Changes in the Dreissenid Community in the Lower Great Lakes with Emphasis on Southern Lake Ontario

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 dreisenid; this trend parallels that observed for dreissenid populations in the Dneiper River basin in the Ukraine.     

The Phytoplankton Zooplankton Link in the Lake Ontario Food Web

Monitoring in Lake Ontario in 1970 and 1982 demonstrated that the zooplankton community was dominated by microzooplankton, which suggested a longer, perhaps inefficient food chain. In this study, annual monitoring of the offshore region of Lake Ontario between 1986 and 1992 was used to determine if microzooplankton were still dominant despite recent changes in nutrient loading and species introductions. Microzooplankton accounted for 49.7% of the total summer zooplankton biomass while small edible phytoplankton accounted for 67.0% of the biomass during the summer. By direct in situ measurement using a Haney grazing chamber, rather than size grazing relationships, the relative impact of micro- and mesozooplankton grazers on phytoplankton production during the summer of 1995 was evaluated. Microzooplankton filtration rates (%/d) for 1995 were significantly higher than mesozooplankton filtration rates. Zooplankton consumed only 17.5% /d of the primary production with microzooplankton grazing representing 69.8% to 93.2% of this amount. Microzooplankton are clearly still dominant and their consumption of primary production in Lake Ontario is low. The major pathway of energy transfer can not be through the classical phytoplankton > large zooplankton > planktivore > piscivore food chain but rather through the phytoplankton > microzooplankton and presumably predacious zooplankton and fish. The longer food chain is a result of the introduction of a size-selective planktivore, the alewive, which has decreased the length and presumably lowered the consumption rate of the entire zooplankton community. This structural impact, a longer food chain, theoretically creates a higher factor of biomagnification of organic chemicals for top-level predators along with lower rates of energy transfer within the food web and suggests lower fish production than in a shorter food web.     

Dynamics of Lake Michigan Phytoplankton: the Deep Chlorophyll Layer

The dynamics of the Lake Michigan deep chlorophyll layer (DCL) were studied from the period of late spring isothermal mixing (May) through mid-stratification (July-August) in 1982–1984. After the onset of thermal stratification, the DCL developed in the 15–30 m region and deepened to 25–50 m in July and 40–70 m in August. Chlorophyll and phytoplankton carbon concentrations in the DCL averaged, respectively, 1.80X and 1.34X epilimnetic concentrations during early stratification (June). Those factors increased to 5.70X and 2.60X during mid-stratification. Although phytoplankton carbon concentrations within the DCL changed on average only 31 % from May through July-August, phytoplankton species composition exhibited pronounced shifts. Measured phytoplankton growth, sedimentation, and zooplankton grazing rates suggest DCL formation was attributable to in situ growth and to a lesser extent to sedimentation and shade adaptation. By July, sedimentation resulted in a net loss from the DCL. With the deepening of the DCL during mid-stratification, the importance of in situ growth decreased while the importance shade adaptation increased. In situ growth was only important in the upper part of the DCL. Zooplankton grazing increased during mid-stratification and was at least partially responsible for phytoplankton concentrations decreases in the 20–50 m region.     

Univariate Step-trend and Multivariate Assessments of the Apparent Effects of P Loading Reductions and Zebra Mussels on the Phytoplankton of the Bay of Quinte,Lake Ontario

The purpose of this work was to identify the apparent effects on the phytoplankton of the Bay of Quinte (northeastern Lake Ontario) of two environmental alterations—a point-source phosphorus loading reduction in the late 1970s and the establishment of invading zebra mussels, Dreissena spp., about 1995. After the P loading reduction, there were significant declines in biovolume of total phytoplankton (−51%), Chlorophyceae (−66%), Dinophyceae (−58%), Bacillariophyceae (−56%), Cryptophyceae (−52%), and Cyanophyceae (−26%). The cyanophyte decline was attributed mainly to non-nitrogen-fixing species. Two dominant diatom genera, Aulacoseira (Melosira) and Stephanodiscus were 53 and 78% lower, respectively. In contrast, after Dreissena spp. establishment, total phytoplankton, Bacillariophyceae, Chrysophyceae, and Cryptophyceae biovolumes did not change significantly; however, total Chlorophyceae, Cyanophyceae, and Dinophyceae showed declines of 42, 51, and 55%, respectively. There were major declines in the diatoms Stephanodiscus (−82%), Synedra (−84%), and Tabellaria (−98%) after the arrival of zebra mussels. Average biovolume of the cyanophyte Microcystis (a potential “bloom”-forming toxin producer) increased 13-fold after the establishment of zebra mussels. Multivariate analyses (ANOSIM tests of within-vs between-group differences), based on phytoplankton community structures defined by the quantitative contributions from 20 phytoplankton taxa, revealed statistically significant differences among communities representing pre- and post P control and pre-and post Dreissena conditions. The shifts in phytoplankton community composition between the identified time periods were attributed to several different taxa, whose relative contributions to the inter-community dissimilarities differed between the upper and lower bay stations.     

Dreissenid phosphorus excretion can sustain C. glomerata growth along a portion of Lake Ontario shoreline

One of the effects of the dreissenid invasion into the Laurentian Great Lakes appears to be a resurgence in the abundance of the nuisance alga Cladophora glomerata which experienced a marked decline following phosphorus abatement in the late 1970s and early 1980s. A subsidy of bioavailable phosphorus excreted by dreissenid mussels could be an important mechanism facilitating the growth of C. glomerata. To assess the importance of phosphorus released by mussels to C. glomerata growth in the nearshore, we conducted a survey of mussel distribution and abundance followed by in situ experiments designed to measure dreissenid phosphorus excretion rates. Average dreissenid mussel abundance in our study area was 3674 individuals/m², with an average biomass of 52.2 g of shell free dry mass/m². The mussels excreted bioavailable soluble reactive phosphorus at an average rate of 7.02 μg SRP/g shell free dry mass/hour, contributing about 11 t of soluble reactive phosphorus to our study area over the C. glomerata growing season. Dreissenids appear to be an important source of recycled bioavailable phosphorus to the nearshore, supplying more soluble reactive phosphorus to our study area than local watercourses and WWTPs, and more phosphorus than is required to sustain local C. glomerata growth.     

Evidence for a Trophic Cascade Effect on North-shore Western Lake Erie Phytoplankton Prior to the Zebra Mussel Invasion

Increasing summer total phosphorus (TP) concentrations measured in samples from a municipal water intake off the north shore of western Lake Erie during 1976 to 1983 were inconsistent with TP loads to the western basin of Lake Erie and with phytoplankton densities in the intake samples, both of which declined over the same time interval. The long-term (1976 to 1988) summer TP data were inversely correlated (r = −0.858) with summer average maximum daily wind velocities, suggesting that low wind velocities contributed to anoxia at the sediment-water interface and high sediment TP release rates in summer. While TP loading reductions in the late 1960s and early 1970s likely contributed to phytoplankton declines, continued phytoplankton declines during the late 1970s to early 1980s could not have been caused by continued reductions in TP loadings while TP concentrations increased. The phytoplankton declines of the 1980s are more likely attributable to changes in the trophic cascade associated with dramatic declines in some species of zooplanktivorous fish during the 1970s and 1980s as a result of a restored walleye population. Long term phytoplankton densities were fit (R² = 0.902) to a multiple regression model with western Lake Erie TP loads and an index of zooplanktivore density as independent variables; the zooplanktivore component of this model was the most significant contributor to the prediction of phytoplankton density. The implications of these findings for maintenance of good lake water quality include the need to maintain strong piscivore populations as well as reduced phosphorus loads.     

Phytoplankton Productivity in Saginaw Bay,Lake Huron: Effects of Zebra Mussel (Dreissena polymorpha) Colonization

Phytoplankton photosynthesis-irradiance parameters, chlorophyll concentrations, underwater extinction coefficients (kPAR), and surface irradiance were determined at 8–10 sites on 27 occasions in Saginaw Bay from spring 1990 through fall 1993 corresponding to a period before and after the establishment of large zebra mussel populations (began in summer 1991). Similar measurements, with the exception of the photosynthetic parameter, α, had also been made in 1974/75 at eight sites on nine occasions. In inner Saginaw Bay where zebra mussels were primarily found, chlorophyll and kPAR values decreased, while the photosynthetic parameters, P_max and α, increased after zebra mussel colonization. At sites in the outer bay where no zebra mussels were found, chlorophyll and kPAR values did not change after zebra mussel colonization, whereas photosynthetic parameters increased. Decreases in chlorophyll and kPAR in the inner bay were related to the zebra mussel, but increases in photosynthetic parameters in both the inner and outer bay were not. Areal-integrated and volumetric phytoplankton productivity decreased by 38% and 37%, respectively, in inner Saginaw Bay after the establishment of zebra mussels; phytoplankton productivity at outer bay control sites was similar during the same period. Decreased phytoplankton productivity in the inner bay was attributable to the large decrease in chlorophyll as increases in underwater irradiance (increased kPAR) and photo synthetic parameters could not compensate for the chlorophyll effect. Increase in underwater irradiance produced a significant increase in light to the benthic region and contributed to increased benthic primary productivity; ratio of photic zone to station depth increased in inner Saginaw Bay, from 0.6–0.8 before the zebra mussel colonization (1974–1990) to 1.1–1.3 after colonization (1992–1993). Overall, primary productivity in the inner bay did not exhibit a notable change after zebra mussel colonization as decreases in phytoplankton productivity were accompanied by increases in benthic primary productivity. Thus, zebra mussels altered inner Saginaw Bay from a pelagic-dominated system to a benthic/pelagic system which will have long-term effects on food web structure and productivity at higher trophic levels.     

Effects of the Zebra Mussel (Dreissena polymorpha Pallas) on Protozoa and Phytoplankton from Saginaw Bay,Lake Huron

Direct effects of the grazing activities of the zebra mussel, Dreissena polymorpha, on the natural assemblage of planktonic protozoa and algae from Saginaw Bay, Lake Huron, were studied in September and October 1994. Water and mussels collected from two eutrophic sites were incubated in an outdoor “natural light” incubator at ambient temperature for 24 hours. Experiments were conducted in 4-L bottles with screened (40 or 53-μm net) or unscreened water and with and without mussels. Despite relatively high growth rates of protozoa on both dates, mussels lowered protozoan numbers by 70–80% and reduced the species richness of the protozoan community by 30–50%. Large heterotrophic flagellates were reduced up to 100% while peritrichous ciliates attached to the colonies of blue-greens were reduced only by 50%. Dreissena selectively removed nanoplanktonic Cryptomonas and Cyclotella, but had no significant effect on the predominant phytoplankton species, Microcystis. Overall, Dreissena clearance rates were low in the presence of this cyanophyte species. We conclude that zebra mussels, in regions where they are abundant, can cause significant changes in composition of both the protozoan and phytoplankton communities.     

Phytoplankton Composition,Abundance, and Distribution: Nearshore Lake Ontario and Oswego River and Harbor

Phytoplankton samples were collected during July, August, and October of 1981 from 16 stations located in the nearshore of Lake Ontario and in the Oswego Harbor and river. Phytoplankton assemblages observed represented many species widely recognized as associated with eutrophic environments and environments possessing high chloride levels. Picoplankton and diatoms were the dominant groups on a numerical and bioyolume basis, respectively. The eutrophic species Stephanodicus tenuis, Fragilaria capucina, and Scenedesmus spp. were present in the Oswego Harbor and river with substantially higher abundances than in the nearshore region of Lake Ontario. A decrease in abundance of the historically prevalent Asterionella and Tabellaria and an increase in cryptomonads is suggested for the Oswego area. Halophilic species dominated the diatom assemblage of the Oswego Harbor and mouth of the Oswego River. The abundance of halophilic species was correlated with high conductivity and chloride levels. Cyclotella atomus, Cyclotella cryptica, and Cyclotella meneghiniana, the dominant halophilic species at the harbor and river stations, accounted for 36.6% of the total diatom abundance.     

Recent Changes in the Near-Shore Phytoplankton of Lake Erie's Western Basin at Kingsville,Ontario

A weekly net phytoplankton monitoring programme begun in 1967 at the Union water treatment plant, Kingsville, Ontario, was expanded in 1976 to include weekly chemical analyses of western Lake Erie water samples. Data from the Union intake monitoring programme have been examined in light of the significant reductions, since 1971, in phosphorus loading from the Detroit River. Total phosphorus concentrations in the “raw water” declined to an average of 30-35 μg P/L during 1977-78 from a pre-P control average of 63 μg P/L during 1967-70. Average annual net phytoplankton density during the pre-P control years averaged about 5000 Areal Standard Units per mL (A.S.U./mL), decreased steadily between 1971 and 1976 to a low of 2900 A.S.U./mL, but then increased again to 4490 A.S.U./mL by 1978. The recent increase was attributed almost entirely to massive mid-summer development of Fragilaria crotonensis. Winter densities of net phytoplankton during 1977 and 1978 were the lowest on record for the 12-year period. Similarly, average densities of green and blue-green algae showed a pre-P control density regime which was clearly higher than the post 1971 densities. Annual average diatom density was negatively correlated with annual average lake levels over the 12-year period and recent anomalous increases in Fragilaria may have been related to the observed drop in lake level which influenced turbulence and resuspension of this diatom.     

Phytoplankton variability in Lake Bourget: Phytoplankton dynamics and meteorology

This paper presents the phytoplankton succession during two successive years in Lake Bourget, a deep subalpine lake recovering from eutrophication. A drastic reduction of the watershed phosphorus loading in 1980 stopped lake eutrophication. At present, the lake has nearly reached constant winter (January–February) nutrient concentrations. However, during the 2 years of the study, the phytoplankton data set shows a great interannual variability, which is analysed on the basis of the meteorological forcing variables.     

Evidence for Remote Effects of Dreissenid Mussels on the Amphipod Diporeia: Analysis of Lake Ontario Benthic Surveys, 1972–2003

The status of invasive dreissenid mussels (Dreissena polymorpha and D. bugensis) and native amphipods (Diporeia spp.) in Lake Ontario was assessed in 2003 and compared with historical data. D. polymorpha (zebra mussels) were rarely observed in 2003, having been displaced by D. bugensis (quagga mussels). D. bugensis expanded its depth range from 38 m depth in 1995 to 174 m in 2003 and this dreissenid reached densities averaging 8,000/m² at all sites < 90 m. During the same time period, Diporeia populations almost completely disappeared from 0–90 m depth, continuing a declining trend from 1994–1997 reported in previous studies. The average density of Diporeia in the 30–90 m depth interval decreased from 1,380/m² to 63/m² between 1997 and 2003. Prior to 2003, areas deeper than 90 m represented a refuge for Diporeia, but even these deep populations decreased, with densities declining from 2,181/m2 in 1999 to 545/m2 in 2003. Two common hypotheses for the decline of Diporeia in the Great Lakes are food limitation and a toxin/pathogen associated with dreissenid pseudofeces. The Diporeia decline in deep waters preceded the expansion of D. bugensis to these depths, and suggests that shallow dreissenid populations remotely influence profundal habitats. This pattern of decline is consistent with mechanisms that act from some distance including nearshore dreissenid grazing and downslope transport of pseudofeces.     

Mirex in the Sediments of Lake Ontario

Analysis of sediment samples collected in Lake Ontario in 1968 revealed the occurrence of mirex in two anomalous zones related to input from the Niagara and Oswego Rivers. These anomalies were confirmed in a 1976 resampling program. A suspended solids sample taken in the Niagara River confirmed an upstream source (Hooker Chemical) of the compound. Bottom sediment samples in the Oswego River identified an industrial source (Armstrong Cork) 14 km upstream of the river mouth. Known use of mirex by this plant indicated a substantial loss some 15 years ago. This was confirmed by sediment coring in the open lake sediments indicating that mirex deposition commenced 7 to 14 years before present.     

Microplastic pollutants in the coastal dunes of Lake Erie and Lake Ontario

Microplastic particles, often studied as aquatic pollutants, have been recovered from coastal dunes along the shores of Lake Ontario and Lake Erie in New York and Pennsylvania. Surface and shallow sub-surface sand samples were collected from coastal dunes in 1 m² areas from 5 locations along Lake Erie: Sunset Bay, Dunkirk Harbor, Point Gratiot, and Canadaway Creek in New York and Presque Isle State Park Beach #11 in Pennsylvania. Samples were also collected from coastal dunes on Lake Ontario at Sandy Island Beach State Park, NY. Abundances, shapes, sizes, textures, and degradation of microplastics were characterized. Twenty-one of 26 samples yielded a variety of microplastics: pellets, fragments, and fibers. Larger microplastics (5.0–1.0 mm) were dominated by spheroidal and disk-shaped pellets with fewer fragments. Smaller microplastics (≤1.0 mm) were predominantly fibers and small fragments. Some microplastic particles exhibited evidence of degradation and weathering as a consequence of transport and exposure to the elements. The presence of microplastics in coastal dunes is attributable to aeolian transport from the adjacent beach.     

Nearshore Benthic Invertebrates of the Ontario Waters of Lake Ontario

Benthic invertebrates were collected by divers along 25 transects (depths of 2, 5, 10, and 20 m) between the Niagara River and Kingston during July 1981. Total standing stocks, which ranged from 1,600 m⁻² to 314,200 m⁻², were greatest on silt and clay, especially near the mouth of the Humber River, and smallest on rock. Of the 196 taxa which were recognized, Vejdovskyella intermedia, immature Tubificidae, Chaetogaster diaphanus, Potamothrix vejdovskyi, Tanytarsus, Pontoporeia hoyi, and Gammarus fasciatus were the most abundant and most frequently collected. Both inspection of the distributions of common taxa and ordination based on abundances of all taxa found at >5 stations indicated that organic enrichment, depth, and susceptibility to upwelling were the primary factors influencing the composition of invertebrate communities. Exposure to wave action was less important, and type of substratum had little effect. Overall, the nearshore fauna of Lake Ontario was less diverse, but more abundant than that of Lake Huron.     

The Impacts of Atlantic Salmon Stocking on Rainbow Trout in Barnum House Creek,Lake Ontario

We compared the impacts of stocking age-0 Atlantic salmon (Salmo salar) at high and low densities, and no stocking on abundance and growth of age-0 rainbow trout (Oncorhyncus mykiss) in Barnum House Creek, Ontario during 1993 to 2005. A similar stream, Shelter Valley Creek, was chosen as an appropriate reference stream where age-0 Atlantic salmon were not stocked. The catches of age-0 rainbow trout in Barnum House and the reference stream were highly correlated (r = 0.96) during years when no stocking occurred; however, this relationship did not persist in years when Atlantic salmon were stocked. The catch of age-0 rainbow trout in Barnum House Creek was significantly lower under both high (P = 0.00026) and low (P = 0.011) density Atlantic salmon stocking treatments compared with the no stocking treatment. The catches of age-0 rainbow trout and age-0 Atlantic salmon were negatively correlated in Barnum House Creek (r = −0.63). The length of age-0 rainbow trout in Barnum House Creek was depressed significantly (P = 0.004), under the high intensity Atlantic salmon stocking treatment, but not under the low intensity treatment (P = 0.20). In contrast, the length of age-0 rainbow trout in Shelter Valley Creek was unchanged over the same period. Restoration stocking of Atlantic salmon in Lake Ontario tributaries may impact rainbow trout abundance and growth.     

The Impacts of the Non-native Macrophyte Cabomba caroliniana on Littoral Biota of Kasshabog Lake,Ontario

An established population of Cabomba caroliniana now covers extensive littoral areas in the shallow waters of Kasshabog Lake (Ontario). This is the first known naturalized population of this non-native aquatic macrophyte, commonly called fanwort, on the Canadian side of the Great Lakes basin, despite the fact that it was first reported in the 1930s. High dispersal potentials combined with the ability to adapt and grow in a range of environmental conditions have made C. caroliniana a nuisance species in Australia, Japan, and parts of the United States. However, little is known about the broader ecological implications of its introduction and establishment. Using a survey approach, we conducted a preliminary assessment of water chemistry, macrophyte, epiphytic algae, and macroinvertebrate communities found in C. caroliniana beds and compared them with native macrophyte beds in Kasshabog Lake. Light penetration was significantly reduced in the C. caroliniana beds and was the only sampled physio-chemical parameter that differed between bed types. We also found several notable differences in the structure and composition of biological communities within macrophyte beds. While native macrophytes were present in dense C. caroliniana beds, abundance was considerably low and unevenly distributed. Significantly more epiphytic algae was present on C. caroliniana plants, however community composition was comparable with epiphytic algae found on native macrophytes. The taxonomic composition of macroinvertebrates was similar between C. caroliniana and native beds, while abundance was substantially higher in C. caroliniana beds, owing to high densities of coenagrionids and chironomids. These differences suggest that C. caroliniana is changing macrophyte community composition in this lake, having an impact on epiphytic algae, and creating a new habitat for some macroinvertebrates. Further studies are required to determine the extent of these ecological impacts.     

Statistical Analysis of the Spatial and Temporal Variability of the Ratio Chlorophyll A to Phytoplankton Cell Volume in Saginaw Bay,Lake Huron

Chlorophyll a has traditionally been used as an indicator of phytoplankton concentration in natural waters. This parameter has also been widely used by phytoplankton modelers for the purpose of comparing model output with field data. Examination of 1974 field data from Saginaw Bay indicated that chlorophyll a concentrations were inconsistent with phytoplankton cell volume concentrations. Statistical procedures were employed to determine the significance and source of the observed variability in the ratio of chlorophyll a to cell volume concentrations.The procedures employed included t-tests, analysis of variance and polynomial regression. The hypothesis that the natural log of the ratio of chlorophyll a to cell volume was constant in time was tested on six different sampling station groupings representing various spatial divisions of Saginaw Bay. In all cases, this ratio was found to vary significantly at the 95% confidence level. The variability of the ratio decreased when only data from the outer portion of Saginaw Bay were used. Greater than 60% of the variability of the ratio can be removed by using a linear correlation of the natural log chlorophyll a to cell volume ratio with the fraction of diatom cell volume.     

Food of Trout and Salmon in Lake Ontario

Stomachs of trout and salmon (n = 1,904) were collected from fish registered at fishing tournaments held in New York State waters of Lake Ontario between April and September 1983 and 1984. Numbers of adult-sized fish containing identifiable food items were 323 lake trout (Salvelinus namaycush), 289 brown trout (Salmo trutta), 24 rainbow trout (S. gairdneri), 164 coho salmon (Oncorhynchus kisutch), and 63 chinook salmon (O. tschawytscha) Proportional similarity in diet between pairs of species was high and normally exceeded 0.70; diet composition of individual species was similar between years. Alewives (Alosa pseudoharengus) were the main prey of all species during all months and were normally 110–149 mm in standard length. Rainbow smelt (Osmerus mordax) was the second most common prey eaten but was generally found in fewer than 20% of the stomachs examined during any month. Diet diversity was generally higher during April-May than during July-September for coho salmon, lake trout, and brown trout. Larger brown trout ate larger alewife in 1983 but not in 1984. Results suggest that the five trout and salmon species in Lake Ontario are potential competitors.