Fernando Rosa

Seasonal Thermal Cycle of Lake Erie

A summary of the seasonal water temperature characteristics of Lake Erie and the 1979 and 1980 thermal structure in the central basin is described. Ice cover extends over 90% of Lake Erie most winters. Minimum surface temperature usually occurs in February (0.1° C) but fully mixed conditions at 1°C or less occur in January with isothermal conditions at (1°C) occurring from mid-February to mid-March. The thermal bar advance lasts about 5 to 6 weeks from April to mid-May and permanent stratification usually begins in mid-June with maximum heat storage in mid-August and overturn in mid-September. The central basin thermocline position varies significantly from year to year, the variability of the upper and lower mesolimnion boundaries being as large as 10 m. Thermocline position shows some dependence on prevailing meteorological conditions and has implications to the development of central basin anoxia. Temperature increases and decreases depicted on isotherm plots for stations in the central basin show correspondence with peak wind stress events. During fragile stability conditions, even moderate wind stresses of less than 0.5 dynes/cm2 are capable of producing upper layer deepening. Episodes of complete vertical mixing in response to high wind stresses of 3 dynes/cm2 during storm periods are observed. Double thermoclines are evident at several locations within the basin and temperature changes resulting from an influx of hypolimnetic water from the Pennsylvania Ridge is documented. Periods of hypolimnetic entrainment are clearly observed along with thermocline tilting of 1 to 2 meters toward the south.

Sedimentation and Sediment Resuspension in Lake Ontario

From March to November 1981, seasonal variations in the weekly sedimentation rates of particulate matter were investigated at an inshore and at an offshore site in Lake Ontario. Sedimentation rates at both sites showed extremely high variability from week to week; minimum to maximum rates varied by a factor of 10 for the offshore site and by a factor of 40 for the inshore site. The variability in the offshore rates was mainly due to the amount of particulate matter in the epilimnion, and in the inshore rates was mainly due to bottom sediment resuspension. Offshore sedimentation rates of total suspended matter measured in the thermocline were a function of the total suspended matter concentrations in the epilimnion. Sedimentation rates measured in the thermocline were higher than those measured at the top of the hypolimnion showing particulate matter retention in the thermocline. Sedimentation rates measured at 5 meters above the bottom at both stations were extremely variable with time and the inshore rates at times exceeded the offshore rates by a factor of 50. This major difference was observed during fall turnover, when previously sedimented material and associated contaminants were resuspended at the inshore site. Since the net downward flux is a function of epilimnetic concentrations, it is clear that inshore resuspension occurs continuously but at varying intensities.

Lake Erie Central Basin Oxygen Depletion Changes from 1929–1980

The hypolimnetic oxygen depletion rates of the Lake Erie central basin have been reassessed using a new approach. In the past, the central basin rates have been calculated using the available data, excluding some areas based on either temperatures and/or depth definition. The high spatial variability in the data caused uncertainties in the mean value which were sufficiently large to prevent a statistically meaningful interval oxygen depletion rate time-trend analysis. The new approach reduces the effect of spatial variability (80%) on the calculation of the interval oxygen depletion rates and hence permits the identification of a time-trend with more precision, particularly when the data are corrected for the effects of vertical mixing, temperature effects on metabolic rates by using Q10 coefficient, variable hypolimnion thicknesses, and seasonal variability. A linear regression analysis of the final corrected depletion rates with time shows a significant increase in the yearly average hypolimnetic oxygen depletion rate of 0.030 gm·m·−3mo·−1 yr·−1 between 1929 and 1980. This increase in the rate accounts for a loss of 4 to 5 gm·m−3 of oxygen from the central basin hypolimnion since the earliest oxygen records in 1929. This increase may be directly related to an increase in the trophic level of the central basin, since most of the major limnological variables which affect the depletion rate have been accounted for.

Effects of abandoned gold mine tailings on the arsenic concentrations in water and sediments of jack of clubs lake, B.C.

Abstract From 1933 to 1964 gold was extracted by underground mining at the northeast shore of Jack of Clubs Lake. At present, tailings and waste rock 4.5 m thick covers approximately 25 hectares of land adjacent to the lake. Arsenic concentrations (>2,000 μg.g‐1) were found in the tailing materials. Two simultaneous processes are controlling the elevated As concentrations in the lake sediments (up to 1,104 μg.g‐1), deposition of suspended particles enriched with As and subsequent diagenetic cycling of sedimentary As. Arsenic concentrations (as high as 556 μg.L‐1) in water samples adjacent to the tailings indicate mobilization of As. However, dissolved As is readily scavenged or coprecipitated with iron hydroxides. The greatest proportion of As in the sediment cores is associated with iron oxides and sulphides. Under oxic conditions the high concentrations of Fe in the tailings is actually beneficial and very effective at limiting the migration of undesirable elements.

Trace elements in water, sediments, porewater, and biota polluted by tailings from an abandoned gold mine in British Columbia, Canada

Abstract The concentrations of major and trace elements in different environmental compartments (e.g., water, suspended and bottom sediments, sediment porewater, and biota) of Jack of Clubs Lake (JCL), Wells, British Columbia (Canada), were determined to assess the biogeochemical effects of abandoned gold mine tailings on the aquatic ecosystem of JCL in the Fraser River drainage basin. Arsenic and Pb were transported from the tailings to the lake, where they accumulated in bottom sediments in concentrations up to 1104 and 281 μ/g, respectively. Although the benthic community in the lake was only partially affected, there was evidence that the tailings inhibited a variety of microbial activities in the lake sediments. The concentrations of As, Cd, Cu and Pb in invertebrates collected from streams flowing through mine tailings, and from vegetation growing on the tailings, suggest a potential for contamination of the food chain of the surrounding ecosystems.

Particulate flux at the bottom of Lake Ontario

Abstract Time-series sediment traps deployed at closely spaced intervals near the bottom of Lake Ontario show that the amount of material collected decreases exponentially with increasing distance from the sediments. At heights of 0.2 and 7 m above the bottom about 85% and 45% respectively of the suspended tripton is recruited from the sediments. The upward flux of particles is strongly correlated with speed of bottom currents, and the turn-over rate for particles at the sediment-water interface is estimated to be only a few hours. It is believed that sediment resuspension is an important phenomenon which can readily account for several puzzling features connected with the distribution of pollutant metals in the Great Lakes sediments.

Distribution of Major and Trace Elements in Sediments and Pore Water of Lake Erie

Abstract Concentration profiles of major and trace elements were determined in sediment cores from the Central Basin of Lake Erie. The concentrations of trace elements (As, Cd, Cr, Cu, Hg, Mn, Ni, Pb, and Zn) in the sediments were greatest at approximately 18 cm sediment depth, corresponding to the early 1960s. The concentrations of all elements in the sediments have considerably decreased in the last decade. However, the concentrations of Hg, Pb, and Cd are still about 10, 5, and 4 times greater, respec, than those in the pre-industrial sediments. The profiles of dissolved concentrations of the trace ele in pore water were characterized by maxima below the sediment-water interface, dropping off rapidly within the zone of reduced sediments. Concentrations of dissolved trace elements below the sediment-water interface are about one order of magnitude greater than those in the lake water. Conservative estimates of benthic fluxes ranged from 0.04 /μg/cm 2 .y to 194 /μg/cm 2 .y for Cd and Fe, respectively. Upward diffusive remobilization from sediments to lake water is a significant transport process in the Central Basin of Lake Erie and may play an important role in the transport of trace elements from the sediments.

Hypolimnion Flow Between the Central and Eastern Basins of Lake Erie During 1977 (Interbasin Hypolimnion Flows)

Abstract The shallow central basin of Lake Erie (mean depth 19 m) is separated from the deeper eastern basin (mean depth 28.5 m) by the Pennsylvania Ridge which extends southward from the base of Long Point to Erie, Pennsylvania. The crest of the ridge lies at a depth of 15 m except for a narrow notch near Erie, Pennsylvania, where communication between the two basins extends to 24 m depth. Both basins are stratified in summer, and the thin (2 m) hypolimnion of the central basin becomes depleted of oxygen by late summer. Flows to the hypolimnion of the central basin from the mid thermocline water of the eastern basin have been thought to be important sources of dissolved oxygen to the central basin. Data from ship cruises and current meter moorings made in 1977 have been used to form an estimate of the hypolimnion flux across the sill area. A strong correlation between winds and mean flux is observed and the dynamic balance appears to be one where surface pressure gradient created by wind stress is opposed by internal pressure gradients and by bottom friction. While the total quantity of oxygen transported by the subsurface flow is significant in terms of the later summer oxygen consumption of the central basin hypolimnion, its effects are confined to the eastern half of that basin due to the relatively weak horizontal diffusion in the mid-basin hypolimnion.

An Improved Dialysis Sampler for the in Situ Collection of Larger Volumes of Sediment Pore Waters

A new Volume Enhanced Sediment Porewater Sampler (VESPOS) for the in situ separation of pore water from aquatic sediments is described. This sampler offers several improvements over conventional in situ sediment pore water sampling devices, particularly for the collection of a large sample volume (30 ml); simplification in the assembly of the sampler and recovery of the sample; and, consequently, minimizing the risks of sample contamination. The large volume of pore water sampled within a 2-cm interval increases the analytical potential. The sampler significantly reduces the labour and equipment involved in sediment pore water sampling, particularly during retrieval of the samples.