Fluvial Lake Research Articles

Relationships among total recoverable and reactive metals and metalloid in St. Lawrence River sediment: Bioaccumulation by chironomids and implications for ecological risk assessment

The availability and bioaccumulation of metals and metalloids, and the geochemical interactions among them, are essential to developing an ecological risk assessment (ERA) framework and determining threshold concentrations for these elements. The purpose of this study was to explore the relationships among total recoverable and reactive metals and metalloid in sediment and their bioaccumulation by chironomids. In the fall of 2004 and 2005, 58 stations located in the three fluvial lakes of the St. Lawrence River and its largest harbour area in Montreal, Canada, were sampled. Nine total recoverable and reactive metals (Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn) and one metalloid (As) were measured in whole sediment using two extraction methods: HCl/HNO 3 and HCl 1N, respectively. The bioaccumulation of six metals (Cd, Cr, Cu, Ni, Pb and Zn) and As by chironomids was evaluated in a subset of 22 stations. Strong collinearities were observed between some total recoverable or reactive metal concentrations in sediment; two principal clusters, including collinear metals, were obtained. The first one included metals of mainly geological origin (Al, Cr, Fe, Mn, Ni), while the second one included As, Cd, Cu, Pb and Zn, which likely derive mainly from point sources of anthropogenic contamination. Each element also showed strong collinearity between their total recoverable and reactive forms (0.65 ≤ r ≤ 0.97). We can conclude that both chemical forms are equivalent for use in statistical models needed to explain biological responses and also in screening risk assessment. However, these relationships are not always proportional. Lower availability percentages were observed for Cd, Cu and Zn in the highly mixed-contaminated area of the Montreal Harbour, even though concentrations in sediment were higher. We observed a significant correlation (0.50 ≤ r ≤ 0.56) between concentrations in chironomids and concentrations of both total recoverable and reactive Cr and Pb in sediment. Arsenic was an exception, with accumulation by chironomids being highly related to reactive sediment concentrations. Finally, we observed variable influences of explanatory factors (e.g. sediment grain size, Al, Fe, Mn, S, TOC), depending on which metal or metalloid was being predicted in chironomids. In this context, it is difficult to choose a universal predictive method to explain the bioaccumulation of specific metals, and more research is still needed into normalization procedures that consider a combination of explanatory factors.

Wither the Saint Lawrence River?

The International Section of the St. Lawrence River, reaching from the head of the St. Lawrence River near Kingston, Ontario to the entry of the river fully into Canada just east of Cornwall, Ontario and Massena, New York (Fig. 1), once contained a 10 mile stretch of white water known as the Long Sault (International) Rapids. The International Rapids is no longer a barrier to navigation. The St. Lawrence River is forced through the 900 megawatt Moses-Saunders power dam (est. 1958) that spans the river between Cornwall, Ontario and Massena, New York, and the St. Lawrence Seaway (est. 1959) guides ocean-going vessels through a fluvial lake over the drowned rapids in what is now called Lake St. Lawrence. Natural water level fluctuations in the river and upstream Lake Ontario have been subdued due to the power dam, and the St. Lawrence Seaway is responsible for many of the invasive species that have colonized the Great Lakes/St. Lawrence River ecosystem. The proximity of ample power provided by the dam stimulated industrial growth in the region, with the unfortunate result of environmental degradation, now identified by the Areas of Concern at Cornwall and Massena. Despite the economic attention bestowed on the International Section of the river, the current level of inattention to the ecology, limnology, and environmental issues of this river will lead to further deterioration of this system. The International Section of the Saint Lawrence River is in a state of neglect; inattention to this system and bypassing real needs have jeopardized its integrity. The Great Lakes community must recognize that many troubles arriving in “our” lakes

Hydrodynamic control of the underwater light climate in fluvial Lac Saint‐Pierre

We measured characteristics of the underwater light spectra (e.g., attenuation of ultraviolet [UV] radiation, photosynthetically active radiation) and select dissolved and particulate physicochemical properties (e.g., chromophoric dissolved organic carbon [CDOM], dissolved and particulate organic carbon, inorganic dry weights, beam attenuation coefficients, particulate absorption coefficients, and nutrients) in different water masses of fluvial Lac Saint‐Pierre (Canada). We used these variables as tracers to reveal the extent and magnitude of spatial and temporal heterogeneity in this large, shallow, fluvial lake of the St. Lawrence River. We superimposed these tracer variables over radiance data obtained from satellite images to identify spatial and temporal changes in the distribution of different water masses and their bio‐optical components. The underwater light environment showed strong horizontal (longitudinal and lateral) variability because of the strong connectivity between the terrestrial and aquatic environments in the lake’s tributaries and adjoining wetlands. Analyzing the downstream distribution of optical and chemical variables as a function of transport time rather than distance from source tributaries allowed us to demonstrate large differences in the age of the different water masses depending on the characteristics of the source tributary, in‐stream processes, and distance from its source. CDOM explained most of the UV attenuation and allowed the greatest discrimination between water masses.

Temporal and spatial distribution and production of dissolved gaseous mercury in the Bay St. François wetland, in the St. Lawrence River, Quebec, Canada

Wetlands are valued for their high biodiversity and for their ecosystem services. However, we still do have a poor understanding of their role in the redox transformation of contaminants such as mercury, particularly in fluvial settings. Seasonal and spatial variations in dissolved gaseous mercury (DGM) distribution and production were studied in the Bay St. François, a wetland in the St. Pierre Lake, a fluvial lake of the St. Lawrence River, in Quebec, Canada. A high spatial resolution for DGM, with samples taken every 10-cm depth, was used in field measurements. Through a series of parallel field and incubation experiments, we assessed the main factors determining Hg(0) transformations as a function of depth, seasons, and presence/absence of macrophyte beds. Besides light penetration in the water column and water temperature, iron and dissolved organic carbon likely stimulated Hg(II) reduction. Inversely, chloride favored Hg(0) oxidation. Macrophytes and associated epiphytes appeared to be important sites of adsorption/absorption of Hg(II) and likely of DGM. It seems however that the effects of macrophytes were restricted to immediately adjacent waters. Near the bottom, under anoxic conditions, the reduction of Hg(II) was highly promoted. In addition, sediments and decomposing macrophytes seemed to release DGM and/or reducible Hg to bottom waters. Overall, differences in DGM between surface and bottom waters tended to be more accentuated than observed differences in DGM between macrophyte beds and sites devoid of plant.

Paleolimnology of a fluvial lake downstream of Lake Superior and the industrialized region of Sault Saint Marie

A paleolimnological study was undertaken to describe the past environment of the St. Mary's River downstream of Lake Superior. Two sediment cores were obtained from the deepest part of Lake George, a fluvial lake in the river system. Rigorous analyses of radionuclides (210Pb,226Ra, and137Cs) and chemical properties provided an accurate sediment chronology. More than 450 diatom species from 47 genera were identified. Diatom and geochemical data indicated gradual environmental change in response to anthropogenic activities, including logging, hydrologic manipulation, and steel, leather, and paper industries. A decline in water quality occurred gradually from the late 1800s through the 20th century in response to human activities, as was apparent from an increase in eutrophic–planktonic diatom taxa. A decline in benthic taxa and an increase in contaminant metals were also contemporaneous with impacts during the 20th century. Subfossil diatoms were similar to those recorded in paleolimnological investigations from the Great Lakes. However, diatom profiles indicate that the algal supply from upstream Lake Superior has been minimal and that the cores mainly reflect environmental characteristics of the near-upstream environment. Despite stochastic sedimentary regimes and complex habitats in the lotic system, this study reinforces the value of river paleolimnology at carefully selected sites.

Fish Assemblages in Oxbow Lakes Relative to Connectivity with the Mississippi River

AbstractThe alluvial valley of the lower Mississippi River contains hundreds of fluvial lakes that are periodically connected to the river during high water, although the frequency, duration, and timing of the connections vary. To help design plans to restore and preserve fish assemblages in these alluvial lakes, this investigation tested whether predictable patterns in lake fish assemblages were linked to the level of connectivity with the river. Results suggested that connectivity played an important role in structuring fish assemblages and that it was correlated with variables such as lake size, depth, distance from the river, and age, which exhibit a continuum of predictable features as the river migrates away from abandoned channels. Annual floods homogenize the floodplain and promote connectivity to various degrees, allowing for fish exchanges between river and floodplain that directly affect fish assemblages. The major physical changes linked to reduced connectivity are loss of depth and area, which in turn affect a multiplicity of abiotic and biotic features that indirectly affect community structure. In advanced stages of disconnection, fish assemblages in oxbow lakes are expected to include largely species that thrive in turbid, shallow systems with few predators and low oxygen content. When the river flowed without artificial restraint, oxbow lakes were created at the rate of 13–15 per century. At present, no or few oxbow lakes are being formed, and as existing lakes age, they are becoming shallower, smaller, and progressively more disconnected from the river. Given that modifications to the Mississippi River appear to be irreversible, conservation of this resource requires maintenance of existing lakes at a wide range of aging phases that provide diverse habitats and harbor distinct species assemblages.

RNA/DNA ratio and total length of yellow perch (Perca flavescens) in managed and natural wetlands of a large fluvial lake

RNA/DNA ratios and total lengths were compared to determine growth patterns of age-0 yellow perch (Perca flavescens) in managed and natural habitats of a large fluvial lake (Lake Saint-Pierre, St. Lawrence River, Quebec, Canada) over seasonal and yearly temporal scales. In 2002, the RNA/DNA ratio responded to degree-days accumulated over periods of 7–8 days before sampling, while in 2003, no relationship with temperature was established. The growth patterns obtained each year probably reflect indices responding to different limiting variables. In 2002, temperature would have been limiting, whereas in 2003, other factors such as prey availability, food quality, and competition may have influenced growth. In addition, the discrepancy between total length and RNA/DNA ratio observed in 2003 may reflect a differential time of response to limiting variables. These results together show that the two indices reflect growth at different time scales and suggest that their combination can help identify shifts between limiting environmental variables. Also, growth in managed wetlands during springtime was systematically superior to that in the natural environment, supporting the contention that managed wetlands are highly productive habitats. In natural habitats, growth rates were higher on the south shore by summer, which is consistent with the established north-south productivity gradient in Lake Saint-Pierre.

Changes in the spectral and chemical properties of a water mass passing through extensive macrophyte beds in a large fluvial lake (Lake Saint-Pierre, Québec, Canada)

Large fluvial lakes, as part of river corridors, are recognized as spatially heterogeneous ecosystems. Due to their shallowness, the littoral zone of these lakes occupies a large proportion of their surface and is extensively covered by macrophytes that are known to affect their physical, chemical and biological environments in various ways. This study documents the small-scale (5 km) bio-optical variations associated with the longitudinal passage of a water mass through macrophyte beds during their maximal growth season. The utilization of a 2D hydrodynamic model allowed us to establish hydrological connectivity between stations within the beds, and thus to identify longitudinal heterogeneity along the macrophyte beds. Significant changes in the inherent properties of the water were observed along the upstream-downstream gradient. Due to their effects on hydrodynamics, macrophytes were responsible for a decrease in particles and dissolved organic carbon (DOC) resulting in an increasing penetration depth of ultraviolet (UV) and photosynthetically active radiation (PAR). Along the transect, chromophoric dissolved organic matter (CDOM) decreased more rapidly than DOC resulting in a decrease of the CDOM to DOC ratio. The observed changes in the DOC pool may be explained by the constant input of non-chromophoric DOC from macrophyte leachates and exudation within the bed and/or the photochemical and microbial degradation of CDOM. The implication of such small-scale heterogeneity on Lake Saint-Pierre productivity is discussed.

Relationship between DOC photochemistry and mercury redox transformations in temperate lakes and wetlands

We compared diurnal variations in dissolved gaseous mercury (DGM) concentration and in losses of dissolved organic carbon fluorescence (DOCF) in four boreal Canadian Shield lakes, in one beaver pond, and in filtered and unfiltered water from a wetland in Lake St. Pierre, a fluvial lake of the St. Lawrence River. These systems were chosen to represent a spectrum of DOC. We also determined the contribution of UVB, UVA and visible light on DGM photo-induced production in the four lakes. Our results showed a strong relationship between DGM concentrations and light intensity and between DGM production and losses in DOCF, in all study sites. We also observed higher rates of DGM formation and of DOCF bleaching in the presence of UV radiation. Under UVB light, production of DGM was higher in clear lakes than in the humic ones. Inversely, in the UVA range, DGM production tended to be higher in humic lakes. We suggest that DOCF bleaching can be used as a proxy for the rate of formation of reactive species that may alter the redox state of mercury in surface waters. We also have indications that DGM production is more important in clear than in humic waters.

PCBs and OCPs in Sediment Cores from the Lower St. Lawrence Estuary, Canada: Evidence of Fluvial Inputs and Time Lag in Delivery to Coring Sites

Three sediment cores were collected along the longitudinal axis of the Laurentian Trough in the Lower St. Lawrence Estuary (LSLE) and an additional one at the junction of the Estuary and the Gulf of St. Lawrence. After core-slicing, each sediment layer was analyzed for polychlorinated biphenyls (PCBs) and some organochlorine pesticides (OCPs) including p,p'-dichlorodiphenyltrichloroethane (DDT) and its metabolites, hexachlorobenzene (HCB) and Mirex. 210Pb activity was also measured in these sediments, which allowed us to confirm that these cores were too much affected by the overall impact of surface mixing to be dated. Nevertheless, POP sedimentary profiles in cores from the LSLE upstream stations showed well-defined subsurface peak concentrations. Apparently, the peak inputs of POPs to these sediment cores had occurred after the years of maximum sales and production of these chemicals in North America, suggesting a time lag in the delivery of POPs to the LSLE sediments. Concentrations of POPs in the LSLE surface sediments as well as POP inventories in sediment cores decreased in the seaward direction, confirming that the head of the LSLE acts as a sink for sediments and associated constituents. Surface concentrations of sigmaPCBs, sigmaDDTs, and HCB in the most upstream core were on average similar to those reported in two fluvial lakes of the St. Lawrence River but were between 12 and 39 times lower than those from Lake Ontario. For Mirex, the surface concentration in that core was 5 and 130 times lower than the average values found in the fluvial lakes and Lake Ontario, respectively. Differences between Lake Ontario sediment cores and the most upstream core from the LSLE were much smaller on the basis of POP inventories than surface concentrations of POPs, but were still important. The total burdens of POPs in LSLE sediments below the 200 m isobath were 8704 kg for sigmaPCBs, 1825 kg for sigmaDDTs, 319 kg for HCB, and 27.5 kg for Mirex. These values represent between 1% and 10% of the total burdens of these POPs in Lake Ontario sediments. The estimated contribution of POPs by direct atmospheric deposition into the water column area of the LSLE represented at most 30% of the total sediment burdens of POPs in the LSLE, illustrating the importance of fluvial inputs.

The oxbow lakes of the central part of the River Labe - comparative study of water quality and heavy metals concentrations in sediment

In the years 2000 and 2002, the research of three fluvial lakes of the central part of the River Labe was carried out. All three localities were chosen to evaluate the environmental state and the impact of human activity. The research included physical, chemical, hydrobiological analyses of water, analyses of sediments and bathymetric measurements. In spite of the identical origin of these lakes, major differences were found e.g. oxygen saturation, BOD5, water loading with nutrients, calcium and chlorides concentrations. Considering the heavy metals in sediments significant differences in concentration - depth relation were determined at each locality as well.

Genetic classification of lakes in the Czech Republic

The paper provides genetic classification of lakes in the Czech Republic. We separate lakes to two groups - natural and anthropogenic origin. The genetic classification is based on the classification of lakes according to the way of origination of the lake basin while the geomorphologic viewpoint prevails: glacial lakes, fluvial lakes, karst lakes, lakes dammed-up by landslides, organogenous lakes and anthropogenic lakes. As the anthropogenic lakes we may regard all the water bodies which originated as a consequence of the human activity - mining of mineral resources.

An evaluation of approaches used to determine the distribution and biomass of emergent and submerged aquatic macrophytes over large spatial scales

We compared the performance of various approaches to determine the distribution and biomass of submerged and emergent aquatic plants in a large fluvial lake. Three empirical models linking local macrophyte biomass to single and multiple environmental variables were applied in a GIS-framework to estimate the spatial distribution and biomass of aquatic macrophytes in Lake St. Pierre, a large (300 km 2), shallow (mean depth: 3 m) and complex widening of the St. Lawrence River (Quebec, Canada). The resulting maps and emergent and submerged macrophyte distributions obtained independently by remote sensing and echo sounding techniques were compared to field data collected in 2000. Maps derived from echo sounding, from a biomass versus depth regression and from a four-variable model (i.e. exposure to wind and waves, plant growth form, water depth and transparency) were the most accurate (55–63% overall agreement with field data). Remote sensing techniques were the least accurate for determining underwater macrophyte distribution in Lake St. Pierre due to the limitations of image-based methods for detecting submerged aquatic vegetation in coloured, turbid waters. This study demonstrates that environmental models in combination with GIS can be used to estimate aquatic macrophyte distribution over larger spatial scales and to examine potential change in macrophyte growth form assemblages arising from different environmental conditions.

Spectral gradients of downwelling light in a fluvial lake (Lake Saint-Pierre, St-Lawrence River)

Large fluvial lakes are understudied with respect to their underwaterlight climates. Fluvial lakes pose unique challenges for photobiologistsinterested in the interactions amongst light climate, nutrients and microbialcommunity structure and biodiversity. This is because fluvial lakes are typifiedby highly dynamic flow regimes often incorporating different inflows anddischarges each characterized by their own unique physico-chemical composition.These compositional characteristics include the concentrations of chromophoricdissolved organic matter (CDOM), suspended solids, and pigments such aschlorophyll. Together these factors contribute to the distribution andcomposition of the water masses that make up fluvial lakes. These water masses,in turn, flow over lakebeds that are typically complex in their morphometry andfeature extensive macrophyte beds, further enhancing the habitat heterogeneityof these ecosystems. We here report on the spectral attenuation of ultravioletradiation (UVR = 280–400 nm) and photosyntheticallyactive radiation (PAR = 400–700 nm) in the three mainwater masses of Lake Saint-Pierre and evaluate the relative contribution ofCDOM, and particulate organic material to UVR attenuation. We demonstrate thatUVR penetrates 18 to 30% of the water column (1% penetration depth) in the LakeSaint-Pierre ecosystem, and show how the underwater spectral UVR varies withinthe three water masses.

Emergence of New Explanatory Variables for 2D Habitat Modelling in Large Rivers: The St. Lawrence Experience

The St. Lawrence River is one of the most important large rivers in North America. This 600-km long watercourse is characterized by a high degree of physical heterogeneity, including fast moving narrow reaches separated by fluvial lakes reaching 10 km in width. The mean annual discharge from the outflow of Lake Ontario is 7500 m3/s and has been managed for hydropower and transportation since the 1960s. With the management plan currently under review an effort is being made to include criteria that take into account the impacts of regulation on the biotic components of the river ecosystem. High resolution 2D spatial modelling of river habitats and floodplains is a powerful tool to make quantitative impact assessments of the biota. Physical variables commonly used in habitat models include depth, velocity and substrate size. In addition, other abiotic variables such as wind-generated wave stress, light penetration, water temperature, sedimentation of fine particles, specific discharge and bottom slope, that define the local ’hydroperiod’ have been suggested. Our proposed approach integrates abiotic data obtained from numerical models, field measurements and biological information to overcome problems inherent in temporally and spatially heterogeneous river systems. This approach was tested with a habitat model applied to submerged aquatic vegetation, various categories of wetlands, benthic organisms and various life stages of a number offish species. Logistic regression is the statistical model currently used to synthesize the relationships between abiotic and biotic factors. The short-term objective of this modelling exercise in the St. Lawrence River is to understand the underlying links between fluvial physics and biota. A longer-term objective is to provide a real-time analysis of key variables and to quantify the links between trophic levels.

The influence of macrophyte beds on plankton communities and their export from fluvial lakes in the St Lawrence River

1. To determine the influence of macrophyte beds on plankton abundance within fluvial lakes of the St Lawrence River, planktonic components (macrozooplankton, heterotrophic bacteria, and phytoplankton as chlorophyll‐a [Chl‐a]) were sampled in Lake St Francis and Lake St Pierre during summer 1998. We tested the hypothesis that the abundance of planktonic components was higher within macrophyte beds in comparison to the more rapidly flushed open water areas of the fluvial lakes. 2. The large cross channel variation in zooplankton biomass was indeed correlated with the presence of dense beds of submerged macrophytes. Total macrozooplankton biomass was nine‐fold greater within the beds (mean=180 μg L−1 dry mass) than in either the open water or areas with only sparse vegetation (mean=20 μg L−1 dry mass). 3. Chl‐a and heterotrophic bacterial abundance were also higher in the beds, but only slightly so. There was no difference in total phosphorus or dissolved organic carbon concentrations between areas of dense vegetation, sparse vegetation or open water. 4. Macrophyte beds on the margins of the fluvial lakes allow the development of high planktonic abundance relative to the fast flowing central channel. Macrozooplankton biomass was much higher at the outflows of the lakes (∼50 μg L−1 dry mass) in comparison to the inflows (<20 μg L−1 dry mass). The increase is due to the transfer of organisms from submerged macrophyte beds into the central channel in the downstream quarter of the two lakes where the marginal littoral waters enter central channel waters. 5. Along rivers, shallow fluvial lakes appear to act as sources of plankton which is exported downstream during years of extensive littoral macrophyte development.

Integrated two-dimensional macrophytes-hydrodynamic modeling

Lake Saint-Francois is the first fluvial lake downstream of the Great Lakes. Aquatic macrophytes are abundant because of water level stabilization and increased nutrient loads. The influence of plants on flow modification is very important and must be considered in order to simulate hydrodynamic conditions. The spatial distribution of plant species is linked to their instream flow preferences. Several abiotic variables are of importance: light penetration, wave energy, current velocity, nutrients in substrate and physical characteristics of the substrate are controlling the species and their biomass. Field characterization of macrophytes was performed using echosounder transects in association with a submersible video camera. This technique allowed the calibration of each echofacies for species identification and their relative proportion, height and density. The main eleven assemblages appear to be strongly correlated with abiotic conditions. A basic interpretation key was set up in order to describe plant distribution over the entire lake. Species, relative proportion, density and plant height were interpolated between transects. This information is used to adjust the Manning's friction coefficient for each assemblage. Simulations of the flow fields with plants and in absence of plants show a contrasted pattern. During the summer (with plants), the flow is mainly concentrated in deep channels where velocities are clearly increased by approximately 20% compared to spring-fall simulation (without plants).

Relationships between total mercury in sediments and methyl mercury in the freshwater gastropod prosobranch Bithynia tentaculata in the St. Lawrence River, Quebec

Total Hg concentrations in sediment and methyl mercury (MeHg) levels in the gastropod species Bithynia tentaculata were evaluated at 21 stations in the fluvial corridor of the St. Lawrence River (Quebec, Canada). In the sediments (n = 21), total Hg concentrations ranged from 34 to 2790 ng·g dry weight-1. In the gastropods, MeHg concentrations varied from 15 to 290 ng·g dry weight-1 in undepurated gastropods (n = 20) and from 41 to 420 ng·g dry weight-1 in depurated gastropods (n = 13; without gut contents). The southern sector of Lake St. Louis, located near the Îles-de-la-Paix, had significantly higher Hg concentrations than any other sectors under study, both for total Hg in the sediments and MeHg in the gastropods. We established linear models to describe the relationships between Hg contamination in sediments and molluscs for the fluvial corridor of the St. Lawrence River and for the Lake St. Louis sector. Total Hg in sediments and MeHg in gastropods were highly correlated for all pooled stations (r = 0.83), and the relationship was even stronger for the Lake St. Louis stations (r = 0.92). Our study suggests that the gastropod B. tentaculata is a promising biomonitor species for assessing Hg contamination in the fluvial corridor and lakes of the St. Lawrence River.

Metal concentrations in two freshwater gastropods (Mollusca) in the St. Lawrence River and relationships with environmental contamination

This study assesses the potential use of two dominant freshwater gastropod species of the St. Lawrence River, Bithynia tentaculata (Prosobranchia) and Physa gyrina (Pulmonata), as biomonitors of metal pollution. Gastropods were collected in the littoral zone of Lake St. Louis and Lake St. Pierre, two shallow fluvial lakes of the St. Lawrence River, at sampling stations chosen to represent a metal concentration gradient in sediments. The soft body tissues of snails were analyzed for Cd, Cu, Ni, Pb, and Zn. Metal concentrations in snail tissues were related to those in macrophytes, on which the snails were collected, and in nearby sediments, where the metal ion concentrations at the water-sediment interface were estimated. There was a significant effect of age on the essential metals Cu and Zn, adults of B. tentaculata showing higher concentrations than juveniles. Significant relationships were obtained between Cd and Zn concentrations in snail tissues and those in the environment (macrophytes or the estimated free metal ion levels at the water-sediment interface). Some other relationships were also obtained with Ni, Pb, and Cu in P. gyrina. Bithynia tentaculata appears to be a promising biomonitor species, particularly for Cd and Zn.

Sediment dynamics in the fluvial lakes of the St. Lawrence River: accumulation rates and characterization of the mixed sediment layer

Permanent sedimentation (210Pb and 137Cs), sediment mixed layer thicknesses, and mixing coefficients (7Be) were measured in the St. Lawrence River in order to evaluate the importance of sediment retention in the particulate matter budget and to characterize the system's resilience to changing contaminant loads. Net sediment accumulation (1 to > 18 kg·m-2·year-1) is observed at most sites deeper than 4.5 m located outside the main channels. Annual sediment retention in the lakes ranges from 1.5% (Lake St. Pierre) to 17% (Lake St. Francis) of their total load of suspended solids. 7Be profiles indicate that the average mixed layer thickness, mixed layer mass, and mixing coefficient are 3.3 ± 0.2 cm, 17.8 ± 1.7 kg·m-2, and 14.9 ± 2.8 cm2·year-1, respectively. The average depth of the long-term (approximately 5 years) mixed layer determined from the 137Cs : anthropogenic Pb ratio is 5.1 ± 0.4 cm, corresponding to 30.6 ± 4.6 kg·m-2. Because the mixing coefficient in superficial sediments is relatively high, and because annual particulate matter loading to the river is comparable with its mixed sediment inventory, the system is expected to have a rather short memory of past conditions and to recover rapidly (2-5 years) following a decrease in contaminant loading.