Protecting freshwater biodiversity is considered an ultimate challenge but depends on reliable surveys of species distribution and abundance which eDNA metabarcoding (environmental DNA metabarcoding) may offer. To do so, a better understanding of the sources of temporal variation among species eDNA abundance and of data transformation in eDNA metabarcoding studies is needed. Here, we show that transformation based on relative abundance is critical to suitable analyses of eDNA metabarcoding data and that Hellinger transformation performed slightly better than other methods. Furthermore, we show that site localities significantly explain eDNA metabarcoding variation, while no variation is explained by time of sampling. This indicates that species communities vary more spatially than temporally within a dendritic system composed of small rivers. We then further documented the community structure in the St. Charles River (Quebec City, Canada) and six of its tributaries. This revealed the existence of eight species communities explaining 82.1% of eDNA read variation within this river network. Moreover, variation in environmental variables among sites explained 53.0% of eDNA reads, while sampling events and temporal environmental variation explained no eDNA metabarcoding variation. Altogether, this supports the claim that eDNA metabarcoding is a powerful tool to document and monitor fish communities in watersheds composed of small river dendritic systems.

Proper environmental DNA metabarcoding data transformation reveals temporal stability of fish communities in a dendritic river system

Understanding the resilience of ectotherms to high temperatures is essential because of the influence of climate change on aquatic ecosystems. The ability of species to acclimate to high temperatures may determine whether populations can persist in their native ranges. We examined physiological and molecular responses of juvenile brook trout (Salvelinus fontinalis) to six acclimation temperatures (5, 10, 15, 20, 23 and 25°C) that span the thermal distribution of the species to predict acclimation limits. Brook trout exhibited an upregulation of stress-related mRNA transcripts (heat shock protein 90-beta, heat shock cognate 71 kDa protein, glutathione peroxidase 1) and downregulation of transcription factors and osmoregulation-related transcripts (nuclear protein 1, Na+/K+/2Cl- co-transporter-1-a) at temperatures ≥20°C. We then examined the effects of acclimation temperature on metabolic rate (MR) and physiological parameters in fish exposed to an acute exhaustive exercise and air exposure stress. Fish acclimated to temperatures ≥20°C exhibited elevated plasma cortisol and glucose, and muscle lactate after exposure to the acute stress. Fish exhibited longer MR recovery times at 15 and 20°C compared with the 5 and 10°C groups; however, cortisol levels remained elevated at temperatures ≥20°C after 24 h. Oxygen consumption in fish acclimated to 23°C recovered quickest after exposure to acute stress. Standard MR was highest and factorial aerobic scope was lowest for fish held at temperatures ≥20°C. Our findings demonstrate how molecular and physiological responses predict acclimation limits in a freshwater fish as the brook trout in the present study had a limited ability to acclimate to temperatures beyond 20°C.

Molecular and physiological responses predict acclimation limits in juvenile brook trout (Salvelinus fontinalis).

Short-term acclimation dynamics in a coldwater fish.

Brook trout (Salvelinus fontinalis) populations are at risk of exposure to high water temperatures in the species native range in eastern North America. We quantified the physiological and molecular responses of juvenile brook trout to six acclimation temperatures that span the thermal distribution of the species (5, 10, 15, 20, 23, and 25{degrees}C). Using quantitative PCR (qPCR), we measured the mRNA transcript abundance of temperature-induced cellular stress genes to identify a potential sub-lethal temperature threshold for brook trout between 20-23{degrees}C. Brook trout exhibited an upregulation of stress-related genes (heat shock protein 90-beta; heat shock cognate 71 kDa protein; glutathione peroxidase 1) and a downregulation of transcription factors and osmoregulation-related genes (Na+/K+/2Cl- co-transporter-1-a; nuclear protein 1) at temperatures [≥]20{degrees}C. We also used respirometry to assess the effects of the acclimation temperatures on oxygen consumption. Standard metabolic rate results indicated that energy expenditure was higher at temperatures [≥]20{degrees}C. We then examined the effects of acclimation temperature on metabolic rate and blood plasma parameters in fish exposed to an acute exhaustive exercise and air exposure stress. Fish acclimated to temperatures [≥]20{degrees}C exhibited elevated levels of plasma cortisol, muscle lactate, and plasma glucose after exposure to the acute stressors. After 24 h of recovery, fish showed longer metabolic recovery times at 15 and 20{degrees}C and cortisol levels remaining elevated at temperatures [≥]20{degrees}C. Our findings suggest that brook trout may have a limited ability to acclimate to temperatures >20{degrees}C and increases in temperatures beyond 20{degrees}C may impact brook trout populations.

/pdf/molecular-and-physiological-responses-predict-acclimation-1q35p84shx.pdf

Molecular and physiological responses predict acclimation limits in juvenile brook trout (Salvelinus fontinalis)

We assessed thermal habitat use of lake-dwelling Brook Trout (Salvelinus fontinalis) populations in 39 lakes of varying size with short-set duration, stratified-random netting surveys in Al...

Thermal habitat of Brook Trout in lakes of different size

We reviewed the literature pertaining to Brook Charr thermal habitat, one of Fry’s original species used in developing his scope for activity, to determine what guidance may be established for defining thermal habitat from the literature. The literature was surprisingly large, with 53 publications estimating Brook Charr thermal habitat. The laboratory-based optimal temperature estimates varied widely but were far more consistent when only acclimation-controlled studies were considered. Acclimation-controlled estimates of optimal temperature matched the peak of the aerobic scope curve published nearly 70 years ago. Confidence in defining optimal thermal habitat is strengthened for Brook Charr based on this match because the peak in aerobic scope has strong mechanistic links between physiology and thermal ecology. However, laboratory-based studies used only sub-adult Brook Charr and may underrepresent Brook Charr thermal habitat in nature due to ontogenetic differences in thermal preference. Optimal temperature estimates from aerobic scope or acclimation-controlled studies should be used to define thermal habitat boundaries for fish but should be treated with caution if based on sub adults. Population level sampling is needed to determine the generality of these literature-derived boundaries among Brook Charr populations.

Temperature selection in Brook Charr: lab experiments, field studies, and matching the Fry curve

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Temperature selection in Brook Charr: lab experiments, field studies, and matching the Fry curve

Thermal habitat of Brook Trout in lakes of different size