Anthropogenic stressors including acid deposition, invasive species, and calcium (Ca) decline have produced widespread damage to Canadian Shield lakes, especially to their zooplankton communities. Here, we review current knowledge on the individual effects on zooplankton by the non-indigenous predator Bythotrephes longimanus and Ca decline; we identify knowledge gaps in this literature and examine the likely interactive impacts of Bythotrephes invasions and Ca decline on zooplankton. The negative impacts of Bythotrephes longimanus on zooplankton communities are well known, whereas current understanding of the effects of declining Ca on zooplankton is restricted to Daphnia spp.; hence, there is a large knowledge gap on how declining Ca may affect zooplankton communities in general. The co-occurring impacts of Bythotrephes and declining Ca have rarely been studied at the species level, and we expect daphniids, particularly Daphnia retrocurva and Daphnia pulicaria, to be the most sensitive to both stressors. We also expect a synergistic negative interaction on cladocerans in lakes with both stressors, leaving a community dominated by Holopedium glacialis and (or) copepods. Our predictions form testable hypotheses but since species and ecosystem response to multiple stressors are difficult to predict, we may actually see ecological surprises in Canadian Shield lakes as Bythotrephes continues to spread and Ca levels continue to fall.

Within the last decade, calcium decline has emerged as a stressor for many soft water lakes. A legacy of long‐term acid deposition and logging, calcium decline has been implicated in the loss of large herbivores, such as Daphnia, and changes in community structure as taxa with presumed low calcium demand also increase. Although lake surveys, paleolimnological studies, and laboratory experiments have provided considerable evidence that declining Daphnia abundances are associated with low calcium concentration, causal relationships for zooplankton response to calcium decline have not been assessed using field experiments with natural zooplankton communities. We conducted a six‐week field experiment under low food conditions typical of Canadian Shield lakes, using four calcium concentrations (0.6 mg/L, 1.0 mg/L, 1.4 mg/L, and 2.4 mg/L) and zooplankton communities originating from eight lakes along a gradient from 1.78 mg Ca/L to 24.8 mg Ca/L, to examine the effect of calcium on growth rates and reproduction of individual and groups of taxa. Population growth rates for daphniids, Bosmina spp., and three cyclopoids declined as calcium concentration decreased. At low calcium, growth rates of Daphnia pulex, Daphnia catawba, and Mesocyclops edax were negative indicating declining populations. There was no effect of calcium on reproduction of the most common cladocerans. Although we detected some variation in growth rates amongst source lakes, zooplankton communities did not differ in their response to low calcium. As calcium concentrations continue to decline in soft water lakes, reduced zooplankton growth rates may result in shifts in zooplankton community structure and overall declines in total zooplankton production.

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.

One possible solution to the recent decline of calcium (Ca) concentrations in Canadian Shield forests and lakes in eastern North America is the addition of Ca-rich wood ash to watersheds. We investigated the feasibility of using small, mainly residential sources of non-industrial wood ash (NIWA) for this purpose by quantifying concentrations of its major nutrients and metals, its toxicity to Daphnia in aqueous extracts, and estimating the amount of NIWA available in the District of Muskoka in central Ontario. Locally collected NIWA averaged 30% Ca, and also contained smaller but significant amounts of K, Mg, Na, and P. Of these, K was so soluble that it was toxic to Daphnia over 48 h in the concentrate and 10-fold dilution; however, sedimented ash was not toxic over 15 d. Most metal levels in NIWA were below targets permitting unrestricted land application. However, Cu and Zn were just above these targets, but well below those for conditional use. Muskoka residents generate about 235 000 kg of NIWA annually, not enough to treat all central Ontario areas affected; however, a NIWA recycling programme implemented across southern Ontario could generate enough ash to solve the Ca decline problem in Muskoka’s forests and lakes.

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Summary An assessment of the invasive risk associated with the establishment and dispersal of plants available in the aquarium and ornamental pond industries in the Greater Toronto area (Canada) was made. In the risk model, sales volumes of individual taxa were used as a proxy for propagule pressure, to assess pathway risk potential. Organism risk potential, the ability to become established and disperse associated with a release, was assessed using an analysis of the biological traits of the species. Discriminant correspondence analysis was used to predict which biological traits were useful in discriminating native plants from alien invasive plants and alien non‐invasive plants. Importantly, a relatively small number of biological traits appear to be useful in predicting whether an alien aquatic plant had the characteristics that would support establishment and/or dispersal in new environments. Aquatic plants distributed by the industries that are cold tolerant, able to propagate by fragments and use a number of dispersal methods are of particular concern as potential invaders. The model identified 11 alien plants in the trade that have a high risk of becoming invasive, and an additional 52 with moderately high risk.

ABSTRACT1. The characterization and classification of the Black River Upper Morass was conducted using the criteria of the three-parameter test for vegetation, soils and hydrology at 14 sites within the study area.2. The Upper Morass exhibited 71% hydrophytic vegetation typical of wetlands (vascular and non-vascular plants including Cladium jamaicense, Phragmites australis, Typha angustifolia and Sagittaria lancifolia) and 29% vegetation atypical of wetlands.3. The Upper Morass is classified as a Palustrine System with sub-classes of Aquatic Bed and Persistent Emergent Wetland.4. The presence throughout the Upper Morass of monoculture stands of the invasive species Eichhornia crassipes and Typha domingensis, which are known to invade disturbed or partially drained wetlands, confirmed that the study area was a disturbed ecosystem.5. The Upper Morass exhibited hydric (flood-water and groundwater) soils, which, in association with its temporarily flooded and saturated wetland hydrology, indicated that it is groundwaterdriven.6. Application of the three-parameter test indicated a linkage between the functions of the Upper Morass and the Lower Morass, and thus a need for conservation of the Black River Morass System as a single unit rather than two independent wetlands. Two key instruments of conservation will be further detailed ecological assessments and the implementation of a management plan.

The purpose of the research was to clarify the common traits of a successful aquatic invasive plant. The biological traits of aquatic plants sold by aquarium and ornamental pond stores in the Greater Toronto Area, Ontario were analysed using logistic regression analyses (LRM). The multinomial LRM successfully predicts group identity 63% of the time, clearly differentiating native invasive and native non invasive from alien invasive (SI), but not ailen non invasive (ANI) from AI. In contrast, the binomial LRM effectively discriminates ANI from AI taxa 77% of the time. The variable that best discriminated between AI and ANI are type of propagation strategies, number of propagation strategies used, number of dispersal mechanisms used, minimum temperature, and sales volume of aquatic plants as a proxy for introduction. Recognizing the common traits of successful invaders allows for the identification of ANI taxa that have the potential to become invaders, prior to introduction, thus preventing future invasions.