rare. Furthermore, no details (other than sampling location) were available for a large portion of the earliest records. Historical data of this nature are extremely valuable, as they provide insights that are not obtainable any other way. However, they must be considered qualitative and cannot be subjected to statistical testing. In their overview of the use of historical information to direct aquatic habitat management in the Great Lakes, Steedman et al. (1996) cautioned that historical information is typically of low resolution, and should be used to "... specify only qualitative generalizations about past ecosystem states and processes." In the present study, generalizations are derived concerning the past and present health of the mussel fauna of the lower Great Lakes drainage basin from the available historical and recent data. Materials and Methods

Study Area The study area consisted of the Canadian waters of the lower Great Lakes (Lake St. Clair, Lake Erie, and Lake Ontario), their connecting rivers, and all watersheds draining into the lakes within the boundaries of the Province of Ontario (Fig. 1).

The Lower Great Lakes Unionid Database All available historical and recent data on the occurrences of mussel species throughout the study area were compiled into a computerized GIS-linked database. Data sources included the primary literature, natural history museums, federal, provincial, and municipal govern- ment agencies (and some American agencies), conservation authorities, Remedial Action Plans for the Great Lakes Areas of Concern, university theses, and environmental consulting firms. Mussel collections held by six natural history museums in the Great Lakes region (Canadian Museum of Nature, Ohio State University Museum of Biological Diversity, Royal Ontario Museum, University of Michigan Museum of Zoology, Rochester Museum and Sci- ence Center, and Buffalo Museum of Science) were the primary sources of information, accounting for over two-thirds of the data acquired. The database was created using Microsoft® Access (Version 7.0), and linked by means of Spansmap® (Version 1.4) to 1:250,000 digital base maps of southwestern Ontario that were provided by the Geomatics Office of Environment Canada, Burlington, Ontario. The database presently consists of over 4,100 unionid records obtained from approximately 1,500 sites between 1860 and 1996; a record is defined here as the occurrence of a given species at a given location on a given date. The database variables include data source, name of collector, collection date, name of waterway, description of sampling location, geographical coordinates, species, condition of specimens at time of collection (living or dead), and number of species collected from a given site. As very few of the data from any source had been geo- referenced, coordinates (latitude and longitude) were assigned to collection sites based on descriptions of site locations. Taxonomy was standardized to the nomenclature most recently adopted by the Freshwater Mussels Subcommittee of the American Fisheries Society Endan- gered Species Committee (Williams et al. 1993).

Limitations of the Data As noted earlier, historical data such as these contain many inconsistencies. For example, the sampling methods used and effort expended by amateurs picking up a few shells for their collections, museum curators conducting extensive surveys, and Ph.D. students sampling intensively for their thesis research are sure to differ but were seldom documented. Further- more, some geographical areas received more attention than others, and some aquatic systems were sampled more frequently or intensively in recent years, while the reverse was true for other systems. The implications of these and other discrepancies will be considered in the Discussion. For over 50% of the records, it was not known whether the specimens were alive or dead at the time of collection. Because mussel shells may persist in the environment for years after the animals themselves have died, the presence of a shell does not necessarily affirm the presence of live animals. The collections of the Royal Ontario Museum, Univer- sity of Michigan Museum of Zoology, and Buffalo Museum of Science were examined and it was found that most specimens were in excellent condition (i.e., nacre still shiny, hinge teeth not worn)— indicating that they had come from live animals or animals that had been dead only a short time. On this basis, all records in the database were treated as occurrences of live species.

Data Analysis To determine if species richness and community composition changed over time throughout the study area, the data were divided into two time periods. The "historical" time period was defined as before 1960 and the "recent" time period as after 1960. The choice of the year 1960 as the demarcation was somewhat arbitrary, but was influenced by the fact that similar numbers of sites were sampled in both periods: 679 sites between 1860 and 1960, and 797 sites between 1961 and 1996. Changes in community structure over time were examined in more detail for three representative rivers. The data for each river were divided into three time periods having relatively even numbers of sampling sites in each period. Of necessity, the time periods differed among rivers (details to follow). The degree of similarity in community composition among time periods was calculated using the overlap index "Cλ" of Horn (1966):

where xi and yi are the proportions of all records in time periods x and y, respectively, represented by species i, and s is the total number of species reported in all time periods. An index value of 1.0 would indicate complete overlap, i.e., identical proportions of records accounted for by each species in each time period, whereas an index value of 0 would indicate no overlap. The index was intended for the measurement of overlap "... in comparative studies of diet, seasonal patterns of abundance, faunal lists, or similar data" (Horn 1966). The analysis of trends over shorter periods of time was precluded by the limitations of historical data that were mentioned above.

Results

Species Richness A total of 40 species of mussels belonging to three subfamilies of the Family Unionidae have been reported from the study area (Table 1). The Family Margaritiferidae is not represented in the basin. All species except Alasmidonta undulata have been found in Lake St. Clair and Lake Erie and/or their drainage basins, whereas only 22 species have been found in Lake Ontario and/or its drainage basin. Seventeen of the 40 species are known in Canada only from the Lake St. Clair and Lake Erie drainages, and an additional 11 species are also found in the Lake Ontario drainage. Six of these 28 species are exceptions, as they are also native to the Red-Assiniboine drainage of southern Manitoba (Clarke 1981). One species (Obovaria olivaria) is disjunct in distribution, being found in the Lake St. Clair and Lake Erie drainages and in the St. Lawrence River, but not in Lake Ontario. The remaining 11 species are more widely distributed throughout eastern Canada, western Canada, or across the country (Clarke 1981). Changes in species richness over time were estimated for ten river systems and/or lakes in the study area for which we deemed there to be adequate data. From west to east in Figure 1, these systems are as follows: Lake St. Clair and the Detroit River (105 records available), Sydenham River (446), Thames River (400), Lake Erie (1,145), Grand River (968), Niagara and Welland Rivers (133), Lake Ontario (241), Trent-Severn Waterway (62), Moira River (213), and Rideau River (138). For each watershed or lake, the total number of species ever reported, the number of species found in the historical vs. the recent time period, and the number of species "lost" or "gained" over time are presented in Table 2. It should be noted that species described as "lost" could still occur, but may have escaped detection due to declines in numbers of individuals, populations, and/or the vagaries of sampling. Similarly, species described as "gained" could have occurred previously, but may have recently become more common or expanded their ranges such that they are now being encountered. Compa- risons of the total numbers of species ever recorded from each system clearly show that the Lake St. Clair and Lake Erie drainages have the capacity to support a much more diverse mussel fauna than does the Lake Ontario drainage.

Comparisons of the historical and recent species richness values in Table 2 suggest that the diversity of mussels has declined over time in most of the systems examined. This is supported by the fact that far more species have been lost from, than gained in, these rivers and lakes. Community Composition Community composition, which in this context refers to the proportion of the mussel community accounted for by each species, is a more sensitive indicator of biological integrity than is species richness. Community composition indicates not only which species are present, but how common or rare they are relative to one another. Data for the entire study area were examined for changes in community composition over time, again using 1960 as the date separating historical from recent times. The proportions of records accounted for by each species in each time period are compared in Figure 2, where species are arranged in order from the most to the least common in the earlier time period such that any changes in dominance will be clearly evident.

The most notable change in the composition of the mussel community over time was a shift in the order of dominance of the two most common species, with Lampsilis siliquoidea accounting for the greatest proportion of records prior to 1960 and Pyganodon grandis consti- tuting the most records after 1960. Lampsilis siliquoidea was found at 30% of the sites in both time periods (data not shown); however, P. grandis was found at 16% of the sites before 1960 and 45% after 1960, indicating that it has become much more common in recent years. Of the 40 species examined, 18 increased in prevalence over time, 21 decreased, and one remained unchanged. The three species experiencing the greatest increases over time were Strophitus undulatus (64% increase in the proportion of records represented by this species in 1961-1996 vs. 1860-1960), Alasmidonta marginata (53%), and Lasmigona complanata complanata (97%). Strophitus undulatus ranked 3rd in dominance after 1960, mo- ving up from 19th place before 1960; A. marginata rose from 23rd to 18th place; and L. с. complanata moved up from 40th to 20th place. The latter species was found at 58 sites after 1960 vs. one site prior to 1960, suggesting that it may be expanding its range. These three species, as well as P. grandis, are members of the Subfamily Anodontinae. Most of the species exhibiting a decrease in dominance over time (13 of 21) are members of the Subfamily Lampsilinae.

Based on total records, the Anodontinae has increased in dominance over time, mainly at the expense of the Lampsilinae (Fig. 3). The proportion of the community accounted for by the Ambleminae has remained relatively constant. Increases in dominance were observed for 50% of the Anodontinae species, 75% of the Ambleminae, and only 30% of the Lampsilinae. In contrast, 65% of species belonging to the Lampsilinae showed declines, as compared with 50% of the Anodontinae and only 25% of the Ambleminae.

Representative River Systems Temporal changes in the richness and composition of the mussel community were examined in detail for three representative river systems: the Grand River (representing the Lake Erie drainage basin), the Thames River (Lake St. Clair drainage basin), and the Moira River (La- ke Ontario drainage basin). It should be noted that several major collections, including Kidd's (1973) 1970-1972 data on the Grand River (349 records from 76 sites), Morris and Di Maio's (1997) and Morris's (1996) 1994-1995 data on the Thames River (193 records from 46 sites), and H.R. Herrington's 1938-1939 data (courtesy of the Royal Ontario Museum and University of Michigan Museum of Zoology) on the Moira River (87 records from 40 sites) dominated the information bases on these systems.

The Grand River—Representing the Lake Erie Drainage Basin The Grand River is a large, well-studied watershed with over 950 mussel records from 1885 to 1996. To assess changes in the community over time, the data were divided into the time periods 1885-1958, 1963-1983 and 1988-1996. The numbers of species reported in each time period were tallied, and the degree of similarity in community composition between time periods was calculated using Horn's (1966) overlap index "Cλ" (Table 3). Species losses were not evident in the Grand River until the 1988-1996 time period, but the most significant change in community composition occurred between the 1885-1958 and 1963-1983 time periods. This suggests that changes in species composition may precede, and thus be predic- tive of, future species losses. If the pre-1960 community composition is considered to be the ideal, then the present community deviates considerably from this objective with an overlap of only Cλ = 0.64.

As illustrated in Figure 4, the structure of the mussel community of the Grand River has changed considerably over time—from a community with a large number of well-represented species up to about 1960, to a community becoming increasingly dominated by fewer and fewer species until eventually some species have been lost. According to Table 4, seven species accounted for 50% of the records prior to 1960, as compared with six species during 1963 to 1983, and only four species in the most recent time period. Also, the community has become dominated by Anodontinae: only three of the eight most common species prior to 1960 belonged to this subfamily, as compared with six species in each of the latter two time periods. Pyganodon grandis, S. undulatus, and Anodontoides ferussacianus ranked 8th, 18th, and 24th in dominance from 1885-1958; 1st, 2nd, and 10th between 1963 and 1983; and 1st, 6th, and 4th between 1988 and 1996.

The Thames River—Representing the Lake St. Clair Drainage Basin The Thames River has received somewhat less attention from malacologists over the years than has the Grand River. Data on the Thames River, which were separated into the time periods 1894-1934, 1935-1988, and 1994-1995, show that the mussel community of this river has experienced a continuous decline in species richness over time (Table 5). The most dra- matic change in community composition occurred after 1988 (Cλ = 0.64), whereas communities in the first two time periods were relatively similar (Сλ = 0.78). Eight species accounted for 50% of the records during the first two time periods, as compared with five species in the latest time period (Table 6). The mussel community of the Thames River, like that of the Grand River, has become increasingly dominated by the Anodontinae over the years. Four of the eight most common species in 1894-1934 and 1935-1988 were members of this subfamily, accounting for 30 to 40% of the total records. In 1994-1995, five of the eight most common species were Anodontinae, and these constituted 60% of the total records.

The Moira River—Representing the Lake Ontario Drainage Basin The Moira River was surveyed for unionids in the fall of 1996. This appears to be the first time in 30 years that data on the mussel community of this river had been collected. The Lower Great Lakes Unionid Database was used to select sites that had been previously sampled, to ensure that the current data would be compatible with the historical data. Although mussel diversity is somewhat limited in rivers of the Lake Ontario drainage, abundance was high throughout the Moira River in 1996. At one site on the lower river, for example, 685 individuals were collected with a sampling effort of 1.5 person-hours (unpublished data). Up to eight species were found at several sites, which is equivalent to the highest diversity previously reported for a site on this river system. It is evident from the overlap index values in Table 7 that community composition in the Moira River has changed very little between 1928 and the present. Fewer species were found in 1996; however, this may not be significant as only eight sites were sampled, including two sites by Schueler (1996). The mussel community of the Moira River is much "simpler" than that of the Grand River or Thames River, with only three or four species accounting for 60% of the records in all time periods (Table 8). The most remarkable change in composition was an increase in the dominance of P. grandis from 8th place prior to 1958 to 3rd place after 1960, where it currently remains. A general increase in prevalence over time was observed for the Anodontinae, although the trend was not as striking as for the Grand and Thames Rivers. Members of this subfamily constituted 18% (4 species), 29% (6 species), and 31% (4 species) of the records in 1928-1958, 1960-1968, and 1996, respectively.

It may appear from Table 8 that Lampsilis radiata radiata has been lost from the Moira Ri- ver, but this is misleading. Many malacologists consider L. r. radiata and L. siliquoidea to be subspecies, whereas others recognize them as full species (Strayer and Jirka 1997). The ranges of these species overlap, and intergrades (possibly hybrids) are known to occur in the La- ke Ontario and St. Lawrence River watersheds (Clarke 1981) and in parts of New York (Strayer and Jirka 1997). Here, all specimens found in the Moira River in 1996 were considered to be L. siliquoidea, based on the presence of females of this species, which are mor- phologically distinct, at most sites. This does not necessarily mean that L. r. radiata is absent from the watershed. Discussion North America is known to be the world center of diversity for freshwater mussels. The area of most intense diversity is the upper drainage basin of the Mississippi and Ohio rivers, with 66 species occurring in the state of Michigan (Barr 1996) and 80 species recorded from both Kentucky (Barr 1996) and Ohio (G.T. Watters, Ohio State University, personal communication, March 1998). Although the lower Great Lakes drainage basin is at the northern periphery of this center, with 40 species of mussels it is by far the richest region in Canada. The distribution patterns of unionids in southwestern Ontario are a result of their post-glacial dispersal routes, which followed those of their host fish. The majority of mussel species (35) re-invaded the Lake St. Clair and Lake Erie drainages from the species-rich Ohio-Mississippi system (Clarke 1981, Barr 1996). Five others (A. undulata, Elliptio complanata, L. r. radiata, Obovaria olivaria, and Pyganodon cataracta) are believed to have originated in the Alta- ntic Refugium, entering the region from the northeast via the Champlain Sea/St. Lawrence River (Barr 1996). These dispersal patterns may partly explain the greater number of species found in the Lake St. Clair and Lake Erie drainages than in the Lake Ontario drainage. There are several possible reasons why many of the mussel species of southwestern origin have not radiated into Lake Ontario and beyond. Either they or their host fish may be at the limit of their thermal tolerance. According to Mandrak and Crossman (1992), most of the 23 fish species restricted to southwestern Ontario originated from the Mississippian Refugium and are probably at their thermal tolerance limits. Further eastward dispersal of these fish species along the north shore of Lake Ontario may also be limited by the absence of suitable habitat in the lake and adjacent watersheds, and/or by the counterclockwise current and wave action in the lake (Simons 1980). Although Lake Ontario has only a slightly lower total fish diversity than Lake Erie (85 vs. 91 native species; Bailey and Smith 1981), warm water habitats in Lake Ontario are extremely limited and may not support sufficient densities of host fish to sustain populations of some mussel species. Water chemistry may also be a de- terminant of mussel distributions. As calcium is an essential element for building shells, Ste- wart (1992) surmised that high concentrations of dissolved calcium in the waters of the Thames and Grand rivers (tributaries to Lake St. Clair and Lake Erie, respectively) account for the rich mussel fauna supported by these systems. A comparison of the number of species found in historical (before 1960) and recent (after 1960) times in ten major watersheds and lakes in the study area revealed a general decline in the diversity of mussel communities throughout the lower Great Lakes drainage basin. In order for such comparisons to be valid, however, sampling effort would have to have been similar in both time periods. Unfortunately, sampling effort is believed to have varied greatly (see Materials and Methods). A measure of sampling effort was often available for the acad- emic surveys, most of which took place after 1960. However, no information of this nature was available for the museum specimens, which constituted the major portion of the historical data. There is some indication that sampling effort was greater after 1960 than before: (1) there were twice as many records after 1960 as before 1960 (2,754 vs. 1,370), even though only 17% more sites were sampled; and (2) the percentage of sites where more than one species was collected was higher after 1960 (60%) than before 1960 (40%). If sampling effort was in fact greater after 1960, then species richness values for the historical time period may have been underestimated relative to the recent time period. It follows that the species losses observed are real and indeed may be underestimates. There is evidence that species richness in the Grand, Thames, and Sydenham rivers, which are the largest and richest river systems within the study area, may have declined precipi- tously in recent years. Mackie (1996) surveyed 70 sites on the Grand River in 1995 and reported a total of 22 species (18 live and 4 dead); Morris (1996) recorded identical numbers for the Thames River in the same year, based on a survey of 30 sites; and Clarke (1992) collected 25 species (23 live and 2 dead) from 16 sites on the Sydenham River in 1991. When total numbers of species (both live and dead) found in the 1990s are compared with the corresponding total, historical, and recent species richness values in Table 2, these findings represent species losses of 29 to 37% from the Grand River, 15 to 31% from the Tha- mes River and 17 to 24% from the Sydenham River. Even by these conservative estimates, i.e., assuming that dead shells represent viable populations of a given species, one-sixth to one-third of the mussel fauna of these rivers has now been lost. As these watersheds represent the last refugia for several rare species (see distribution maps in Metcalfe-Smith et al. 1997), the results warn of an alarming trend toward increasing numbers of species extirpa- tions throughout the basin. The composition of the mussel community in the lower Great Lakes drainage basin appears to have changed substantially over time. Species of the Lampsilinae, which dominated the community during the historical time period, are now being overtaken by species of the Ano- dontinae such as P. grandis, S. undulatus, A. marginata, and L. с. сотplanata. Members of the Anodontinae are generally thin-shelled mussels that are commonly referred to as "floater" mussels. They can survive in soft, silty substrates and are usually considered to be pollution-tolerant. Morris and Corkum (1996) reported that rivers in southwestern Ontario with narrow, grassy riparian zones were characterized by P. grandis and S. undulatus, whereas rivers with forested riparian zones were characterized by species of Lampsilinae and Ambleminae. They concluded that "... increasingly agricultural activity is resulting in a shift towards dominance by a single common species in rivers of open riparian zones, with P. grandis representing over 60% of individuals in these rivers." In a related study, Morris (1994) found that L. c. complanata also increased significantly in abundance as the land- scape shifted from forest-dominated to predominantly agricultural. On the other hand, Hogg- arth et al. (1995) demonstrated that agricultural activities have seriously threatened the endangered purple catspaw, Epioblasma obliquata obliquata (Subf. Lampsilinae), in Ohio. Mackie (1996) extensively surveyed the Grand River watershed for unionids in 1995, and compared diversity and abundance with the results of earlier surveys on that system. He determined that the subfamily with the greatest percentage of species at risk in this watershed was the Lampsilinae (88%), followed by the Ambleminae (60%), with the Anodontinae being the most resilient group (only 22%). These findings support the results of the present study, which demonstrated an increase over time in the occurrence of Anodontinae species relative to Lampsilinae species for the lower Great Lakes drainage basin as a whole. Sampling effort has been consistently greater over time in the Lake St. Clair and Lake Erie drainages than in the Lake Ontario drainage, and this has implications for interpreting temporal trends for certain species. Differences in sampling effort between these two areas were more pronounced in recent years: 42% of the 679 sites sampled prior to 1960 were in the Lake Ontario drainage vs. just 23% of the 797 sites sampled after 1960. Viewed another way, about two-thirds as many sites were sampled in the Lake Ontario drainage after as before 1960 (182 vs. 285), while the reverse was true for the Lake St. Clair and Lake Erie drainages (615 vs. 394). Because of this sampling artifact, an apparent decrease in prevalence over time would be expected for those species with ranges predominantly in the Lake Ontario drainage, namely A. undulata, E. complanata, L. r. radiata, and P. cataracta. In fact, such a decrease was observed (see Fig. 2). However, the fact that declines were observed for several species known only from the Lake St. Clair and Lake Erie drainages (e.g., Epioblas- ma torulosa rangiana, Epioblasma triquetra, Pleurobema coccineum, Simpsonaias ambigua, Truncilla donaciformis, and Villosa fabalis), even though more time was spent searching for them in recent years, suggests that these mussels are in serious trouble. A more detailed examination of changes over time in the occurrences of individual species is presented in Metcalfe-Smith et al. (in press). The trends toward species losses and increasing domination of the mussel community by members of the Anodontinae over time, which were observed in the data for the entire study area, were also observed separately for the Grand, Thames, and Sydenham rivers. In the early 1970s, Kidd (1973) reported major increases in the numbers of P. grandis, S. undulatus, and Lasmigona costata in the Grand River. Most recently, Mackie (1996) found this river to be dominated by floater species, including Lasmigona compressa, L. costata, P. grandis, and S. undulatus. Mackie (1996) attributed both temporal and spatial declines in diversity to a variety of anthropogenic impacts, including agricultural runoff, roadway crossings, cattle crossings, industrial discharges and storm sewer discharges. Kidd (1973) blamed dam construction for changes in mussel distributions, especially in the lower reaches of the river, noting that dams may limit the migration of the mussels' host fish. According to Watters (1996), dams as small as 1 m high may prevent the upstream movement of some fishes, and hence limit the distributions of the unionids that use these fish as hosts. Over 95% of the Grand River watershed is agricultural, with less than 5% forest cover (Daw-son 1963). The Thames River also primarily drains agricultural land (Messih 1976). An increase in the pre- dominance of Anodontinae species in the Moira River is more difficult to explain, as two- thirds of the watershed is on the Canadian Shield and agricultural activity is limited (Terry Sprague, Moira River Conservation Authority, personal communication, March 1997). Con- tamination of the river with metals due to a long history of mining and smelting activities in the basin (Mudroch and Capobianco 1980) would be the main source of stress to the aquatic community. A recent study suggests that floater mussels may be less sensitive to metals than are other mussels. Jacobson et al. (1997) determined the toxicity of copper to released glochidia of five species of unionids, and found the common floater (P. grandis) to be the most resistant. The 24-hour LC50 for copper was up to an order of magnitude greater for P. grandis (347 (g Cu/L) than for the wavy-rayed lampmussel, Lampsilis fasciola (26 (g Cu/L). The relative stability of the mussel community of the Moira River may be due to the absence from the Lake Ontario drainage of many of the more ecologically fragile mussel species.

Conclusion This study examined changes in the diversity and composition of freshwater mussel communities throughout the Canadian waters of the lower Great Lakes drainage basin during the period 1860 to 1996. For this purpose, thousands of collection records obtained from numerous sources were compiled into a database (The Lower Great Lakes Unionid Database) and examined together for the first time. Analysis of these data revealed a pattern of species losses and changing community composition throughout the basin, particularly in the for- merly species-rich Lake Erie and Lake St. Clair drainages. River systems that once supported numerous species characteristic of a wide variety of habitats are now dominated by fewer siltation- and pollution-tolerant species of the Anodontinae. The fact that trends observed on a basin-wide basis were also observed on an individual basis for several representative river systems, despite the inherent variability of historical data, serves to emphasize the signifi- cance of these findings. The results of this study provide compelling evidence that the steady decline in freshwater mussel diversity that has been documented for the United States (Neves 1997), is also occurring in Canada.

Acknowledgments

Many agencies and individuals contributed data, contacts, advice and, in some cases, collections of specimens to this project. The authors would particularly like to thank the following museum curators for allowing us access to their data and/or specimens: J.B. Burch, Museum of Zoology, University of Michigan; D.L. Calder, Curator in Charge, Department of Invertebrate Zoology, Royal Ontario Museum; J.- M. Gagnon, Chief, Invertebrate Collection Management, Canadian Museum of Natu- re; W.K. Gall, Curator of Entomology, Division of Invertebrate Zoology, Buffalo Mu- seum of Science; G.C. McIntosh, Curator of Geology, Rochester Museum and Scien- ce Center; and D.H. Stansbery, Division of Bivalve Mollusks, Ohio State University Museum of Biological Diversity. F.W. Schueler, Research Associate, Canadian Muse- um of Nature, D. Coulson, H. Schraeder and A. Timmerman, Ontario Ministry of Natural Resources, and the late W.G. Stewart were especially helpful in contributing data and/or specimens. Ms. M. Villella, NWRI, assisted with the task of assigning geographical positions to the collection sites. We are grateful to the Great Lakes 2000 Program and the Ecological Monitoring Coordinating Office, Environmental Conservation Service, Environment Canada, for their financial support.