Gerald L. Mackie

Biology of the exotic zebra mussel, Dreissena polymorpha, in relation to native bivalves and its potential impact in Lake St. Clair

The zebra mussel, Dreissena polymorpha, is a new exotic species that was introduced into the Great Lakes as early as the fall of 1985. It differs markedly from native species of bivalves in its: (i) shell form; (ii) mode of life; (iii) reproductive potential; (iv) larval life cycle; (v) population dynamics; (vi) distribution, (vii) dispersal mechanisms; (viii) physiology; (ix) potential impact on the ecosystem; and (x) impact on society and the economy. In body form, it has an anterior umbone, a flat ventral surface with permanent aperature for the byssal apparatus and a shape that together make the animal well adapted for life on a hard surface. The shell has a zebra-stripe pattern, a heteromyarian muscle condition and lacks hinge teeth which make it easily identifiable from native bivalves. The zebra mussel is strongly byssate and has an epifaunal mode of life not seen in native bivalves. The species is dioecious and has external fertilization, the eggs developing into pelagic veligers which remains planktonic for approximately 4 weeks. Gametogenesis begins in late winter to early spring, veligers appear in the water column in late May to early June and disappear in mid to late October in Lake St. Clair. Adults live for about 2 years and have very rapid growth rates. Maximum shell lengths average 2.3 to 2.5 cm. Standing crops as high as 200 000 m-2 are present in the 1-m depths of the Ontario shores. Infestations may be interfering with the normal metabolism of native unionid clams and there is potential of the unionid clam populations being reduced or even eliminated from Lake St. Clair.

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

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 Canadas 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.

Effect of sampling effort on the efficiency of the timed search method for sampling freshwater mussel communities

Qualitative sampling is recommended over quantitative sampling when the objective of a mussel survey is to find rare or endangered species. Although advice on how to conduct quadrat surveys is available, there is little information on the influence of sampling effort on the effectiveness of timed searches. We conducted timed searches for mussels at 28 sites on 5 rivers in southwestern Ontario using 4.5 person-hours (p-h) of sampling effort/site. The survey period was divided into 3 equal time intervals, and the number of species and individual mussels collected within each interval were recorded. Increasing the sampling effort dramatically improved the detection of rare species. Sampling for 1.5 p-h, which is the level of effort commonly used in timed search surveys, would have resulted in <½ of the rare species being found and would have underestimated species richness by an average of 37%. Seventy percent of encounters with 3 federally endangered species took place during the 2nd and 3rd time intervals. Increasing the sampling effort from 1.5 to 4.5 p-h/site resulted in all but 2 of the 28 species being found at more sites. Even 4.5 p-h of effort may be insufficient for detecting all species at all sites or estimating species richness for a river. Researchers who rely on qualitative surveys to determine the presence of rare and endangered mussel species should consider the amount of time they spend at their sites because sampling effort can significantly affect their ability to detect these species.

Total cadmium concentrations in the water and littoral sediments of Central Ontario Lakes

Lakes within 20 km of Sudbury, Ontario, have significantly higher Cd concentrations in surface waters (geometric mean 122 ng L−1; n = 7) than lakes elsewhere in central Ontario (10.8 ng L−1; n = 57). Cadmium concentrations in water from lakes beyond the Sudbury halo were negatively correlated (r = 0.797; p < 0.001) with pH. A weak correlation between fluoride and Cd concentrations leads to speculation that some Cd may be mobilized from watersheds with Al. Cadmium concentrations in littoral sediments are not elevated near Sudbury. The geometric mean Cd concentration of littoral sediments in central Ontario lakes is 0.08 mg Cd kg−1 dry mass (n = 75). Cadmium concentrations in littoral sediments are strongly correlated with sediment loss on ignition (r = 0.860; p < 0.001). After correction for differences in organic content, littoral sediments are less enriched with Cd than profundal sediments, as reported in the literature. The difference between littoral and profundal sediments, and the sensitivity of Cd concentrations in water to pH, may be due to the importance of Cd binding by Fe/Mn hydrous oxides in the profundal zone, while organic matter binds Cd in the littoral zone. The lack of sensitivity of Cd concentrations in littoral sediments to acidification may be due to the incorporation of much of the Cd in those sediments into organic particulates.

A Refuge for Native Freshwater Mussels (Bivalvia: Unionidae) from Impacts of the Exotic Zebra Mussel (Dreissena polymorpha) in Lake St. Clair

The introduction and spread of the exotic zebra mussel (Dreissena polymorpha) throughout the Great Lakes has decimated native unionid populations. However, significant communities have continued to survive in several nearshore areas of Lake Erie. This study documents the discovery of a “refuge” site for unionids in Lake St. Clair. Ninety-five sites in various areas around the lake were surveyed between 1998 and 2001, and 2,356 live unionids of 22 species were found alive at 33 of these sites. Almost all sites (31) were in shallow (mainly < 1 m) waters of the St. Clair delta, in habitats similar to refugia in Lake Erie, i.e., nearshore areas with firm sandy substrates and marshy bays with soft, muddy sediments. Species richness ranged from 1 to 12 species per site, and relative abundance ranged from 2 to 302 unionids per person-hour of sampling effort. Densities at nine sites ranged from 0.03 to 0.07 per m2. Five species considered to be at risk were found alive. Infestation rates at sites near the St. Clair delta ranged from 0 to 286 zebra mussels per unionid, which is slightly higher than rates at other known refuge sites. The community is now dominated by thick-shelled species such as Fusconaia flava and Lampsilis cardium, which are known to be least susceptible to zebra mussels. Further studies are needed to determine if unionid populations in the delta are stable, and to understand the mechanisms responsible for unionid survival at this and other refugia. Such information could be used to predict the locations of other natural sanctuaries and to guide their management for the preservation of the Great Lakes unionid fauna.

Sensitivity of the glochidia (larvae) of freshwater mussels to copper : Assessing the effect of water hardness and dissolved organic carbon on the sensitivity of endangered species

The assessment of the potential impact of waterborne contaminants on imperilled freshwater mussels is needed. Acute copper toxicity was assessed in a standardized soft water (hardness 40-48 mg CaCO(3)equivalents L(-1)) using the larvae (glochidia) from three common and six (Canadian) endangered mussel species. The resulting 24h EC50s ranged from 7 to 36 microg Cu L(-1), with the EC50s of two endangered species <10 microg Cu L(-1). Acute copper sensitivity was also determined in Ptychobranchus fasciolaris, a species that employs conglutinates (packets of glochidia) in its reproductive strategy. Conglutinates were found to provide significant protection from acute copper exposure as the EC50 of the encased glochidia was more than four-fold higher than freed glochidia (72.6 microg Cu L(-1) vs. 16.3 microg Cu L(-1)). The glochidia from two endangered species, Epioblasma triquetra and Lampsilis fasciola, were used to examine the influence of water hardness and dissolved organic carbon (DOC) on copper sensitivity. Exposures in moderately-hard water (165 mg CaCO(3) L(-1)) demonstrated that an increase in water hardness resulted in a significant reduction in copper sensitivity. For example, in L. fasciola the 24 h EC50s were 17.6 (14.2-22.6) microg Cu L(-1) and 50.4 (43.5-60.0) microg Cu L(-1) in soft water and moderately-hard water, respectively. The addition of DOC (as Aldrich Humic Acid) also resulted in a significant decrease in Cu sensitivity, such that a 10-fold increase in the EC50 of E. triquetra was observed when the reconstituted soft water was augmented with 1.6 mg DOC L(-1). To determine if current water quality regulations for copper would protect all glochidia, the USEPAs Biotic Ligand Model (BLM) was used to derive water quality criteria for these exposures. While BLM-derived criteria for the soft water exposures indicate that protection would be marginal for the sensitive endangered species, the criteria derived for the DOC exposures suggest that the natural complexity of most natural waters in Southern Ontario (Canada) will provide glochidia with protection from acute copper exposure.

Seasonal patterns in metal levels of the net plankton of three Canadian shield lakes

Abstract Seasonal changes in the levels of Ca, Sr, Ba, Mn, Fe, Al, Mg, Zn and Cd in the net plankton (> 250 μm) of three Canadian Shield lakes are described, and major temporal, multielemental patterns are extracted using non-metric multidimensional scaling (NMDS). Changes in Ca, Sr, Ba and Mn produced the strongest temporal pattern summarized in the first NMDS axis. Its scores were correlated with zooplankton taxonomic composition, not with water quality data, suggesting that the pattern was attributable to seasonal succession of zooplankton species. This suggestion was verified by the good match of observed seasonal changes in net plankton Ca and Mn levels with levels predicted using the Ca and Mn concentrations of individual zooplankton species measured on a single occasion, and temporal changes in their proportional contribution to total net plankton biomass. Seasonal variability of seven of the nine metals, all but Fe and Mn, was greatest in Plastic Lake, the most acidic of the lakes. A comparison of net rates of accumulation of Zn and Cd in the sediments of the lakes with estimates of metal flux through the zooplankton indicated that zooplankton may be quantitatively important in the downward transport of Cd in many Canadian Shield lakes.

Changes Over Time in the Diversity and Distribution of Freshwater Mussels (Unionidae) in the Grand River, Southwestern Ontario

Abstract The Grand River, a major tributary to Lake Erie in southwestern Ontario, historically supported a diverse and abundant freshwater mussel fauna, with 34 species recorded from the system since 1885. A recent study suggested that the number of species inhabiting the Grand River has declined over time. The present study provides a detailed assessment of changes over time in the diversity and distribution of mussels throughout the Grand River, by comparing the results of surveys conducted at 94 sites in 1995 and 1997–98 with those from a survey conducted 25 years earlier and with the historical data. Timed searches were conducted in both 1995 and 1997–98, using sampling efforts of 1.5 and 4.5 person-hours, respectively. Only 17 species had been found alive in 1970–72, probably because of the impacts of sewage from a rapidly-growing human population. At that time, only six species occurred in the lower reaches of the main stem. Mussel populations have since rebounded, with 25 species found alive throughout the system in 1995/1997–98, including 21 in the previously impoverished lower reaches. This recovery is attributed to significant improvements in water quality over the past 25 years. The recent addition of fishways to some of the dams and weirs on the river should improve the reproductive success of mussels, by eliminating barriers to the movement of host fishes. Although environmental conditions in the Grand River appear more favorable now for mussels than they have in decades, there is concern that the growing pressures of urbanization and agriculture may slow, stop, or even reverse these hard-won gains.

Effects of salinity on the condition and survival of zebra mussels (Dereissena polymorpha)

The objectives of this study were to determine the tolerance of various life stages of zebra mussels to salinity; determine the extent to which acclimation events in estuarine systems, affect tolerance of zebra mussels; and determine the effects of salinity on health or condition of adult zebra mussels. At high temperatures (18–20°C), the condition of zebra mussels is reduced at salinities above 1%.. However, at lower temperatures (3–12°C), the optimum salinity for zebra mussels is 2–4%.. The incipient lethal salinity of post-veligers is near 2%., of larger adults (5–15 mm) between 2%. and 4%., and of veliger larvae near 4.5%.. Zebra mussels are able to acclimate to slowly changing salinities (i.e., 1%. d−1) such that the time to 50% mortality of a population should be greater than 1 yr at temperatures near 20°C and salinites up to 8%.

Pliocene and Pleistocene geologic and climatic evolution in the San Luis Valley of south-central Colorado

Rogers, K.L., Larson, E.E., Smith, G., Katzman, D., Smith, G.R., Cerling, T., Wang, Y., Baker, R.G., Lohmann, K.C., Repenning, C.A., Patterson P., and Mackie, G., 1992. Pliocene and Pleistocene geologic and climatic evolution in the San Luis Valley of south-central Colorado. Palaeogeogr., Palaeoclimatol., Palaeoecol., 94:55 86. Sediments of the Alamosa Formation spanning the upper part of the Gauss and most of the Matuyama Chrons were recovered by coring in the high (2300 m) San Luis Valley of south-central Colorado. The study site is located at the northern end of the Rio Grande rift. Lithologic changes in the core sediments provide evidence of events leading to integration of the San Luis drainage basin into the Rio Grande. The section, which includes the Huckleberry Ridge Ash (2.02 Ma) and spans the entire Matuyama Chron, contains pollen, and invertebrate and vertebrate fossils. Stable isotope analyses of inorganic and biogenic carbonate taken over most of the core indicate substantially warmer temperatures than occur today in the San Luis Valley. At the end of the Olduvai Subchron, summer precipitation decreased, summer pan evaporation increased, and temperatures increased slightly compared to the earlier climate represented in the core. By the end of the Jaramillo Subchron, however, cold/wet and warm/dry cycles become evident and continue into the cold/wet regime associated with the deep-sea oxygenisotope Stage 22 glaciation previously determined from outcrops at the same locality. Correspondence between the Hansen Bluff climatic record and the deep-sea oxygen- isotope record (oxygen-isotope stages from about 110-18) is apparent, indicating that climate at Hansen Bluff was responding to global climatic changes.