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Cladocera serve as important bio‐ and paleo‐indicators of lake food webs and environmental conditions. The ecological optima of cladocerans are often established by regional‐scale calibration sets, with subsequent comparisons to limnological variables. However, due to logistical constraints when sampling large numbers of lakes, this approach often limits the length of the environmental gradients that can be examined. To extend spatial and limnological gradients, we combined 20 datasets (388 lakes) containing both cladoceran and environmental data, spanning multiple ecoregions across Canada and Alaska. These data were collected over c. 20 years using similar techniques in a single laboratory. Our continental‐scale analysis examined the main chemical and physical variables that influenced cladoceran assemblages, and identified critical environmental thresholds structuring assemblages.Multivariate analyses were used to examine the influences of six environmental variables (depth [either maximum depth or coring depth, which are very similar], surface area, pH, calcium [Ca], total phosphorus [TP], and dissolved organic carbon) and ecoregion classifications on cladoceran assemblages. Gradients of pH and Ca were strongly related. Daphnia longispina spp. and Chydorus brevilabris/biovatus were associated with higher pH and Ca concentrations, while Bosmina spp. and Holopedium spp. were more common in lakes with lower pH and Ca. Dissolved organic carbon was highly correlated with TP. Chydorus brevilabris/biovatus was associated with higher‐nutrient systems. Littoral chydorids, D. longispina spp., Holopedium spp., and Daphnia pulex spp. were associated with intermediate TP and dissolved organic carbon concentrations, and Bosmina spp. was more closely associated with oligotrophic systems.Physical limnological variables influenced taxonomic composition on a continental scale. For example, D. longispina spp., D. pulex spp., Bosmina spp., and Holopedium spp. were associated with larger and/or deeper systems, and littoral chydorids were associated with smaller and/or shallower lakes. A multivariate regression tree identified two thresholds important for structuring assemblages based on pH (<7 and ≥7, probably closely tied to lakewater Ca concentrations) and depth (<6.45 m and ≥6.45 m at pH < 7, and <4.75 m and ≥4.75 m at pH ≥ 7).Our results highlight the sensitivity of several cladoceran taxa to multiple chemical and physical gradients. We demonstrate that the ecological responses of cladocerans to environmental variables—often established through regional‐scale surveys—are applicable across ecoregions and broad limnological gradients, reinforcing the value of cladocerans as bioindicators of key chemical and physical variables in freshwaters and as paleoindicators for assessing human impacts on aquatic systems through time. [ABSTRACT FROM AUTHOR]

Many lakes across Canada and northern Europe have experienced declines in ambient phosphorus (P) and calcium (Ca) supply for over 20 years. While these declines might create or exacerbate nutrient limitation in aquatic food webs, our ability to detect and quantify different types of nutrient stress on zooplankton remains rudimentary. Here, we used growth bioassay experiments and whole transcriptome RNAseq, collectively nutrigenomics, to examine the nutritional phenotypes produced by low supplies of P and Ca separately and together in the freshwater zooplankter Daphnia pulex. We found that daphniids in all three nutrient-deficient categories grew slower and differed in their elemental composition. Our RNAseq results show distinct responses in singly limited treatments (Ca or P) and largely a mix of these responses in animals under low Ca and P conditions. Deeper investigation of effect magnitude and gene functional annotations reveals this patchwork of responses to cumulatively represent a co-limited nutritional phenotype. Linear discriminant analysis identified a significant separation between nutritional treatments based upon gene expression patterns with the expression patterns of just five genes needed to predict animal nutritional status with 99% accuracy. These data reveal how nutritional phenotypes are altered by individual and co-limitation of two highly important nutritional elements (Ca and P) and provide evidence that aquatic consumers can respond to limitation by more than one nutrient at a time by differentially altering their metabolism. This use of nutrigenomics demonstrates its potential to address many of the inherent complexities in studying interactions between multiple nutritional stressors in ecology and beyond. [ABSTRACT FROM AUTHOR]

High dietary protein has been hypothesized to cause lower bone mineral density (BMD) and greater fracture risk. Previous results are conflicting and few studies have assessed potential differences related to differing protein sources. Participants/Measures: Protein intake was assessed as percent of total energy intake (TEI) at Year 2 (1997-99) using a food frequency questionnaire (N=6510). Participants were contacted annually to ascertain incident fracture. Total hip and lumbar spine BMD was measured at baseline and Year 5. Analyses were stratified by group (men 25-49 y, men 50+ y, premenopausal women 25-49 y, and postmenopausal women 50+ y) and adjusted for major confounders. Fracture analyses were limited to those 50+ y. Results: Intakes of dairy protein (with adjustment for BMI) were positively associated with total hip BMD among men and women aged 50+ y, and in men aged 25-49. Among adults aged 50+ y, those with protein intakes of <12% TEI (women) and <11% TEI (men) had increased fracture risk compared to those with intakes of 15% TEI. Fracture risk did not significantly change as intake increased above 15% TEI, and was not significantly associated with protein source. Conclusions: In contrast to hypothesized risk of high protein, we found that for adults 50+ y, low protein intake (below 15% TEI) may lead to increased fracture risk. Source of protein was a determinant of BMD, but not fracture risk. [ABSTRACT FROM AUTHOR]

Multiple natural and anthropogenic stressors pose a serious threat to the health and diversity of biological communities. A growing body of research suggests that one consequence of these stressors has been a regional-scale loss of crayfish populations across south-central Ontario. The mechanisms contributing to the imperilment of crayfish probably include historical acidification and metal contamination, shoreline development and habitat loss, increased water temperature, declining [Ca] in lake water, and invasion by nonnative species (i.e., Smallmouth Bass and the rusty crayfish). Long-term monitoring data of crayfish populations in Algonquin Provincial Park (Ontario) have documented marked declines in Cambarus bartonii abundance despite the relatively isolated nature of the study lakes. Furthermore, monitoring data indicate that the onset of this decline may predate the monitoring record, thereby creating a need for the use of paleolimnological techniques to infer past conditions. We used a multiple proxy approach (i.e., diatoms and cladocerans) to reconstruct key environmental variables (e.g., pH) and long-term changes in lake chemistry (e.g., [Ca]) as they pertain to crayfish population loss. We found that interactions between pH and [Ca] have led to the decline in crayfish abundance and to the continued lack of recovery of these populations. Furthermore, lake-water [Ca] in all 4 Algonquin Park lakes are now <2.0 mg/L, the lowest hypothesized requirement for freshwater crayfish (2–10 mg/L), suggesting that short-term recovery of crayfish populations is unlikely, despite pH recovery. [ABSTRACT FROM AUTHOR]

Lake water calcium (Ca) decline has recently been recognized as a stressor impacting softwater lake districts that have experienced long-term patterns of acid deposition and/or timber harvesting. Declining aqueous Ca levels may impact the survival of aquatic biota, particularly Ca-rich cladoceran taxa such as daphniids. Daphnia pulex are sensitive to laboratory Ca levels below 1.5 mg l; however, responses of cladoceran communities to Ca decline in natural environments require further study. Dickie Lake (Ontario, Canada) is the site of an inadvertent natural experiment, providing insight into the effects of changing aqueous Ca availability upon cladoceran communities, as the lake has a history of acidification, followed by recent (1990s) Ca additions to the watershed via applications of calcium-rich road dust suppressants. Paleolimnological analyses were used to examine changes in cladoceran community structure (with a focus upon Ca-rich daphniids) from pre-industrial times to present day. Three distinct temporal stages were apparent in Dickie Lake's daphniid community: 1870-1950, 1950-1990, and 1990-present. The daphniid community of the pre-industrial assemblages was dominated by members of the Daphnia longispina species complex, but shifted in the late 1950s to more acid- and Ca-insensitive members of the D. pulex species complex. During the most recent stage, coincident with dust suppressant applications, both daphniid complexes are well represented. Observed transitions between daphniid species complexes provide further evidence of the influence of Ca availability upon cladoceran community structure, indicating the potential importance of the controlled addition of Ca to freshwater systems (i.e., liming) as a mitigation/recovery strategy as Ca declines continue. [ABSTRACT FROM AUTHOR]

The acute toxicities of waterborne Cd, Cu, Pb, Ni, and Zn were determined in the first-instar larvae (generally considered to be the most sensitive) of Chironomus riparius, under standardized conditions. Toxicity tests were conducted in soft water (hardness, 8 mg/L as CaCO3 equivalents) in the absence of food and were limited to 24 h to avoid control mortality associated with food deprivation. For each metal, a logarithmic range of concentrations was tested between 0 and 25 mg/L. First-instar C. riparius are most sensitive to Pb, with a 24-h LC50 of 0.61 mg/L (CI of 0.26–1.15 mg/L), and to Cu, with a 24-h LC50 of 2.09 mg/L (CI of 1.57–2.96 mg/L). The LC50 for Cd was 9.38 mg/L, while the LC50s for Zn and Ni were both higher than the highest tested concentration of 25 mg metal/L. Our results found that even first-instar chironomid larvae are well protected by both the current CCME Canadian water quality guidelines for the protection of aquatic life and the USEPA Water Quality Criteria, as LC50s were at least 25 times higher than the guideline concentrations. [ABSTRACT FROM AUTHOR]

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 (SO), calcium (Ca2+), pH, maximum lake depth ( Z max) 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. [ABSTRACT FROM AUTHOR]