Central Ontario Lakes Research Articles

Lead biogeochemistry in a central Ontario Forested watershed

To determine the sources and sinks of atmospherically deposited Pb at a forested watershed (Plastic Lake) in central Ontario, Canada, Pb pools and fluxes through upland, wetland and lake compartments were measured during 2002/2003 and compared with previous measurements taken between 1989 and 1991. In 2002/2003, annual bulk deposition of Pb was 0.49 mg m−2 compared with 1.90–1.30 mg m−2 in 1989–1991. Annual Pb concentrations in stream water draining the upland part of the catchment were very low (0.04 μg l−1) and were approximately half those measured in 1989–1991 (0.11–0.08 μg l−1). Leaching losses in stream water were small and mass balance estimates indicate almost complete retention (>95%) of atmospherically deposited Pb in upland soils. In contrast, annual Pb concentrations in stream water draining a wetland were between 0.38 and 0.77 μg l−1, with the highest concentration occurring in 2002/2003 and mass balance calculations indicate that the wetland is a net source of Pb in all measured years. Lead concentrations in the lake outflow were low and the average Pb concentration measured in 2002/2003 (0.09 μg l−1) was approximately half the value recorded in 1989–1991 (0.19 μg l−1 both years). Annual mass balance estimates indicate that the lake retained between 2.47 mg m−2 (1989/1990) and 1.42 mg m−2 (2002/2003) and that in 2002/2003 68% of the Pb input to the lake is derived from the terrestrial catchment. These estimates are higher than sediment core records, which indicate around 18 mg m−2 Pb was retained in sediment during the 1990s. Nevertheless, Pb concentrations decrease with sediment depth and 206Pb/207Pb concentrations increase with depth, a pattern also observed in mineral soils that reflects the substantial contribution of anthropogenic Pb to the watershed. Lead isotope data from soil and sediment indicate a recent anthropogenic Pb signal (206Pb/207Pb ∼ 1.185) in upper soils and sediments and an older anthropogenic signal (206Pb/207Pb ∼ 1.20) in deeper soil and sediment. Lead isotope data in sediment and vegetation indicate that practically all the Pb cycled in the forest at Plastic Lake is anthropogenic in origin.

Manganese cycling in central Ontario forests: Response to soil acidification

Soil acidification results in enhanced mobilization of many potentially phytotoxic metals, including Mn. In Canada, there is also concern that emissions of Mn from the gasoline additive methylcyclopentadienyl manganese tricarbonyl (MMT) may contribute to the environmental risk posed by Mn. As a first step toward understanding the risk posed by Mn in acidifying forest systems in Ontario, the authors sampled foliage and soil from 46 forest plots in central Ontario that encompassed a soil pH range of 3.9–8.1. Increased availability of Mn at lower soil pH was clearly demonstrated, as water-extractable Mn increased and molar ratios of Ca:Mn in foliage decreased at sites with lower soil pH, although Mn availability appears to be determined by both soil pH and Mn content in soil. The cycling of Mn was studied in detail at a representative acidified coniferous forest catchment (Plastic Lake) in central Ontario. Between 1980 and 2002, Mn bulk deposition at PC ranged from 3 to 7 mg m −2 a −1 and there was no evidence that Mn deposition increased over this time period despite increased usage of MMT. Temporal patterns of Mn concentrations in stream water draining the Plastic Lake catchment were highly correlated to SO 4 2 - levels in stream water ( r 2 = 0.90), indicating that acid leaching is currently driving Mn losses. Furthermore, Mn outputs in stream water draining the terrestrial catchment exceeded bulk deposition inputs, whereas the lake was a net sink for Mn, due to retention in lake sediment. Because Mn availability is determined by soil pH and the Mn content in soil, net losses of Mn from the terrestrial catchment indicate that Mn levels in soil and stream water will ultimately decline over the long-term, and therefore Mn is unlikely to have a negative impact on forests in this region.

Acidification at Plastic Lake, Ontario: Has 20 Years Made a Difference?

In response to reduced sulphur emissions, there has been a large decrease in sulphate (\( {\text{SO}}^{{2 - }}_{4} \); −0.97 μeq l−1 year−1) and hydrogen (−1.18 μeq l−1 year−1) ion concentration in bulk precipitation between 1980 and 2000 at Plastic Lake in central Ontario. The benefit of this large reduction in \( {\text{SO}}^{{2 - }}_{4} \) deposition on stream water chemistry was assessed using the gauged outflow from a conifer-forested catchment (PC1; 23.3 ha), which is influenced by a small wetland located immediately upstream of the outflow. Sulphate concentrations declined, but not significantly due to large inter-annual variation in \( {\text{SO}}^{{2 - }}_{4} \) concentration. Between 1980 and 2000, there were significant increases in dissolved organic carbon, ammonium and potassium concentration likely reflecting increased mineralisation in the wetland. Calcium concentrations in PC1 decreased during the two decade period (−2.24 μeq l−1 year−1), as a consequence there was no improvement in stream pH and the Ca:Al ratio in PC1 continued to decline. A similar response was noted in an upland-draining sub-catchment of PC1-08 that has been monitored since 1987. Despite large reductions in \( {\text{SO}}^{{2 - }}_{4} \) deposition and almost complete retention of nitrogen in soil, there has been no improvement (in terms of pH) in stream water at PC1 due to a combination of soil acidification and climatic (droughts, increased mineralisation) perturbations.

Long‐term trends in catchment export and lake retention of dissolved organic carbon, dissolved organic nitrogen, total iron, and total phosphorus: The Dorset, Ontario, study, 1978–1998

Annual catchment export of total phosphorus (TP), dissolved organic carbon (DOC), total iron (Fe), and dissolved organic nitrogen (DON) to seven lakes in central Ontario was measured between 1978 and 1998. Fluctuations in annual water discharge and total DOC load (including precipitation) to the lakes over the 20‐year period were similar in the seven study lakes. DOC export to the lakes responded proportionally to changes in discharge, decreasing during drier and warmer years. There were similar but less accentuated variations in annual DOC lake concentrations. There were no clear regional trends evident during the 20‐year period toward drier or wetter conditions, less DOC load, clearer lakes, etc., that could be interpreted as signaling a shift toward a different equilibrium state. The fraction of the DOC load retained by lakes (transferred to sediments and the atmosphere) increased during an extended dry period. Fe, TP, and DON export decreased more than DOC export during the extended dry period. Runoff appears to affect Fe, TP, and DON export first by controlling export of organic matter and second by affecting water table position and thus redox levels in the surface layer of peatlands. Permanently drier conditions with less runoff would likely lead to clearer lakes that are less productive. Conversely, increased runoff would lead to more colored and productive lakes. Both scenarios have implications for subsistence and sport fishing economies.

PREDICTING OCCURRENCES AND IMPACTS OF SMALLMOUTH BASS INTRODUCTIONS IN NORTH TEMPERATE LAKES

Smallmouth bass and other warmwater littoral piscivores are presently expanding their geographic range northward into lakes across southern Canada. Smallmouth bass introduction can dramatically reduce minnow abundances, causing native lake trout to shift to low quality, invertebrate-based diets. Here we develop models to predict future occurrences and impacts of smallmouth bass in central Ontario, with the goal of identifying “vulnerable” lakes in order to better guide prevention efforts. Using local and regional environmental variables for 3046 central Ontario lakes, an artificial neural network was used to predict lakes that are likely to be invaded by bass. Smallmouth bass can significantly influence the occurrence and abundance of small-bodied fishes (mainly minnows), and stable isotope analysis of food webs in 18 lakes revealed that lake trout are buffered from impacts of bass on minnows in lakes containing pelagic prey fishes. In the absence of pelagic prey fishes, the trophic niche of lake trout depends on the presence of bass; lake trout feed primarily on zooplankton in the presence of bass, and minnows in the absence of bass. Of the 3046 lakes, the 788 lake trout lakes in central Ontario were classified according to their vulnerability to bass invasion based on the predictability of bass occurrence and their subsequent impacts, and mapped in a Geographic Information System (GIS). Only 48 lake trout lakes (6%) were classified as “high vulnerability”—predicted to be invaded and impacted by bass. Another 301 lakes had a sensitive food web structure but were not predicted to support a bass population. Based on this information, efforts to prevent further impacts can be optimized by focusing on this vulnerable subset of lakes.

Effects of Recreational Fishing on the Population Dynamics of Lake-Dwelling Brook Trout

Data collected over 7 consecutive years were used to examine the population dynamics of the exploited brook trout Salvelinus fontinalis of Meach Lake in central Ontario. Numbers of age-1 and older trout averaged 975 among years. The largest trout were 490 mm long and weighed 1,807 g. Few trout lived beyond age 4. Annual mortality rates varied among age-classes and between sexes; average total instantaneous mortality was 0.95 and average total and fishing mortalities were 78% and 44%, respectively. Exploitation was greatest for age-2 and age-3 brook trout of both sexes. Catchability was uncorrelated with abundance. There was evidence of density-dependent growth at age-2 or age-3 but no indication of density-dependent mortality. A “hockey stick” model appeared to best describe the stock–recruitment relationship. Abundance of potential competitors was not correlated with brook trout size, although abundance of white suckers Catostomus commersoni was correlated with numbers of age-2 male brook trout. We conclude that total mortality rates are naturally very high and existing levels of harvests in this lake may not be increasing total mortality. To ensure the sustainability of brook trout fisheries, lake-dwelling populations should be managed to maintain total mortality rates below those observed in this study.

The influence of smallmouth bass (Micropterus dolomieu) predation and habitat complexity on the structure of littoral zone fish assemblages

Fish assemblages in small lakes ([Formula: see text]50 ha) in central Ontario were characterized to determine the impact of smallmouth bass (Micropterus dolomieu) predation and habitat complexity on the structure of littoral zone fish assemblages. Data were collected employing minnow traps and visual assessment. Although species richness did not differ between lakes with and without smallmouth bass, species composition and relative abundance did differ. We identified two distinct fish assemblage types: one characterized by small-bodied species, mainly cyprinids, and a second by large-bodied centrarchid species, e.g., smallmouth bass. Smallmouth bass appear to reduce abundance, alter habitat use, and extirpate many small-bodied species such as brook stickleback (Culaea inconstans), fathead minnow (Pimephales promelas), pearl dace (Margariscus margarita), and Phoxinus spp.

Staying Cool: Behavioral Thermoregulation during Summer by Young-of-Year Brook Trout in a Lake

Abstract Thermal habitat selection and behavior by young-of-year brook trout Salvelinus fontinalis was studied in a lake in central Ontario, Canada. In May, trout foraged actively within 2 m of shore in the warmest water available (∼15°C). In early June, trout foraged near the bottom within 4 m of shore, where bottom water temperatures were near, or at, the upper thermal tolerance, for trout, of 20°C. In July, when ambient water temperatures ranged from 23°C to 27°C, trout lay on the bottom in the coldest water available (18–20°C) in discrete areas 3–8 m from shore. Flow rate of cold groundwater accounted for 87% of the variance in trout density in these areas, and the data suggest that a minimum flow rate of 125 mL·m−2·min−1 is required for trout to take up station. When trout were displaced from holding positions, sites with greater groundwater flow were more quickly reoccupied by trout than sites with lower flow. Experimentally created trough-like depressions at these sites attracted higher densities o...

UVB effects on a plankton community: results from a large-scale enclosure assay

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 16:189-198 (1998) - doi:10.3354/ame016189 UVB effects on a plankton community: results from a large-scale enclosure assay Isabelle Laurion1,*, David R. S. Lean2, Warwick F. Vincent1 1Département de Biologie & Centre d'études nordiques, Université Laval, Ste-Foy, Québec G1K 7P4, Canada 2Department of Biology, University of Ottawa, Box 450, Station A, Ottawa K1N 6N5, Canada *Present address: Institute of Zoology and Limnology, University of Innsbruck, Technikerstr. 25, A-6020 Innsbruck, Austria. E-mail: isabelle.laurion@uibk.ac.at ABSTRACT: The long-term effects of natural and enhanced ultraviolet-B (UVB) exposure on picocyanobacteria, size-fractionated chlorophyll a (chl a), primary production (photosynthesis-irradiance parameters) and grazing activity on phytoplankton were investigated over a 20 d period in a mesotrophic lake in central Ontario, Canada. Large volume enclosures (20000 l) open to the sediments were installed in the littoral zone and sampled at 6 d intervals. Four radiation treatments were run: UVB-screened giving 0% incident UVB, natural ambient (100% UVB), and 2 lamp-enhanced treatments giving 112% biologically weighted UVB (range of 109 to 126% depending on cloud cover) and 118% biologically weighted UVB (114 to 138%). The light utilization efficiency of the phytoplankton (a) decreased under enhanced UVB, but there was no significant decrease in biomass (chl a or cell counts). Populations of the nanoflagellate Ochromonas sp. significantly increased under enhanced UVB; after 20 d the cell concentration of this species was ca 3.6 times higher relative to the natural UVB control. However, differences between treatments in picocyanobacteria, <2 μm chl a, other nanoplanktonic groups, maximum photosynthetic rates, sensitivity to photoinhibition and grazing activity were generally small relative to the variability between duplicate enclosures. These results indicate a low impact of natural and increased levels of incident UVB on the inshore plankton community in this type of lake environment. KEY WORDS: Ultraviolet-B · Photosynthesis · Phytoplankton · Enclosure · Temperate lake · Ochromonas · Grazing · Mixotroph Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 16, No. 2. Publication date: November 27, 1998 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 1998 Inter-Research.

Effect of landscape form on export of dissolved organic carbon, iron, and phosphorus from forested stream catchments

Predicting the effects of climate change and atmospheric deposition on water quality requires predicting the effects of landscape form on export of substances downstream. In this paper, we present dissolved organic carbon (DOC), total phosphorus (TP), and iron (Fe) export data (1980–1992) for 20 relatively undisturbed, forested catchments draining into seven lakes in central Ontario and develop regression models of chemical export as functions of landscape composition. The extent of wetlands was correlated with export of DOC and TP; the proportion of the catchment covered by peatlands accounted for 78% of the variance in a regression model of long‐term average DOC export and 76% of the variance in a model of color “export.” Peatland coverage and Fe export together explained 76% of the variance in a long‐term average TP export model, which is consistent with published experimental evidence that Fe facilitates P complexation with DOC in surface waters. The long‐term average Fe export model was not significant when all 20 catchments were included. However, Fe export from the 14 catchments with thin tills was a function only of peatland coverage (R2 = 0.71), suggesting that Fe export is dependent to a large extent upon either the export of organic material or the reducing conditions. The long‐term export models worked well when export of a substance was dominated by peatlands but was not very sensitive to the influence of mineral soils. The long‐term average TP/DOC ratio was remarkably constant among most whole‐lake catchments, ranging from 1.4 to 2.0 mg P/g C, the exception being the catchment of anthropogenically acidified Plastic Lake with a ratio of 0.8. Fe export to Plastic lake was also enriched relative to TP export compared with the other lakes. Therefore TP export to Plastic Lake may be limited by some mechanism related to acidification.

Use of a Remotely Operated Vehicle to Study Habitat and Population Density of Juvenile Lake Trout

Abstract We determined the feasibility of using a remotely operated vehicle (ROV) to observe juvenile lake trout Salvelinus namaycush. The ROV was equipped with a high-resolution, low-light, black-and-white video camera and two halogen headlamps and was tethered by a 152-m umbilical cable. We used the ROV to sample two central Ontario lakes: Source Lake in summer and fall and Lake Opeongo in summer. We surveyed the lake bottom at depths of 2.5–40 m in both lakes during day and night. The ROV traveled slightly above the substrate recording a field of view 1.8 m wide and 0.3–0.5 m high at a distance of 1.8 m in front of the ROV. Overall, 54,594 m2 of lake bottom were sampled, and 114 juvenile lake trout (<300 mm, total length) were observed. Juvenile lake trout exhibited minimal avoidance of the ROV with some individuals being observed for several minutes (mean, 41.6 s). The observed distribution suggested movement to shallower habitat at night. Mean (±SE) lake trout densities varied between the study lakes...

Coping with Water Quality Problems due to Hypolimnetic Anoxia in Central Ontario Lakes

Abstract Many lakes experience oxygen depletion in their hypolimnia during summer or winter stratification. This study investigates the remedial actions that are available to combat the harmful effects of hypolimnetic anoxia in three types of Central Ontario lakes with different trophic states. In more eutrophic lakes, e.g., Lake Wilcox, southern Ontario, much phosphorus accumulates in the hypolimnion during anoxia and represents ca. 60% of the annual P budget. To diminish the detrimental effects of such a high internal phosphorus load, withdrawal of the hypolimnetic waters, together with damming of the surface water outflow, has been suggested. In oligo- to mesotrophic Chesley Lake, on the Niagara Escarpment, phosphorus accumulation in the hypolimnion is only slight and metals with a binding capacity for phosphorus are available. Here a hypolimnetic oxygenation has been suggested to alleviate the stress on fish and also to prevent further P release from the sediments. Lakes in the District of Muskoka are softwater lakes because of their location on the Canadian Shield. Although the geochemistry of the catchment typically renders these lakes nutrient poor and oligotrophic, long-term development on their shores has led to phosphorus accumulation in the sediments that is released under anoxic conditions in some lakes. To prevent the deterioration of the water quality in these lakes, water quality models to manage and control future development in its watersheds, as those of the district municipality, should include anoxia and internal P recycling in a quantitative manner.

THE EFFECT OF EL NIÑO-RELATED DROUGHT ON THE RECOVERY OF ACIDIFIED LAKES

Although SO2 emissions and deposition rates havedeclined substantially since the implementation of sulphuremission control programmes in North America [1], recovery(measured as decreases in $${\text{SO}}_{\text{4}}^{{\text{2 - }}} $$ concentrations) of affected lakes in central Ontario has been much less substantial thananticipated based on the decrease in deposition. The slowrecovery is attributed to the reoxidation and release of storedsulphur in catchments. Reduced sulphur retained in previousyears when sulphur deposition was higher is exposed to air andoxidized during severe droughts, then exported duringsubsequent wet periods. Elevated stream $${\text{SO}}_{\text{4}}^{{\text{2 - }}} $$ concentrations and export rates occur in the autumns of yearswith prolonged severe droughts, particularly in catchments withextensive wetlands. Drought in our study catchments occurred inyears following strong El Nino events. When the SouthernOscillation Index (SOI) was strongly negative (1976–77, 1982–83,1986–87, 1991–92, 1993–94) the frequency of occurrence ofdrought the following summer in small catchments with shallowoverburden was extremely high. A lake‘s rate of recovery fromacidification depends upon the amount of excess reduced Sthat has been stored in anoxic zones in the catchment (largely afunction of the extent of wetlands) during years of elevated Sdeposition rates, and the frequency and severity of droughts. Iflong-term changes in global or regional climate alter thefrequency or magnitude of El Nino-related droughts, therecovery of acidified lakes will be affected.

Leeches as indicators of dietary mercury exposure in non-piscivorous waterfowl in central Ontario, Canada

We collected and analysed 113 leeches (Hirudinea) from 17 small lakes in the acid-stressed Muskoka region of central Ontario, Canada to examine the relationship between lake chemistry and mercury (Hg) concentrations in leeches, and thus determine whether leeches and other benthic invertebrates posed a dietary risk of Hg exposure for non-piscivorous waterfowl. Hg concentrations in leeches were generally low and only a few-fold above the detection limit (0.78 ng g −1 wet weight (ww)). Mean Hg concentration in the bloodsucker Macrobdella decora was 6.94 ± 0.78 SE ng g −1 ww ( n = 49) and was 5.98 ± 0.46 ng g −1 ww ( n = 64) in the scavenger Percymoorensis marmoratis. Leech Hg concentrations were correlated with calcium and dissolved organic carbon concentrations in the water, respectively. These data suggest that leeches are not suitable monitors of Hg (usually as methylmercury) biomagnification in central Ontario lakes, and do not pose a dietary risk to non-piscivorous waterfowl.

Dissolved organic and inorganic carbon mass balances in central Ontario lakes

Mass balances of dissolved organic carbon (DOC) and dissolvedinorganic carbon (DIC) based on stream and precipitation inputs andoutflows were measured for seven unproductive lakes in central Ontariobetween 1981 and 1989. Net annual CO2 evasion occurred in sixof the seven study lakes with minor net invasion in the seventh. Atmosphericinvasion might have been significant at certain times of the year, particularlyduring the growing season. Net evasion rates were greater than DIC loadingrates, indicating partial mineralization of the terrestrially-derived DOC in thelakes. A steady state mass balance model adequately described the variationin DOC retention between lakes. Net annual carbon accumulation of forestcommunities based on estimates of net ecosystem production may beoverestimated because of significant export of carbon to lakes via streamsand groundwater, particularly in catchments with extensive peatlands.

Long-term phosphorus budgets and an examination of a steady-state mass balance model for central Ontario lakes

The objective of this study was to evaluate, modify if necessary, and calibrate a mass balance steady-state total phosphorus (TP) model for use in the Precambrian Shield area of Ontario. Hydrologic data, TP mass balances and lake concentrations data were collected between June 1978 and May 1992 from seven oligotrophic and oligomesotrophic lakes located on the Precambrian Shield in central Ontario. Two of the lakes (Dickie and Harp) had substantial shoreline development; four lakes had alkalinities <25 μ eql −1. The gross settling or loss coefficient, v, was underestimated when internal load was ignored. Long-term mean v ranged about three-fold, from 4.7 m yr −1 in Plastic Lake to 13.7 m yr −1 in Dickie Lake with an average for six lakes of 7.9 m yr −1. The exception was Harp Lake where v was 23.2 m yr −1. The difference in v between heavily developed Dickie and Harp Lakes may have been due to the unusually thick glacial tills in the Harp catchment and their likely ability to retain at least some of the TP from septic effluent. The current inability to predict septic TP retention rates by soils/tills, the highly variable nature of soils/tills, and the decades required to reach equilibrium after installation leave environmental managers with little choice but to apply the “precautionary principle” to ensure adequate protection of inland recreational waters and assume that soils do not retain septic TP. There was no evidence that enhanced sedimentation of TP (“oligotrophication”) occurred in any of the three most acidic lakes. In fact, losses via sedimentation were lowest in these lakes. Average retention, R ((input-output)/input) of TP during the ice-free months of May to November was much higher than average R in winter (December–April) in all lakes. Lack of wind-induced mixing, low biological activity and thermal stratification caused by ice cover make it more likely that stream loads are discharged directly from lakes during winter. Seasonal variation in retention suggests that R and v should be higher in warmer climatic zones. There may be no globally valid v, although the data presented here suggest that a regionally applicable estimate of v may be appropriate. The model should only be calibrated with a long-term data set and should be only used to predict long-term, average responses to changes in average TP loading.

The Anoxic Factor, a Quantitative Measure of Anoxia and Fish Species Richness in Central Ontario Lakes

The detrimental effect of oxygen depletion on fish is well known. Here a quantitative measure of anoxia is compared with the number of fish species in 52 south-central Ontario lakes. The anoxic factor (AF, days per year or per season) estimates the number of days that a sediment area equal to the lake surface area is overlain by anoxic water. It can be expressed separately as summer or winter AE Stepwise multiple-regression models were constructed with AF and common physical and chemical variables to predict fish species richness. Because some of the parametric regression assumptions were violated, the traditional statistics were evaluated with randomization tests to confirm significance. The best regression models included summer AF or lake surface area, or winter AF and mean depth, for subsets of acidic and circumneutral lakes; these models explained up to 75% of the variation in fish species richness. One of the models can be used to estimate the likelihood of winterkill. Anoxic factor was also correlated with the number of coldwater fish species present. The concept of AF can be used to model the distribution and number of fish species in stratified lakes and should be useful in the management of oxygen-dependent aquatic animals (e.g., finfish and shrimp) in freshwater and marine habitats.

Quantifying anoxia in lakes

The anoxic factor (AF, days per year or per season) can be used to quantify anoxia in stratified lakes. AF is calculated from oxygen profiles measured in the stratified season and lake surface area (A0) as urn:x-wiley:00243590:media:lno19954061100:lno19954061100-math-0001 AF represents the number of days that a sediment area, equal to the whole‐lake surface area, is overlain by anoxic water. Average AF for 56 central Ontario lakes, 19 additional eastern North American lakes, including the Laurentian Great Lakes and several acidified lakes, ranged from 0 to 83 d per summer. Of this variation, 65% (54%) is accounted for by average phosphorus (nitrogen) concentration and an indicator of lake shape, z̄/A00.5 (where z̄ is mean depth). Only 17% on average was explained by annual dissolved organic C (DOC) concentration. Annual variation within lakes was large and partly ascribed to variable annual loading of DOC. AF was not correlated with areal hypolimnetic oxygen depletion, because of the opposing dependency on lake shape, but was correlated with Reckhow’s probability of anoxia in eutrophic lakes. AF was also correlated with the redox potential of anoxic water at the end of summer stratification, especially after inclusion of an estimate of iron concentration in the sediment.

Trace Element Patterns in Lichens Following Uranium Mine Closures

Instrumental neutron activation analysis was used to determine trace elements in Cladina mitis (Sandst.) Hale & Culb. along transects extendingfrom uranium mines at Elliot Lake and Agnew Lake in central Ontario, Canada. Levels of 11 elements were reported and the presence of uranium (U) was confirmed, although U concentrations were much less than in Cladina rangiferina 10 years earlier. Among the elements identified in lichen thalli was Th, which occurred in higher concentrations than U. All trace elements, including the two radionuclides, were found in deterio- rating thallus parts as well as living podetia, and five of these seem to have originated as airborne particulates from minesites. In spite of mine closures, levels of Th and U remained higher near sources of ore dust and there was little relationship between radionuclide concentrations in thallus

Effects of Experimental and Cultural Lake Acidification on Littoral Benthic Macroinvertebrate Assemblages

A simple method for the assessment of littoral benthic macroinvertebrate (BMI) assemblages in lakes was developed at the Experimental Lakes Area (ELA) in northwestern Ontario and applied to 64 lakes in central Ontario. The presence (1) or absence (0) of BMI taxa was established at five sites on each lake, using a kick-and-sweep net technique. Summing presence/absence scores across sites ranked the importance of each taxon in each lake on a scale of 0 to 5. Relationships between the BMI assemblages were assessed using nonmetric multidimensional scaling (NMDS), based upon the Kendall's τ correlation matrix. Correlations between NMDS dimension 1 and 2 scores and environmental variables identified factors that may structure BMI assemblages. The BMI assemblages of three experimentally acidified and seven reference lakes at the ELA were correlated strongly with lake pH. The BMI assemblage structure of central Ontario lakes was predicted by lake area and sensitivity to acidification (NMDS dimension 1) and by lake elevation (NMDS dimension 2). The BMI assemblages of small or Ca-poor and acidic lakes include fewer oligochaetes, mayflies, and Cryptochironomus and Stictochironomus (chironomids) and more Odonata, Trichoptera, Chironomus, Conchapelopia, Microtendipes, and Procladius (chironomids), and Crangonyx (Amphipoda) than other central Ontario lakes.