South-central Ontario Research Articles

Trends in submersed aquatic plant communities in a large, inland lake: impacts of an invasion by starry stonewort (Nitellopsis obtusa)

Ginn BK, Dias EFS, Fleischaker T. 2021. Trends in submersed aquatic plant communities in a large, inland lake: impacts of an invasion by starry stonewort (Nitellopsis obtusa). Lake Reserv Manage. 37:199–213. Aquatic plant and macroalgae (collectively, macrophyte) communities from Lake Simcoe (Ontario, Canada) were studied in lakewide, >200 site surveys in 2008, 2013, and 2018. Over this period, mean macrophyte biomass increased 5-fold, from 29.9 g (dry)/m2 in 2008 to 153.9 g (dry)/m2 in 2018, due to the arrival and expansion of invasive starry stonewort (Nitellopsis obtusa). First recorded in Lake Simcoe in 2009, starry stonewort has greatly altered the macrophyte community, particularly in shallow (<3 m) water where it outcompeted invasive Eurasian watermilfoil (Myriophyllum spicatum). By 2018, starry stonewort comprised 67.6% of the total macrophyte biomass in Lake Simcoe. In shallow, mesotrophic Cook’s Bay, comparison to studies from the 1980s shows an increased plant biomass due to increased water clarity, from phosphorus (P) abatement and invasive dreissenid mussels, with further increases after 2011 due to starry stonewort. Starry stonewort may continue to impact nearshore ecology, with shallow-water fish species losing habitat and refugia as the “forest-like” structure of the plant community is replaced by large, dense aggregations of starry stonewort. Recreational uses will also be impaired and landowner complaints of macrophyte wash-ups will increase, with municipalities and lake-based businesses bearing the cost of mitigation and control strategies. Future research should consider the impacts of starry stonewort to P cycling as, unlike aquatic plants that uptake sediment P, macroalgae use dissolved P as a nutrient source. A lack of communication and reporting on starry stonewort has enabled its spread through south-central Ontario and the Great Lakes Region. Moving forward, we need a better understanding of starry stonewort biology and need to develop effective control and management strategies.

Effects of road-salt application on Cladocera assemblages in shallow Precambrian Shield lakes in south-central Ontario, Canada

Road-salt application is an integral part of winter road maintenance and safety in many cold regions. However, chloride (Cl−) runoff from salting activities may have adverse ecological effects. Lakes affected by road salt, particularly in urban areas, may have Cl− concentrations up to 300% higher than natural conditions. Zooplankton are sensitive to changes in water chemistry; however, little research has been conducted thus far into the biological effects of elevated Cl− concentrations from road-salt runoff. Using paleolimnological analyses, we studied the long-term effects of Cl− from road salt runoff on shallow, soft-water lakes within the southern Canadian Shield. Specifically, we examined Cladocera subfossils in dated sediment cores from 6 lakes within the Muskoka River Watershed of south-central Ontario, Canada. We compared long-term changes in cladoceran assemblages in the 5 lakes with the highest regional Cl− concentrations (32.8–90.9 mg/L) with a reference lake (0.91 mg/L). Prior to salting (ca. 1950), all of the lakes were dominated by Bosmina spp., and cladoceran assemblages showed little variation over time. After salting began, species assemblages in the 5 impacted lakes showed distinct taxonomic shifts, including increases in relative abundances of Chydorus brevilabris (Frey, 1980), Eurycercus spp., and the Daphnia pulex complex, and decreases in the relative abundance of Bosmina spp. and Alona spp. In 2 of the lakes, these changes were distinct and coincided with the onset of salting activities. In contrast to the species changes observed in the affected lakes, we recorded minimal directional changes in cladoceran assemblages in the nearby reference site. Overall, biological changes in the impacted lakes were best explained by changing Cl− concentrations associated with road-salt application. Among lakes, the severity of impact may have been indirectly mediated by differences in food quantity through its effect on Cl− toxicity.

Contrasting long-term trends of chloride levels in remote and human-disturbed lakes in south-central Ontario, Canada

Yao H, Paterson AM, James AL, McConnell C, Field T, Ingram R, Zhang D, Arnott SE, Higgins SN. 2020. Contrasting long-term trends of chloride levels in remote and human-disturbed lakes in south-central Ontrario, Canada. Lake Reserv Manage. XX:XXX–XXX. We examined a 41 yr data record from 4 monitored lakes in south-central Ontario, Canada, to demonstrate and explain long-term changes in chloride (Cl) loading and in-lake concentrations in relation to the degree of watershed disturbance. A mass balance calculation for each lake was used to determine ungauged shoreline loads. Correlations of lake Cl concentrations with each of 3 major contributing sources (inputs from inflowing streams, atmospheric deposition, and ungauged shoreline areas) revealed the contribution of each source to Cl trends at each lake. Areal loads (Cl load per year per unit area) showed the relative intensity of the contributions. Two lakes (Red Chalk, Plastic) with relatively undisturbed watersheds showed declining trends in atmospheric deposition, stream loads, lake export, and lake Cl concentrations, whereas 2 lakes (Harp, Dickie) with disturbed watersheds showed increased stream loads, lake export, and lake concentrations, despite long-term declines in atmospheric deposition. This contrasting trend was related to increased Cl concentrations in inflowing streams, which were caused by increased residential development and road salt applications. The ungauged shoreline loads in the disturbed watersheds made a contribution as high as 80% to total Cl loads. As increasing Cl concentrations have implications for aquatic biota, even at the relatively low concentrations observed in the study lakes, these findings highlight the need for improved management of road salt applications. Our results confirm that even relatively remote lakes within the Canadian Shield are potentially at risk from watershed disturbances that are disrupting the natural loading of Cl to inland waters.

Population movements of the Huron-Wendat viewed through strontium isotope analysis

Environmental isotopes can provide information about the composition of groups and the movement of people across landscapes. The archaeological record of Huron-Wendat communities in south-central Ontario is one of numerous drainage-based sequences of small villages among which families or larger population segments moved. These villages amalgamated in the early to mid-sixteenth century into fewer, larger communities. Strontium isotope values (87Sr/86Sr) are used to test hypotheses about these early localized interactions and later amalgamations. There is little prior information about strontium values from this region which was recently glaciated and receives ample precipitation. From the late thirteenth century onward, ancestral Huron-Wendat communities had distinct burial practices of primary burial followed by secondary, collective ossuary burial. Strontium values from tooth enamel of 118 human first permanent molars from 15 archaeological contexts spanning four centuries are interpreted in a framework of archaeologically derived deer (N = 34), small (N = 35) and other large (N = 7) mammals. Reflecting their local origins, 87Sr/86Sr values of small mammals from three geological substrates differ significantly from one another. Each small mammal group clusters more tightly than those of deer. Most human 87Sr/86Sr values agree with small mammal values, by region. Three sites, out of six with more than 10 data points, show mean human 87Sr/86Sr values that differ significantly from the small mammal values of their geological substrate, signaling community movement and individual in-migration. Interpretation of individual human values outside local ranges is informed by information from dietary isotopes. Environmental isotopes substantiate and enhance our prior understandings of the ancestral Huron-Wendat.

Spatio-temporal patterns of occupation and density by an invasive fish in streams

Since its introduction to North America in the 1990s, the round goby has spread throughout the Great Lakes and inland movement into small tributaries, a new environment for this species in both its native and invaded ranges, is becoming more prevalent. We assessed spatial and temporal occupation of round gobies in four small streams in two systems in south-central Ontario, Canada to determine which habitat variables are the best predictors of round goby density. Two streams are tributaries of Lake Ontario and two are tributaries of the Otonabee River; all of these streams have barriers preventing upstream migration. Occupation and density patterns differed between the systems. In the Lake Ontario system, density was highest near the longitudinal centre of the stream between the mouth and first barrier, and round goby appeared to occupy these streams mainly from spring to fall. In the Otonabee River system, round goby occupied the streams year-round, and the most important factor determining adult density was distance from a barrier to upstream movement, with density highest next to the barriers. Adult density in Lake Ontario tributaries was highest in sites with a high percentage of cobble/boulder and a low percentage of gravel substrate, while substrate was less important in Otonabee River tributaries. Date was the strongest predictor of juvenile density, with density increasing through the sampling period in both systems. Occupation and density patterns may differ due to contrasting environmental conditions in the source environments and distance to the first barrier preventing upstream movement. This study shows diversity in invasion strategies, and provides insight into the ability of invasive species to change their occupation strategies based on the different environments they encounter.

Modifying SWAT-CS for simulating chloride dynamics in a Boreal Shield headwater catchment in south-central Ontario, Canada

Rising chloride concentrations in surface water due to applications of deicing practices is proving detrimental to aquatic systems. In this study, a new chloride module is developed for a version of the Soil and Water Assessment Tool specially designed for Canadian Shield catchments (SWAT-CS) to model long-term chloride dynamics in a headwater catchment in south-central Ontario, Canada. In this modified model (SWAT-CS-CL; extended SWAT-CS model for chloride), chloride sources, sinks, internal storages or pools, and movement between these components are depicted. Performance of SWAT-CS-CL is assessed using a two-stage evaluation process based on the generalized likelihood uncertainty analysis (GLUE) framework. SWAT-CS-CL was found to perform moderately well, with simulated monthly chloride in streams and lake outflow following overall chloride trends and capturing regular chloride dynamics. However, simulations fail to consistently reproduce some instances of large or low chloride fluxes. Limitations in simulating large chloride fluxes may be attributed to the inadequate ability for SWAT-CS-CL to closely simulate snowpack and snowmelt processes. Parameter transferability among sub-catchments does suggest that there is a potential to extend SWAT-CS-CL to other Canadian Shield catchments for chloride modelling. Further improvements are needed through more trials to other catchments in a same or different landscape, and by modifying the simulation structure, especially representation of snow hydrology and chloride inputs.

Long-term trends in hypolimnetic volumes and dissolved oxygen concentrations in Boreal Shield lakes of south-central Ontario, Canada

Temperature–oxygen profiles, collected biweekly to monthly for ∼40 years, were used to calculate end-of-summer volume-weighted hypolimnetic oxygen (VWHO) concentrations in six small lakes located in south-central Ontario, Canada. Coherent decreases in thermocline depth and increases in hypolimnetic volume, mean hypolimnetic dissolved oxygen (DO) concentration, and VWHO were observed in five of the six study lakes. All lakes underwent an abrupt increase in VWHO and mean hypolimnetic DO after 2010. In four of the six study lakes, the highest hypolimnetic DO concentrations were observed in years where chlorophyll a concentrations were low, whereas at five of the six study lakes the highest hypolimnetic volumes were observed when dissolved organic carbon concentrations were relatively high. Warmer spring or winter air temperatures were associated with higher hypolimnetic DO concentrations at two sites, and longer ice-free periods were associated with smaller hypolimnetic volumes at two sites. These results suggest that the recent VWHO increases in the studied south-central Ontario lakes may be a function of multiple drivers that include changes in primary production, lake water transparency, and regional climatic factors.

Importance of rainfall partitioning in a northern mixed forest canopy for soil water isotopic signatures in ecohydrological studies

AbstractThe forest canopy can play a significant role in modifying the amount and isotopic composition of water during its passage throughout the near‐surface critical zone. Here, partitioning of gross rainfall into interception, throughfall, and stemflow and its implications for the amount and isotopic composition of soil water was studied for red oak, eastern white pine, and eastern hemlock trees in a northern hardwood‐conifer forest in south central Ontario, Canada. Stemflow production was greatest for red oak as a result of its upward‐projecting branches and least for eastern white pine due to its horizontal branches and rougher bark. These stemflow contributions to the near‐bole soil surface failed to produce consistently wetter soils relative to distal locations from the bole for all tree species. There was also no consistent evidence of isotopic enrichment of throughfall and stemflow relative to gross rainfall or of stemflow relative to throughfall for red oak or eastern hemlock. However, there was isotopic enrichment of both throughfall and stemflow for eastern white pine with increasing maximum atmospheric vapour pressure deficit, which may reflect the potential for evaporative fractionation as a result of retention and detention of water moving through the canopy by the rougher bark of this species. Dry soil conditions limited sampling of mobile soil water during the study, and there was no consistent evidence that either throughfall or stemflow fluxes controlled temporal changes in the isotopic signature of soil water beneath the tree. Thus, the potential for throughfall and stemflow fluxes in northern hardwood‐conifer forests to modify the isotopic composition of water taken up by the tree via transpiration remains an open question.

Explaining the Non-significant Changes in Ice-off Date over Six Decades at Lake of Bays and Lake Nipissing, South-Central Ontario

The phenomenon of non-significant trends in ice-off date under a warming climate was quantitatively explained by three efforts: exploring possible driving factors where possible and defining new factors to represent snow conditions, identifying the contributing factors through correlation and trend tests, and evaluating relative contributions through partial Mann-Kendall method. Why the ice-off became only slightly earlier over 62 years at Lake of Bays has been satisfactorily assessed: the increased winter temperature, increased total rain and decreased days of snow on ground acted as three promoting drivers to earlier ice-off date, but their promoting functions were effectively offset by adverse changes in four other factors (snowfall slope, precipitation slope, snowpack slope, and last day of snow). The ice-off date at Lake Nipissing did not have a significant trend over 58 years, although there were five factors contributing to the ice-off decline without sufficient offsetting, suggesting that the ice-off of this lake may not be sensitive, or basically elastic, to the climatic variation stressor. Relative contributions of drivers as calculated helped explain how much they contributed to ice-off trends or how much they offset the influences.

Just How Competitive Are We? Managing Organizational Culture in a Canadian University Athletics Department

Christiaan Jacobs is the new Dean of Student Affairs at the University of South Central Ontario, which puts him in charge of the Department of Athletics and Recreation. Jacobs has learned that the hypercompetitive environment established by the athletic director, Nathan Scott, has been causing friction in many areas of the department, potentially resulting in the resignation of several long-term employees. As part of an organizational audit, he interviewed many employees and had them complete the Competing Values Framework questionnaire, the results of which were troubling. How should Jacobs lead this department forward and can he count on Scott to be supportive of the direction that he wants it to go? The purpose of this case is to introduce students to the importance of organizational culture and challenges to organizational change. Students will learn about the Competing Values Framework, change management, and have the opportunity to analyze qualitative and quantitative data in formulating responses to the case-guiding questions. This decision-focused case is suitable for use with upper division undergraduate and graduate sport management students in courses such as Organizational Behavior, Strategic Management, Collegiate Athletics Administration, and Critical Issues in Sport.

1205. Emergence of Carbapenemase Producing Enterobacteriaceae in South Central Ontario, Canada

BackgroundThe spread of CPE is an increasing global threat to patient safety. We describe the introduction and evolution of CPE in south-central Ontario, Canada.MethodsThe Toronto Invasive Bacterial Diseases Network has performed population based surveillance for CPE in metropolitan Toronto and Peel region from first identified isolates in 2007. All laboratories test/refer all carbapenem non-susceptible Enterobacterial isolates for PCR testing for carbapenemases. Demographic and medical data and travel history are collected from chart review and patient/physician interview.ResultsSince 2007, 659 patients have been identified as colonized/infected with CPE; 362, 57%) have at least one clinical isolate. Annual incidence has increased from 0 in 2006 to 1.3 per 100,000 in 2016/17 (Figure 1). First bacteremia occurred in 2010, the incidence in 2017 was 0.14 per 100,000 population. 388 (59%) patients were male, median age was 70 years (range 3 months–100 years). Most common genes among first isolates were NDM (306, 46%), OX48 (149, 23%), KPC (122, 19%). Most common species were K. pneumoniae (268, 41%) and E. coli (259, 39%). Over time, second species/same gene were identified in 113 (16%) patients. In addition, 34/xxx patients with isolates with NDM and/or OXA-48 subsequently had a second isolate with a different gene/gene combination. Of 518 patients whose travel and hospitalization history are available, patients with VIM were less likely than other patients to have a foreign hospitalization or travel history (9/28 vs. 341/490, P < 0.0001). Patients with KPC were more likely to have a hospitalization history outside Canada and the Indian subcontinent (25/70, 36%), in Canada (47/164,29%) than to have no hospitalization in the last year (13/93, 14%), or a history of hospitalization in the Indian subcontinent (2/191, 1%) (P < 0.001). The number of incident patients with different hospitalization and travel history over time is shown in Figure 2.ConclusionCPE is increasingly recognized in southern Ontario, both in patients with a history of exposure in healthcare in other countries, and to healthcare in Canada. Intensification of control programs is urgently needed.Figure 1.Incidence of clinical isolates of CPE over time.Figure 2.Number of incident CPE cases with different hospitalization (H) and travel (T) history over time.Disclosures S. Poutanen, MERCK: Scientific Advisor, Speaker honorarium. COPAN: Speaker(but not part of a bureau), Travel reimbursement. Accelerate Diagnostics: Investigator, Research support. Bio-Rad: Investigator, Research support. bioMérieux: Investigator, Research support.

Emergence of Carbapenemase-Producing Enterobacteriaceae, South-Central Ontario, Canada1.

We analyzed population-based surveillance data from the Toronto Invasive Bacterial Diseases Network to describe carbapenemase-producing Enterobacteriaceae (CPE) infections during 2007–2015 in south-central Ontario, Canada. We reviewed patients’ medical records and travel histories, analyzed microbiologic and clinical characteristics of CPE infections, and calculated incidence. Among 291 cases identified, New Delhi metallo-β-lactamase was the predominant carbapenemase (51%). The proportion of CPE-positive patients with prior admission to a hospital in Canada who had not received healthcare abroad or traveled to high-risk areas was 13% for patients with oxacillinase-48, 24% for patients with New Delhi metallo-β-lactamase, 55% for patients with Klebsiella pneumoniae carbapenemase, and 67% for patients with Verona integron-encoded metallo-β-lactamase. Incidence of CPE infection increased, reaching 0.33 cases/100,000 population in 2015. For a substantial proportion of patients, no healthcare abroad or high-risk travel could be established, suggesting CPE acquisition in Canada. Policy and practice changes are needed to mitigate nosocomial CPE transmission in hospitals in Canada.

Drumlinized tunnel valleys in south-central Ontario

A network of Late Wisconsin valleys deeply incise thick successions (up to 200 m) of Quaternary sediment and Paleozoic bedrock across south-central Ontario. The valleys have previously been interpreted as an integrated network of tunnel channels recording catastrophic releases of subglacial meltwater across the bed of the Laurentide Ice Sheet (LIS). Recent geologic investigations in the western part of the valley network integrate information gained from surficial sediment and landform mapping, continuously-cored boreholes that penetrate the entire Quaternary sediment succession in valleys as well as intervening uplands, and both water-borne and land-based geophysical surveys. Together, these data provide new insights and a refined interpretation of the genesis, timing and paleoglaciological significance of these valleys.Valleys within the study area are some of the largest observed within the regional network – up to 30 km long, 7 km wide and >175 m deep. They are oriented both north-south and in a radial pattern, varying from NE-SW in the southern part of the study area, to ESE-WNW in the north. An undulating Late Wisconsin till sheet (Newmarket Till) caps the regional uplands and is commonly observed along the flanks and floors of the valleys – all of which show evidence of a drumlinization phase that postdates excavation of most of the valleys. At least two generations of valleys are interpreted from cross-cutting relationships, with abrupt heads and locally perched downflow ends of some valley forms. Radiocarbon age determinations of organic material from both below and above the Newmarket Till constrain the timing of valley excavation to between 28.05 and 12.81 14C kyr BP. Borehole and outcrop data within the valleys reveal four stratigraphic units (SU1-SU4) that form the floor and sediment infill of the valleys. The Newmarket Till (SU1) is commonly underlain by deformed substrates. The till is absent in parts of some valleys and the lowermost parts of the valley fill commonly consist of coarse-grained gravels (SU2) that pass upwards into sands (SU3). The bulk of the sediment infill consists of glaciolacustrine silt-clay rhythmites (SU4) that pass upwards into organic- and mollusk-bearing nearshore and fluviodeltaic deposits that record drainage of proglacial lakes from the study area during deglaciation.It is proposed that these features are a previously unrecognized type of tunnel valley that developed subglacially during Late Wisconsin ice cover. Meltwater surpluses would be generated at the ice-bed interface due to the low-transmissivity substrate in the study area with flow preferentially routed into lows on the bed. Downward incision into confined and pressurized aquifers likely facilitated rapid headward erosion of the valleys enhanced by piping. Waning meltwater flow velocities/discharges allowed ice creep to re-occupy newly-formed valleys permitting renewed till deposition and subsequent drumlinization of the valleys. The valleys also provided efficient routing for meltwater draining the retreating ice marginal zone during deglaciation and allowed localised sand and gravel deposition as eskers and ice-proximal subaquatic fans in the Oak Ridges Moraine and within proglacial lakes that developed during ice retreat. However, there is no evidence for regional-scale catastrophic release of subglacial meltwater in the valley infills. This integrated dataset represents one of the most complete characterizations of subglacial valleys in a terrestrial setting in North America.

Conceptual hydrogeological model of the Yonge Street Aquifer, south-central Ontario: a glaciofluvial channel–fan setting

The Yonge Street Aquifer (YSA) in the Greater Toronto Area of south-central Ontario is a prolific municipal supply aquifer. It has been considered to be channelized sand and gravel linked to a bedrock valley. Despite considerable work, the fundamental conceptual model for the YSA is not well developed and documented. Based on high-quality data, a revised conceptual model of the aquifer is presented. Seismic profiles define the geometry of the regional stratigraphy with four distinct units: bedrock, Lower sediments, Newmarket Till, and Oak Ridges Moraine (ORM) sediment. Seismic data reveal two generations of roughly north–south channels: older sub-Newmarket Till channels within Lower sediments (termed Thorncliffe channel) and ORM-related channels (termed ORM channel) that incise both Newmarket Till and Lower sediments. The YSA is interpreted to occur within a Thorncliffe channel, with possible vertical connection to younger ORM channels and lateral connection to inter-channel Lower sediments. Thorncliffe channel deposits consist of fining-upward transitions from coarse gravel, to sand, to rhythmically bedded mud interpreted to be deposited within a channel – esker – subaqueous fan complex. Upper Thorncliffe channel mud facies and overlying Newmarket Till provide a capping aquitard. The YSA conceptual model benefits from a strong understanding of facies changes in the Thorncliffe Formation. The deposits with highest permeability occur within up to 80 m thick gravel and sand sequences at the base of the Thorncliffe channel, with transmissivity ranging from 1500 to 4500 m2/day. Groundwater level response to municipal pumping confirms connection along the channel with muted hydraulic response laterally. Thorncliffe channels are interpreted to be up to 20 km long and approximately 2 km wide.

New range extensions for the Canadian Chironomidae fauna from two urban streams

We report on 2 first records of Chironomidae for Ontario, Rheosmittia spinicornis (Brundin, 1956) and Sublettea coffmani (Roback 1975), and confirm a previous record of Odontomesa fulva (Kieffer 1919) from Ontario. Specimens of S. coffmani have been only reported from New Brunswick whereas R. spinicornis has records from northern, western and central provinces and territories. Reports of O. fulva were not con-firmed in Ontario. Here we report its occurrence for the second time in south central Ontario, confirming its occurrence within the province.

Increase in woody debris nutrient pools in stream channels following selection harvesting in a northern hardwood forest

Abstract Timber harvesting can influence the rate of transfer of organic matter from terrestrial catchments to streams, which may affect in-stream nutrient pools and dynamics. In the interest of developing sustainable forestry practices, the continued study of the effects of forestry on nutrient dynamics in aquatic systems is paramount, particularly in sensitive nutrient-poor oligotrophic systems. The goal of this study was to investigate the impacts of harvest-related woody debris on stream nutrient status in six streams located in the Canadian Shield region of south-central Ontario, Canada. Surveys showed greater large (>10 cm) and small (

Phosphorus Deposition in a Low-Phosphorus Landscape: Sources, Accuracy and Contribution to Declines in Surface Water P

Atmospheric deposition can be an important source of phosphorus (P) to watersheds in geologically low-P landscapes like the Precambrian Shield, in south-central Ontario. Recent observations in this region of declining total P (TP) concentrations in streams and lakes, as well as P limitation in vegetation, prompted this investigation into the potential contributing role of atmospheric deposition. Long-term (1980–2014) measurements of bulk TP deposition at four monitoring stations, as well as wet-only deposition at one of the sites (2012–2014) and pollen measurements in 2014, were used to assess the potential contribution of biogenic sources to TP deposition in the region. Ratios of phosphorus to potassium (P:K) in historical bulk deposition and variability in P deposition among the four sites indicated that continuously open, bulk deposition collectors are enriched in ‘local’ biogenic sources of P, particularly in the summer, fall and spring months. Furthermore, in 2014 pollen could account for as much as 30% of bulk TP deposition, and pine trees (Pinus strobus) were the most important source of pollen (>75% of collected pollen). In contrast, winter bulk deposition was the least affected by local, biogenic sources, and likely best reflects net P inputs to this region from long-range (that is, external to the watershed) sources. Catchment input–output budgets for P that use measured annual bulk deposition totals as inputs are generally strongly positive (indicating net retention/accumulation of P within watersheds), whereas annual budgets calculated using extrapolated winter deposition are in approximate balance. Balanced P budgets are more consistent with regional observations of declining surface water TP and P limitation in terrestrial vegetation. Importantly, there was no significant decline in winter TP deposition over the period of record (1980–2014), which suggests that changes in regional deposition are not responsible for declines in surface water TP. Although we recommend extrapolating winter bulk deposition measurements to approximate annual atmospheric TP input to terrestrial ecosystems in non-dusty, oligotrophic landscapes such as this, local sources of P, particularly pollen, are likely important seasonal sources of P nutrition to lakes.

Exotic earthworm (Oligochaeta: Lumbricidae) assemblages on a landscape scale in central Canadian woodlands: importance of region and vegetation type

A growing understanding about the impacts of earthworms (Oligochaeta: Lumbricidae) on ecosystem processes and forest restoration necessitates an examination of their role in Canadian forests where they have become invasive. Little is known about the landscape-scale responses of earthworm populations to different regional characteristics and vegetation types within Canada’s central woodlands. We examined the regional variation of earthworm species richness, biomass, and assemblage composition across a range of four municipal regions (from south to north: Halton, Wellington, York, and Simcoe) and four habitat types (deciduous forest, mixed forest, tree plantation, and meadow) with varying soil characteristics in woodlands of south-central Ontario, Canada. In general, earthworm communities differed by region but not by habitat type. The most southern regions supported the highest earthworm species richness, biomass (i.e., Lumbricus and Octolasion), and density, and this was associated with a south–north gradient in soil characteristics. Assemblage composition differed by region but not by habitat type. The observed south–north gradient suggests an underlying effect of invasion spread associated with human settlement and density. Our results provide baseline information about earthworm communities in south-central Ontario forests and will enable managers to plan for the increasing role of earthworms in Canada’s future forests.

Calcium decline reduces population growth rates of zooplankton in field mesocosms

Regional calcium (Ca) decline, a legacy of acid deposition and logging, is a potential threat to aquatic organisms. Lake surveys and laboratory studies indicate that Ca-rich daphniids are likely most susceptible, allowing for competitive release of other taxa with low Ca demand. Indeed, dramatic shifts in zooplankton community structure have been documented in lakes where Ca has declined, amid multiple other stressors. Given the perceived threat of this large-scale stressor, manipulative studies are needed to evaluate causal relationships between Ca decline and zooplankton community structure. We analysed per capita growth rates of zooplankton from three independent mesocosm experiments where we manipulated aqueous Ca concentrations to reflect current and future Ca concentrations. In two experiments where Ca concentration was reduced to 0.6 or 0.9 mg/L, we observed reduced growth rates for several taxa, including daphniids, bosminids, and copepods. No effect of Ca was detected in the experiment where Ca concentrations ranged from 1.2 to 2.5 mg/L, a gradient representing 68% of lakes in south-central Ontario. These results suggest that future Ca decline in soft-water Canadian Shield lakes may be accompanied by shifts in community structure and overall declines in zooplankton production.

Can recovery from disturbance explain observed declines in total phosphorus in Precambrian Shield catchments?

The plausibility of land disturbance as a cause of declining phosphorus (P) concentrations in oligotrophic lakes within south-central Ontario, Canada, is evaluated using the process-based model INCA-P. The model was calibrated upon three catchments in the Muskoka–Haliburton region (MHR): Harp (HP), Dickie (DE), and Plastic (PC), which have varying degrees of declining P export and different forms of historic disturbances (timber harvesting, tree death, and soil acidification, respectively). Hindcasts (1978–2007) were run with and without simulated disturbances. Model performance of both DE and HP was greatly improved when effects of wetland tree deaths (DE) and harvesting (HP) were included. In PC, with no record of timber harvesting and relatively minor declines in P, initial hindcasts successfully accounted for the majority of interannual P fluxes, and performance was only marginally improved through the simulation of soil acidification. Vegetation decay, harvesting, and catchment acidification accounted for 63%, 24%, and 0.6%, respectively, of P export over the past 30 years. Of all disturbances, wetland vegetation death had the highest impact on areal P exports, indicating that riparian stability is particularly important.