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Bythotrephes cederströemi are a predatory cladoceran zooplankter that have invaded numerous inland lakes in North America, many of which are stratified and support offshore fishes like Cisco (Coregonus artedi). While changes in zooplankton community composition following Bythotrephes invasion predict an increase in Cisco mercury concentrations (Hg), this phenomenon was not detected from a survey evaluating temporal changes in Cisco Hg across a broad range of lakes varying in the presence or absence of Bythotrephes. Here, we compare temporal changes in Cisco bioaccumulation slopes (i.e., slopes of relationships between Cisco Hg and trophic position) from lakes experiencing Bythotrephes invasion over the study period to those already invaded (as a reference) over similar time periods. Our results show that bioaccumulation slopes after Bythotrephes invasion either changed direction entirely (from positive to negative relationships) or decreased in elevation relative to those prior to invasion. No such pattern was observed in previously invaded reference lakes. Reductions in Cisco bioaccumulation slopes and/or intercepts following Bythotrephes invasion suggest that conversion efficiency (and therefore growth) of Cisco increased after invasion (i.e., less Hg accumulates in fish at an equivalent trophic position after vs. before invasion). Back-calculated Cisco growth rates and size-at-age from a second complimentary study were greater in the presence of Bythotrephes than without, further supporting the hypothesis that changes in Hg bioaccumulation are likely due to increased conversion efficiency among invaded populations. These findings highlight the potential importance of foraging energetics over and above shifts in trophic position in modifying fish contaminant concentrations. [ABSTRACT FROM AUTHOR]

Recreational boating activities enable aquatic invasive species (AIS) dispersal among disconnected lakes, as invertebrates and plants caught on or contained within watercraft and equipment used in invaded waterbodies can survive overland transport. Besides simple preventive measures such as "clean, drain, dry", resource management agencies recommend decontaminating watercraft and equipment using high water pressure, rinsing with hot water, or air-drying to inhibit this mode of secondary spread. There is a lack of studies assessing the efficacy of these methods under realistic conditions and their feasibility for recreational boaters. Hence, we addressed this knowledge gap via experiments on six invertebrate and plant AIS present in Ontario. Washing at high pressures of 900–1200 psi removed the most biological material (90%) from surfaces. Brief (< 10 s) exposure to water at ≥ 60 °C caused nearly 100% mortality among all species tested, except banded mystery snails. Acclimation to temperatures from 15 to 30 °C before hot water exposure had little effect on the minimum temperature required for no survival. Air-drying durations producing complete mortality were ≥ 60 h for zebra mussels and spiny waterfleas, and ≥ 6 days among plants, whereas survival remained high among snails after a week of air-drying. Hot water exposure followed by air-drying was more effective than either method separately against all species tested. [ABSTRACT FROM AUTHOR]

Adaptation to one stressor can influence organismal responses to future environmental changes, either to the same or a novel stressor. But there is a lack of research on this topic, particularly in the context of freshwater habitats. Fluctuating salinity and heatwaves that are becoming more frequent and intensified are affecting freshwater ecosystems. We applied an experimental evolution approach to examine the influence of adaptation to elevated salinity on the population's responses to subsequent salt and heat stress. We conducted lab experiments using Daphnia pulicaria cultured from individuals with previous 8-week exposure history to two salinity treatments (6.5 or 350 mg Cl−/L). Iso-female lines with or without prior exposure to elevated salinity were assayed along a salt gradient (18.5 to 1500 mg Cl−/L) or an acute heat gradient treatment (20 to 35 °C). Our results showed that Daphnia survival, fecundity, and body length growth rate declined with increased salt concentration, with survival and fecundity being most sensitive. The treatment group with previous salt exposure history had higher survival and fecundity than the naïve treatment group when treated with salt, without loss of fitness under low-salinity conditions. Daphnia survival and growth rates were reduced in temperatures higher than 30 °C. Despite the fact that the two stressors can induce similar defense mechanisms, previous exposure history to salt did not prevent Daphnia populations from experiencing reduced survival and growth rates under heated conditions. Our work demonstrates that organisms can rapidly adapt to a stressor that protects them from later exposure to increases in this stressor, without a trade-off in fitness under undisturbed conditions, but this evolved tolerance cannot protect them from all levels of this stressor or alleviate damage by a novel one. [ABSTRACT FROM AUTHOR]

Determining which non-natives are likely to be introduced is integral for understanding and predicting biological invasions. However, the hypotheses and research regarding invasive species have largely focused on processes occurring post-introduction. Improving predictions of non-native transport and generating new hypotheses about the drivers of species invasion requires a better understanding of the 'pre-introduction' mechanisms that determine whether propagules successfully enter, survive, and exit human vectors. We propose that the subset of non-natives successfully introduced are determined by two primary filtering mechanisms: (1) the characteristics of organisms, and the way in which these characteristics are shaped by and interact with their environment; and (2) the attributes, movement, and behavior of human vectors. We review how species distribution, individual phenotype, environmental conditions, and ecological interactions filter organisms between each pre-introduction stage of non-native transport. Additionally, we apply a modified version of the vector science framework to elucidate mechanisms driving patterns in human movements, which also determine the subset of individuals transported and introduced as non-natives. Our framework distills the human-mediated transport process to its most critical components, providing a simple approach for creating new hypotheses of the drivers of biological invasions. [ABSTRACT FROM AUTHOR]

Bythotrephes longimanus is an invasive zooplankton predator, negatively impacting zooplankton abundance and diversity in North American lakes. Previous studies have shown that Daphnia populations in lakes move to deeper waters during the day, in the presence of Bythotrephes, a visual predator occupying well-lit regions. However, Daphnia vertical position can be influenced by a variety of biotic and abiotic factors. We conducted a survey to determine (1) if Daphnia daytime vertical position differed between invaded and uninvaded lakes and (2) if Daphnia vertical position in invaded lakes was affected by water chemistry variables linked to water clarity, UV exposure, and phytoplankton production, and Bythotrephes density. Invaded lakes had a lower proportion of epilimnetic Daphnia as compared to uninvaded lakes. Daphnia vertical position was species-dependent, and with a lower proportion of epilimnetic Daphnia mendotae and greater proportion of epilimnetic D. longiremis observed in invaded lakes. D. mendotae were deeper in the water column in high Bythotrephes density lakes with low dissolved organic carbon levels. Our results show Daphnia vertical position response to Bythotrephes is species specific, dependent on Bythotrephes density and influenced by local abiotic conditions, with important implications for community structure and ecosystem function in invaded lakes. [ABSTRACT FROM AUTHOR]

Calcium decline is an emerging environmental concern for thousands of soft-water lakes. Daphnia, an important freshwater herbivore, has reduced reproduction, a weaker, less-rigid carapace, and decreased survival under low aqueous calcium concentrations, but there has been little attention to study how calcium decline will influence predator-prey relationships. We conducted a mesocosm experiment to determine the joint effects of calcium concentration (0.3, 1.2, and 2.3 mg/l) and kairomones from Chaoborus americanus, a ubiquitous invertebrate predator, on Daphnia minnehaha populations. In the presence of Chaoborus kairomones, chemical signals of Chaoborus presence, Daphnia populations had delayed peak abundance and grew antipredator neck spine defenses. However, aqueous calcium concentration influenced the strength of the response. Daphnia in both 0.3 and 2.3 mg Ca/l had stronger neck spine responses and larger reproductive delays than Daphnia in 1.2 mg Ca/l. Our results suggest that as lake calcium concentrations continue to fall, Daphnia populations will respond in complex, nonlinear ways. [ABSTRACT FROM AUTHOR]

Habitat connectivity and regional heterogeneity represent two factors likely to affect biodiversity across different spatial scales. We performed a 3 × 2 factorial design experiment to investigate the effects of connectivity, heterogeneity, and their interaction on artificial pond communities of freshwater invertebrates at the local (α), among-community (β), and regional (γ) scales. Despite expectations that the effects of connectivity would depend on levels of regional heterogeneity, no significant interactions were found for any diversity index investigated at any spatial scale. While observed responses of biodiversity to connectivity and heterogeneity depended to some extent on the diversity index and spatial partitioning formula used, the general pattern shows that these factors largely act at the β scale, as opposed to the α or γ scales. We conclude that the major role of connectivity in aquatic invertebrate communities is to act as a homogenizing force with relatively little effect on diversity at the α or γ levels. Conversely, heterogeneity acts as a force maintaining differences between communities. [ABSTRACT FROM AUTHOR]

We employed zooplankton resting eggs to track population-level shifts in acid tolerance over the last century in a boreal shield lake recovering from acidification. Shifts in mean and variance of ecological tolerances of species that occupy ecosystems recovering from anthropogenic stress are important to consider because of their potential to influence eco-evolutionary dynamics at community and ecosystem levels. In a laboratory experiment, we compared juvenile survival of Leptodiaptomus minutus copepods hatched from resting eggs from three time periods (80- to 100-year- old: pre-industrial, 20- to 50- year- old: lake acidification, and 8- to 10-year- old: recovery of lake-water back to pH ≥ 6.0) under several pH treatments. Mitochondrial DNA was used to confirm species identity and to test for population bottlenecks as a possible mechanism to explain our results. We expected that nauplii hatched from eggs deposited prior to industrialization (lake-water pH ≈ 6.0) and from the period of pH recovery (lake-water pH ≥ 6.0) would have lower mean and more variable survival at acidic pH compared to nauplii hatched from the period of peak lake acidification (lake-water pH ≈ 4.7). Our results, which are likely a combination of both genetic and environmental effects, suggest support for this hypothesis. Nauplii hatched from eggs deposited during the period of acidification in George Lake had reduced variation in pH tolerance compared to the recovery period. This was likely driven by strong selection rather than genetic drift because we found no evidence of a population bottleneck. However, we could not detect differences in the variance of naupliar survival between pre-industrial and acidic time periods. Trends in mean naupliar survival from different time periods matched findings from other field-based studies that detected a relationship between lake acidification history and acid tolerance in L. minutus. [ABSTRACT FROM AUTHOR]

Bythotrephes longimanus is an invertebrate predator that has invaded the North American Great Lakes and a number of inland lakes, where it preys on crustacean zooplankton. We examined the effect of Bythotrephes on two measures of ecosystem function during a four-month observational study of freshwater lakes on the boreal shield. Bythotrephes-invaded lakes had significantly lower epilimnetic zooplankton abundance and production compared to reference lakes. On average, Bythotrephes consumed 34% of zooplankton production when it was present in lakes. There was some evidence of changes in the timing of zooplankton production, as well as shifts to cooler, less productive habitats, which may lessen the overall effect of the invader on the transfer of energy to higher trophic levels. We experimentally demonstrated a weak trophic cascade where invader predation reduced zooplankton biomass, and subsequently increased phytoplankton growth. However, the response was small in magnitude and not biologically relevant at the whole lake-scale. The most conspicuous effect of Bythotrephes that we measured was a diversion of energy away from native predators at higher trophic levels. [ABSTRACT FROM AUTHOR]

Dispersal can be an important determinant of local diversity and species composition, but evidence for effects of the regional species pool on local zooplankton communities has been mixed. Theory and experiments suggest that immigration will be necessary for maintenance of community diversity and functioning during periods of environmental change; conversely, fluctuating resource levels may increase the likelihood of invasion success. We conducted a factorial-design mesocosm experiment to test the effects of a nutrient pulse and weekly immigration from other lakes on the diversity and composition of a pelagic zooplankton community. Contrary to expectations, there were no interactive effects of nutrient enrichment and immigration on any measure of diversity, and the initial shift in community composition in response to the nutrient pulse did not depend on the introduction of new species or genotypes from more productive lakes. Although immigration increased species richness in enclosures, success of most colonising species was poor. However, the dispersal treatment appears to have enabled a stronger predator response to increased herbivore numbers in nutrient-pulsed enclosures, leading to an eventual decline in the abundance of some herbivorous species in response to immigration. We conclude that community invasibility was not influenced by productivity, and that dispersal limitation did not strongly constrain the response of the zooplankton community to our applied disturbance. This indicates an unexpected resistance to change in species composition and diversity in spite of disturbance, and suggests that, in our study system, changes in the abundance of resident species are more important than introductions of new species in the community response to short-term environmental change. [ABSTRACT FROM AUTHOR]

Quantifying chemical variability in different lake types is important for the assessment of both chemical and biological responses to environmental change. For monitoring programs that emphasize a large number of lakes at the expense of frequent samples, high variability may influence how representative single samples are of the average conditions of individual lakes. Intensive temporal data from long-term research sites provide a unique opportunity to assess chemical variability in lakes with different characteristics. We compared the intra- and inter-annual variability of four acidification related variables (Gran alkalinity, pH, sulphate concentration, and total base cation concentration) in four lakes with different flushing rates and acid deposition histories. Variability was highest in lakes with high flushing rates and was not influenced by historic acid deposition in our study lakes. This has implications for the amount of effort required in monitoring programs. Lakes with high flushing rates will require more frequent sampling intervals than lakes with low flushing rates. Consideration of specific lake types should be included in the design of monitoring programs. [ABSTRACT FROM AUTHOR]

Climate change is expected to have important impacts on aquatic ecosystems. On the Boreal Shield, mean annual air temperatures are expected to increase 2 to 4 °C over the next 50 years. An important challenge is to predict how changes in climate and climate variability will impact natural systems so that sustainable management policies can be implemented. To predict responses to complex ecosystem changes associated with climate change, we used long-term biotic databases to evaluate how important elements of the biota in Boreal Shield lakes have responded to past fluctuations in climate. Our long-term records span a two decade period where there have been unusually cold years and unusually warm years. We used coherence analyses to test for regionally operating controls on climate, water temperature, pH, and plankton richness and abundance in three regions across Ontario: the Experimental Lakes Area, Sudbury, and Dorset. Inter-annual variation in air temperature was similar among regions, but there was a weak relationship among regions for precipitation. While air temperature was closely related to lake surface temperatures in each of the regions, there were weak relationships between lake surface temperature and richness or abundance of the plankton. However, inter-annual changes in lake chemistry (i.e., pH) were correlated with some biotic variables. In some lakes in Sudbury and Dorset, pH was dependent on extreme events. For example, El Nino related droughts resulted in acidification pulses in some lakes that influenced phytoplankton and zooplankton richness. These results suggest that there can be strong heterogeneity in lake ecosystem responses within and across regions. [ABSTRACT FROM AUTHOR]

Examines conceptual frameworks in order to understand the sequence of ecological steps needed for the recovery of biotic communities and on where and how to intervene in this process should recovery stall. Description of decision tree, highlighting the sequence of processes necessary for ecological recovery; Analysis of process bottlenecks such as inadequate water quality; Details of the framework for the integration of ecological processes in a larger management context.

Assesses the importance of rotifers in lake ecosystems which increases the acidicity level during the experimental manipulation in Little Rock, Arkansas. Nature of species-specific responses among habitats; Observation of the total rotifer biomass; Indication of hydrogen-ion concentration.