South-central Ontario Research Articles

Predicting tree survival in Ontario sugar maple (Acer saccharum) forests based on crown condition

Decline index (indicator of crown condition) data from 102 forest plots (approximately 10 000 trees) during 1986–2004 were compiled to derive survival models for south-central Ontario, Canada. The dominant species was sugar maple ( Acer saccharum Marsh.) with approximately 75% occurrence (n = 7640). The predictor variables for sugar maple survivorship included the decline index of 1 or 2 years prior to the beginning of the modelled period and ecological region (Algoma, Georgian Bay, Huron–Ontario, and Upper St. Lawrence). The observed crown condition of sugar maple improved significantly over the study period; in contrast, short-term mortality rate did not improve. The risk of sugar maple mortality could be predicted from decline index data for a single year indicating that the risk of tree death increases with higher decline index values (declining crown condition). Moreover, using 2 years of decline index data indicated that the risk of tree death also increased with the length of consecutive time individual trees have higher decline index values. Trees in the Algoma region, which represent the northern limit of sugar maple distribution in Ontario, were significantly more likely to die than trees in Huron–Ontario region.

Effectiveness of Piscicides for Controlling Round Gobies ( Neogobius melanostomus)

Round gobies (Neogobius melanostomus) were introduced to the Great Lakes presumably as a result of ballast water releases from seagoing freighters returning from European water bodies. These unwelcome fish have become established in the Great Lakes region and are expanding their range to suitable portions of other interior drainage basins including the Mississippi River traversing the central United States and the Trent-Severn waterway spanning south-central Ontario. If the invasion continues, use of chemical toxicants as a control measure may be necessary. Toxicity tests of the currently registered piscicides antimycin, rotenone, 3-trifluoromethyl-4-nitrophenol (TFM), and Bayluscide ® were conducted with three fish species native to the Great Lakes and round gobies collected from the Illinois Waterway. Tests indicated that round gobies are sensitive to all of the piscicides, however, the level of sensitivity is similar to that of the native fish species tested. Therefore, currently registered piscicides have limited potential to selectively remove round gobies. Bottom-release formulations of Bayluscide ® and antimycin were also evaluated as control agents for the normally bottom-dwelling round goby. Avoidance behavior tests demonstrated that the round goby did not react to the presence of either chemical. Therefore, the bottom-release formulations may have some application for the selective removal of round gobies, and may be one of the few tools presently available to fishery managers to help limit the range expansion of this invasive fish.

Response of benthic macroinvertebrate communities to El Niño related drought events in six upland streams in south-central Ontario

We examined the effects of extreme drought events on benthic macroinvertebrate (BMI) community structure in six forested upland streams in south-central Ontario, Canada, during a 9-year period. Variation in the mean winter El Niño – Southern Oscillation Index was strongly correlated with drought conditions (zero flow days) in the study streams. Drought onset and duration varied among study streams and among years. Below-average precipitation coincided with the occurrence of drought, although it remains unclear if snowfall and rainfall contributed equally to the impact of decreased precipitation. Increased relative abundance of Ephemeroptera, Plecoptera, and Trichoptera (EPT) one year following drought and decreased relative abundance two years after drought indicated high resistance but poor resilience. In contrast, chironomids showed poor resistance and high resilience. Although these patterns were not consistent across all streams, temporal coherence among streams was found in percent EPT, percent chironomids, and percent dipterans, suggesting that drought acts as a disturbance mechanism that simplifies benthos community assemblages. Biocriteria developed from 22 nearby reference streams indicated that abnormal BMI communities occurred only after recurring episodes of drought, indicating that the effects of drought are cumulative. Headwater streams may prove to be sentinel ecosystems for monitoring the impacts of climate change.

Influence of seasonal changes in runoff and extreme events on dissolved organic carbon trends in wetland- and upland-draining streams

Dissolved organic carbon (DOC) fluxes at eight headwater basins in south-central Ontario were strongly related to seasonal streamflow, and extreme events contributed to both interannual and intercatchment variability. Six catchments with high stream DOC and greater peatland coverage exhibited a different seasonal pattern of DOC concentration compared with two catchments with low DOC and less wetland influence. In wetland-dominated catchments, DOC concentrations decreased during fall wet-up and spring melt, and because of the dominance of the spring melt period in annual budgets, variations in spring flow explained 39%–48% of the intervariability in DOC concentration. Significant increases in average DOC concentration between 1980 and 2001 at all six wetland-dominated catchments were driven by relatively high DOC concentrations in the latter years of record, consistent with low spring flow in these years, and were not translated into greater DOC export to downstream lakes. Localized rainstorms in summer and fall resulted in differences in DOC export among adjacent catchments, and a single fall storm in September 1998 was only detected at one of six catchments draining into Harp Lake but accounted for one-quarter of the annual tributary DOC load to the lake.

Estimating calcium weathering rates and future lake calcium concentrations in the Muskoka–Haliburton region of Ontario

Calcium (Ca) concentrations in surface waters on the Precambrian Shield are determined primarily by the Ca weathering rate in soil, which requires extensive soil data that generally do not exist. From a water chemistry database comprising approximately 550 lakes in south-central Ontario, Canada, 130 lakes were selected with low Ca concentrations (Ca < 75 µmol·L–1). Calcium weathering is primarily dominated by silicate minerals such as plagioclase and hornblende, allowing the use of Ca–sodium (Na) ratios in lake water to estimate Ca weathering rates. Soil profile data at seven sites indicated that the Ca–Na ratio from mineral weathering is 0.86; correspondingly, Ca weathering rates in lakes ranged from 0.04 to 0.24 kmol·ha–1·year–1 (median of 0.09 kmol·ha–1·year–1). This compares with a range of 0.06–0.24 kmol·ha–1·year–1 (median of 0.14 kmol·ha–1·year–1) obtained using the steady-state water chemistry model. Using these methods to bound potential weathering rates, Ca concentrations in individual lakes at steady state are predicted to decline by 10%–40% compared with current values. Dynamic soil hydrochemical model predictions indicate that Ca concentrations in lakes will be considerably lower than these steady-state predictions within decades if timber harvesting occurs in the drainage basins.

Ecosystem and Seasonal Control of Stream Dissolved Organic Carbon Along a Gradient of Land Use

We examined the influence of watershed land use and morphology on dissolved organic carbon (DOC) concentration in 32 south-central Ontario streams having varying agricultural land-use intensities in their catchments. For streams in this region, both univariate and multivariate regression models identify the proportion of the watershed with poorly drained soils (r2 up to 0.67) as a better predictor of stream DOC concentrations than any other landscape characteristic, including the proportion of the watershed as wetland. Agricultural land use did not strongly influence DOC concentrations in our study area; however, we do show that land-use changes could significantly alter the delivery of DOC to streams in the region. We also identify how landscape–DOC relationships change over a 2-year time period, as related to season, regional climatic conditions, soil moisture, and hydrology. Our results indicate that the relationships between landscape predictors and stream DOC concentrations are temporally dynamic. Strong temporal trends are shown seasonally and in association with climate, through its control of modelled soil moisture conditions. During periods of positive and negative deviation from normal soil moisture conditions, the relationships of DOC concentrations with landscape characteristics become less predictable. We show that these dominant patterns are likely a function of varying flow paths and that anthropogenic changes that affect soil moisture conditions or flow path will in turn strongly influence DOC dynamics.

Providing supportive care to cancer patients: a study on inter-organizational relationships

Supportive cancer care (SCC) has historically been provided by organizations that work independently and possess limited inter-organizational coordination. Despite the recognition that SCC services must be better coordinated, little research has been done to examine inter-organizational relationships that would enable this goal. The purpose of this study was to describe relationships among programs that support those affected by cancer. Through this description the study objective was to identify the optimal approach to coordinating SCC in the community. Senior administrators in programs that provided care to persons and their families living with or affected by cancer participated in a personal interview. South-central Ontario, Canada. administrators from 43 (97%) eligible programs consented to participate in the study. Network analysis revealed a diffuse system where centralization was greater in operational than administrative activities. A greater number of provider cliques were present at the operational level than the administrative level. Respondents identified several priorities to improve the coordination of cancer care in the community including: improving standards of care; establishing a regional coordinating body; increasing resources; and improving communication between programs. Our results point to the importance of developing a better understanding on the types of relationships that exist among service programs if effective integrated models of care are to be developed.

FROG VIRUS 3-LIKE INFECTIONS IN AQUATIC AMPHIBIAN COMMUNITIES

Frog virus 3 (FV3) and FV3-like viruses, are members of the genus Ranavirus (family Iridoviridae), and they have been associated with infectious diseases that may be contributing to amphibian population declines. We examined the mode of transmission of an FV3-like virus, and potential hosts and reservoirs of the virus in a local amphibian community. Using the polymerase chain reaction to detect infected animals, we found an FV3-like virus in south-central Ontario, Canada, amphibian communities, where it infects sympatric amphibian species, including ranid and hylid tadpoles (Rana sylvatica, Hyla versicolor, and Pseudacris spp.), larval salamanders (Ambystoma spp.), and adult eastern-spotted newts (Notophthalmus viridescens). The high prevalence of FV3-like infections in caudate larvae suggests that salamanders are likely to be both hosts and reservoirs. In laboratory FV3 challenges of R. sylvatica, the rate of infection was dependent on the amount of virus to which the animals were exposed. In addition, although vertical transmission was suspected, horizontal transmission through exposure to infected pond water is the most likely route of infection in tadpoles. Based on our observations, a simple model of FV3/FV3-like virus transmission postulates that, in aquatic amphibian communities, transmission of the virus occurs between anuran and urodele species, with ambystomatid salamanders the most likely reservoir for the ranavirus in our study.

Long-term trends in dissolved organic carbon concentration: a cautionary note

Dissolved organic carbon (DOC) plays an important role in surface water chemistry and ecology and trends in DOC concentration have been also associated with shifts in terrestrial carbon pools. Numerous studies have reported long-term trends in DOC concentration; however, some studies consider changes in average measured DOC whereas other compute discharge weighted concentrations. Because of differences in reporting methods and variable record lengths it is difficult to compare results among studies and make regional generalizations. Furthermore, changes in stream discharge may impact long-term trends in DOC concentration and the potentially subtle effect of shifts in stream flow may be missed if only measured DOC concentrations are considered. In this study we compare trends in volume-weighted vs. average measured DOC concentration between 1980 and 2001 at seven headwater streams in south-central Ontario, Canada that vary in wetland coverage and DOC (22-year mean vol. wt.) from 3.4 to 10.6 mg l−1. On average, annual measured DOC concentrations were 13–34% higher than volume-weighted values, but differences of up to 290% occurred in certain years. Estimates of DOC flux were correspondingly higher using measured concentration values. Both measured and volume-weighted DOC concentrations increased significantly between 1980 and 2001, but slopes were larger in measured data (0.04–0.35 mg l−1 year−1 compared with 0.05–0.15 mg l−1 year−1) and proportional increases at the most wetland-influenced sites ranged from 32 to 43% in volume-weighted DOC and from 52 to 75% in measured DOC. In contrast, DOC flux did not change with time when estimated using either method, because of the predominant influence of stream flow on DOC export. Our results indicate that changes in stream flow have an important impact on trends in DOC concentration, and extrapolation of trend results from one region to another should be made cautiously and consider methodological and reporting differences among sites.

Morphology and Composition of Two Late Wisconsinan Soils Forming in Till and Lacustrine Deposits, Scarborough Bluffs Area, South-Central Ontario

Halton Till and Glacial Lake Iroquois lacustrine sand and gravel deposits are the major surficial materials exposed at the surface of Scarborough Bluffs in South-Central Ontario. Luvisols formed in these deposits have different morphologies, including depth of weathering, complexity of horizonation, and strength of structural grades which result from parent material differences and pedogenesis. Particle size variations between the two paleosols result, in part, from different modes of deposition, and show that variable amounts of clay were produced pedogenically in the two systems. Clay mineral genesis, involving the transformation of illite and illitesmectite to vermiculite, appears to be restricted to the Iroquois sand paleosol, while some chloritization of illite occurs in both profiles. Changes in the primary mineral contents in the two paleosols suggest a similar magnitude of weathering in both systems. Distributions of vermiculite and dithionite-extractable Fe suggest some preweathering effects in the Halton Till paleosol. Morphological, mineralogical and some soil chemical properties are closely related to the physical attributes of the two different parent materials (till vs lacustrine sand and gravel).

Soil Development as a Function of Time in the Rouge River Basin, South-Central Ontario

Three soils of postglacial age, representing a chronosequence in the Rouge River Basin of south-central Ontario, were studied to determine variations in morphological, physical, chemical, and mineralogical properties. These soils, forming in alluvium of mixed mineralogy represent the Late HoIocene (Entisol), Middle Holocene (lnceptisol),and Early Holocene (lnceptisol) surfaces. Horizon differentiation and soil thickness increase with greater age, while pH drops slightly in the sola of the older profiles. Cation exchange capacity (CEC) and organic matter in the surface horizons increases slightly with age, while, for the most part, CaCO3 and Ca/ Mg ratio decrease. Significant changes occur over time, especially with an increase of dithionite-extractable iron with age (from ~ 0.20 in the Late Holocene soil to ~ 0.70 in the Middle to Early Holocene profiles). Illite and illite-smectite tend to decrease with age, being replaced by small amounts of chlorite and vermiculite. In spite of some variability in parent materials due to stratification and preweathering, and minor changes in species composition, the changes in soil properties are attributed to processes of soil formation acting over time.

The Bedrock Surface of the Western Lake Ontario Region: Evidence of Reactivated Basement Structures?

Lower Paleozoic bedrock strata, in south-central Ontario and the adjacent part of New York State are covered by a thick (100m+) blanket of Pleistocene glacial and interglacial sediments. The form of the buried bedrock surface has been reconstructed from 70,000 waterwell boreholes that extend through the entire Pleistocene cover using GIS data processing techniques. The sub-drift bedrock surface shows linear channels that connect the basins of lakes Huron, Ontario and Erie and which form part of an ancestral mid-continent Great Lake drainage system prior to modification and infilling during successive Pleistocene glaciations. This relict drainage system is cut across Lower Paleozoic carbonates and elastics up to 500 m thick, but the position of several channels is aligned above terrane boundaries, faults and other deep-seated and poorly understood geophysical anomalies in underlying mid-Proterozoic Grenville basement rocks. Other channels are controlled by a dominant northwest and northeast trending regional joint system. A close relationship among deeply seated geophysical lineaments, basement structures and topographic lineaments cut across thick Paleozoic cover strata suggests a history of Phanerozoic reactivation and upward propagation of fractures from the Precambrian basement. Several basement structures and lineaments are seismically active suggesting ongoing neotectonic activity across the 'stable' craton of south-central Ontario.

Stratigraphic and Structural Framework of Upper Middle Ordovician Rocks in the Head Lake-Burleigh Falls Area of South-Central Ontario

Field investigations in the Head Lake-Burleigh Falls area of south-central Ontario, that focused mainly on the Covey Hill(?), Shadow Lake, Gull River and Coboconk formations along the Paleozoic escarpment, provided clear evidence of faulting. Observed and inferred structural deformation, coupled with faciès changes within the Shadow Lake and lower Gull River, point to a succession of basement movements during the Phanerozoic. These range from Hadrynian-Early Cambrian, through Middle Ordovician to post-late Middle Ordovician times. Some of the earlier movements (Hadrynian-Early Cambrian to late Middle Ordovician) appear to be coincident with, and probably related to, plate tectonic events and the associated Taconian orogeny that were in progress along the southeastern margins of the North American continent. Post-Middle Ordovician block faulting, on the other hand, may have been triggered by any number of epeirogenic events related to late stage Taconian, Acadian or Alleghanian orogenies in Late Ordovician to Carboniferous times, or possibly to rifting associated with continental break-up and initiation of seafloor spreading processes in the early Mesozoic. Manuscrit révisé accepté le 5 août 1993

Cladocera assemblages from the surface sediments of south-central Ontario (Canada) lakes and their relationships to measured environmental variables

Fossil cladoceran remains preserved in surface sediment samples from 44 oligotrophic lakes in south-central Ontario were examined to evaluate the relationships between species assemblages and measured environmental variables. Differences in cladoceran assemblages were related to physical and chemical variables using multivariate techniques. Redundancy Analysis (RDA) identified five environmental variables as significantly influencing assemblage composition: sulphate (SO42−), calcium (Ca2+), pH, maximum lake depth (Zmax) and dissolved organic carbon (DOC). There was a distinct separation of lakes and taxa along the ion gradient based on SO4, Ca and pH. Additionally, cladoceran communities in coloured, shallow lakes had relatively higher abundances of littoral chydorid species and the pelagic taxa Holopedium spp., and the Daphnia pulex complex. Deep, clear lakes had relatively higher abundances of other pelagic taxa. Predation by fish (measured as presence–absence) and Chaoborus (measured as density) were less significant than some of the physico-chemical variables in influencing cladoceran assemblage structure. However, this could be due to the limited resolution of the predation data that was available at the time of this study. The distribution of cladocerans in the surface sediment, and their relation to these important environmental variables, suggests that there is considerable potential for the use of sedimentary cladoceran remains as environmental indicators in south-central Ontario lakes.

Land use controls nutrient excretion by stream invertebrates along a gradient of agriculture

We examined the relationship between land use in the watershed and nutrient release by mayflies in streams in southcentral Ontario (Canada). Mayfly excretion rates and molar ratios of dissolved organic C (DOC), NH4+, and soluble reactive P (SRP) were measured in 6 streams flowing through watersheds with contrasting agricultural land use. Mass-specific release rates of NH4+ and SRP by mayflies and % agricultural land use (% agriculture) in the watershed were strongly positively related (r2 = 0.48, p < 0.01 and r2 = 0.58, p < 0.05, respectively). Changes in excretion rates and ratios probably were caused by lower periphyton C:N and C:P ratios associated with elevated total dissolved N and total P concentrations (r2= 0.30, p < 0.05 and r2= 0.38, p < 0.001, respectively) in streams in watersheds with high % agriculture. Mayfly mass-specific excretion rates and periphyton elemental composition were related to % agriculture, but the relatively constant body C, N, and P content in mayflies across all streams suggests strong homeostasis in these consumers. Watershed land use alters nutrient loading, which influences periphyton elemental composition and, ultimately, stoichiometry of nutrient fluxes through invertebrate consumers.

The contribution of rain-on-snow events to annual NO 3-N export from a forested catchment in south-central Ontario, Canada

The contribution of rain-on-snow (ROS) events to NO 3-N levels in stream water has received relatively little research attention. However, individual ROS events during January and February contributed up to 40% of annual NO 3-N export from a forested catchment in south-central Ontario between 1980 and 2000, but comprised less than 10% of annual precipitation. Nitrate-N concentrations in stream water increased rapidly following ROS events, and were similar to the concentrations in incident rainfall and the accumulated snow pack, likely due to limited contact of runoff with mineral soil under snow cover and low winter biological activity. Increased NO 3-N associated with ROS events resulted in substantial depressions in stream pH and alkalinity, which may delay the biological recovery from acidification. The contribution of ROS events to annual or winter NO 3-N export has been generally greater in recent years, although there is a considerable year-to-year variation. As a result, ROS events contribute to inter-annual variability in stream NO 3-N concentrations and will have a strong effect on apparent temporal trends. The contribution of ROS events to annual NO 3-N export should be considered when assessing surface water recovery from acidification and the N-status of forests, particularly if climate change projections for winter warming result in a greater proportion of winter precipitation occurring as rain.

Drought-induced sulphate release from a wetland in south-central Ontario

Increased sulphate (SO(4)) export from wetlands following summer droughts in central Ontario, Canada has been associated with the delayed chemical recovery of downstream surface waters following decreased sulphur (S) emissions. Prolonged summer droughts result in a decrease or cessation of stream flow, declines in wetland water table level and oxidation of reduced S compounds to SO(4), which is subsequently flushed into drainage streams when stream flow resumes. Sulphate input-output budget calculations (1983-1995 and 1999-2001) at a conifer Sphagnum swamp in the Plastic Lake catchment, indicate that SO(4) is retained in most years but is exported on a net basis following particularly severe summer droughts that result in the cessation of stream flow for more than 54 days (95% CI: 41-72 days). Hindcast calculations using long-term (1916-2000) stream discharge records from a nearby station indicate that while droughts occurred frequently in south-central Ontario over the past 85 years, sufficiently dry conditions to cause net SO(4) export occurred in only 18 of the past 85 years, and indicate a cumulative positive SO(4) balance for the swamp (i.e. net SO(4) retention). Furthermore, the S pool at the Plastic Lake swamp has been estimated to be approximately 1500 kg S/ha in the upper 40 cm peat layer, which is large compared to the amount of net SO(4) export that occurs even in years with particularly dry summers (e.g. -43 kg S/ha in 1987/88). Together, these data suggest that the wetland S pool at Plastic Lake has not been depleted by previous droughts and will continue to sustain episodic drought-related SO(4) export for the foreseeable future.

The Rise and Fall of the Waterloo-Wellington Ale Trail: A Study of Collaboration within the Tourism Industry

Partnerships are becoming increasingly important in the tourism industry especially for small and medium sized enterprises. The development of an ale trail, similar to a wine route, represents an important tool to facilitate the building of partnerships for organisations linked to culinary tourism. The purpose of this paper is to investigate the rise and fall of the Waterloo-Wellington Ale Trail located in south central Ontario, Canada. Qualitative analysis of key informant interviews is presented to examine why the decision was made to discontinue the Ale Trail despite its success.

Ethnicity and Class, Transitions Over a Decade: Ontario, 1861‑1871

Suite aux travaux pionniers de Pentland, un grand nombre d'historiens canadiens du 19e siècle ont postulé l'existence d'un marché du travail partagé pendant la période cruciale s'étendant de 1850 à 1870. D'après cette hypothèse, on assiste à cette époque à la création d'un prolétariat sans terres, élément essentiel à la transformation du pays, d'une économie agricole et commerciale à une économie industrielle et capitaliste. Ce groupe naissant était partagé suivant l'ethnie, les travailleurs Anglais et Ecossais occupant les emplois spécialisés, les Irlandais étant relégués aux emplois non spé- cialisés. On soutient que ce partage interne, s ajoutant aux salaires élevés versés aux travailleurs les plus prévilégiés, permit au Canada d'éviter en grande partie le radi- calisme ouvrier, alors que la réserve de personnel compétent épargna aux industries canadiennes les coûts élevés de la formation. Cependant une étude des recensements de 1861 et 1871 dans le centre-sud de l'Ontario indique que, en majorité, les immigrants irlandais catholiques et protestants travaillaient non pas à des emplois industriels peu rémunérés, mais bien à Vagriculture. Une enquête informatisée sur 10,000 hommes cou verts par les deux recensements et dont l'emploi est connu, fournit des renseignements sur un certain nombre de questions. L'analyse des données révèle les différences suivantes dans la composition de la population active masculine entre 1861 et 1871: a) la proportion de personnes nées en Ontario passer de 27 à 40 pour cent, alors que le nombre de personnes nées en Irlande baisse de 24 à 15 pour cent; la proportion de personnes nées en Ecosse et en Angleterre demeure essentiellement inchangée: b) pour ce qui est des emplois, il y aun mouvement du travail d'ouvriers vers iagriculture et, dans une moindre mesure, vers les occupa- tions artisanales et non-manuelles; la grande majorité des fermiers (84 pour cent en 1871) est propriétaire et non locataire; c) ni l'ethnicité ni la religion ne déterminaient l'occupation; à l'exception d'une concentration d'Irlandais d'origine dans les milieux ouvriers, la population active ne semble pas avoir été départagée suivant l'ethnicité ou Vappartenance religieuse. Une proportion relativement élevée de travailleurs nés en Ontario (30 pour cent en 1861 et 25 pour cent en 1871) était constituée de fermiers et de leurs fits; d) pendant cette période les différences entre nationalités au sein des groupes occupationnels s'aténuent, quoique les Irlandais catholiques continuent d'être sur- représentés chez les ouvriers, tout comme les Anglais d'origine dans les occupations bourgeoises et artisanales. S'il y a une certaine spécialisation occupationnelle par groupe ethnique, la concertration n'est pas assez prononcée pour appuyer l hypothèse selon laquelle la population était profondément partagée suivant l'ethnicité; e) une analyse des emplois par groupes d'âge laisse croire qu'il n'y a pas de mouvement d'abandon de l'agriculture et, conséquemment, pas de prolétarisation de la population au cours des années 1860. Au contraire, il y eu un accroissement de la proportion de fermiers et une diminution de la proportion d'ouvriers. Chaque groupe occupationnei suit un modèle de croissance particulier, avec des variations dans le temps; f) i établisse- ment de liens entre les données des deux recensements permet certaines conclusions sur la question de la permanence dans l'emploi. Près de 90 pour cent de ceux qui étaient fermiers en 1861 se réclamaient du même groupe occupationnei une décennie plus tard, alors que seulement un tiers des ouvriers faisaient de même. Dans l'ordre, les artisants, les professionnels et les marchands/fabricants se situent au centre de l'échelle pour ce qui est de la permanence. Un nombre relativement peu élevé de fils de fermiers (7 pour cent) sont devenus ouvriers ou artisans. Le groupe occupationnei le moins stable est celui des travailleurs non-manuels, où seulement 30 pour cent ont gardé le même genre d'emploi. Les données indiquent une mobilité occupationnelle assez élevée dans le temps, et l'absence de dépeuplement rural ou d'une crise de l'agriculture; g) sauf pour les travailleurs non-spécialisés, la mobilité semble dépendre plus de l'occupation que de l'âge. Le taux de mobilité est le plus élevé pour les travailleurs âgés de moins de 25 ans, particulièrement chez les ouvriers, alors qu'il est moins prononcé entre 25 et 54 ans. Au delà de cet âge, seuls les artisans ont une certaine mobilité. En tenant compte de l'ethnicité dans les calculs, un seul phénomène important peut être identifié: les ouvriers Irlandais catholiques avaient plus fortement tendance à demeurer dans cette catégorie pendant assez longtemps. A une époque où Vindustrialisation urbaine prenait de l'ampleur, la production indé- pendante de produits de base pas des familles d'agriculteurs devint également plus courante. La mobilité occupationnelle était généralisée, ce manifestant particulière- ment du groupe des ouvriers, vers celui des agriculteurs.

Climate variability and forecasting surface water recovery from acidification: Modelling drought-induced sulphate release from wetlands

Climate-induced drought events have been shown to have a significant influence on sulphate (SO 4 2−) export from forested catchments in central Ontario, subsequently delaying recovery of surface waters from acidification. Field and modelling studies have demonstrated that water table drawdown during drought periods promotes oxidation of previously stored (reduced) sulphur (S) compounds in wetlands, with subsequent efflux of SO 4 2− upon re-wetting. Although climate-induced changes in processes are generally not integrated into soil-acidification models, MAGIC (Model of Acidification of Groundwater in Catchments) includes a wetland compartment that incorporates redox processes driven by drought events. The potential confounding influence of climate-induced drought events on acidification recovery at Plastic Lake, south-central Ontario (under proposed future S emission reductions) was investigated using MAGIC and two climate scenarios: monthly precipitation and runoff based on long-term means (average-climate scenario), and variable precipitation and runoff based on the past 20 years of observed monthly data (variable-climate scenario). The variable-climate scenario included several periods of summer drought owing to lower than average rainfall and higher then average temperature. Nonetheless, long-term regional trends in precipitation and temperature suggest that the variable-climate scenario may be a conservative estimate of future climate. The average-climate scenario indicated good recovery potential with acid neutralising capacity (ANC) reaching approximately 40 μmol c L − 1 by 2020 and 50 μmol c L − 1 by 2080. In contrast, the forecasted recovery potential under the variable-climate scenario was very much reduced. By 2080, ANC was forecasted to increase to 2.6 μmol c L − 1 from − 10.0 μmol c L − 1 in 2000. Elevated SO 4 2− efflux following drought events (introduced under the variable-climate scenario) has a dramatic impact on simulated future surface water chemistry. The results clearly demonstrate that prediction of future water quality, using models such as MAGIC, should take into account changes or variability in climate as well as acid deposition.