Pelagic and benthic algal biomass and pelagic algal community structure were measured in Boreal Shield lakes impacted by forest harvesting and wildfires (Haute-Mauricie, Québec). Sixteen reference lakes in which the watershed has been unperturbed for at least 40 years, seven harvested lake watersheds (logged in 1995), and nine lake watersheds burnt in 1995 were sampled for 3 years following harvesting or wildfires. From 1996 to 1998, repeated-measures ANOVA showed significant effects between treatment and sampling years for pelagic chlorophyll a (Chl a) and biomass, but for 1997-1998 benthic Chl a, repeated-measures ANOVA showed only significant treatment effects. Chl a concentrations increased 1.4- to 3-fold in perturbed lakes as compared with reference lakes. Areal pelagic Chl a (milligrams per square metre) was lower than estimated littoral Chl a in perturbed lakes. The pelagic algal community was dominated by mixotrophic nanoflagellates in reference lakes. Watershed perturbation induced differential changes in pelagic algal communities: mixotrophic nanoflagellates increased in harvested lakes and photoautotrophic diatoms in burnt lakes. Considering only perturbed lakes, algal biomass was proportional to the fraction of the catchment area perturbed divided by the surface area of lakes in the catchment.

Eleven headwater lakes in Alberta's Boreal Plain were monitored for nutrients and plankton 2 years before and 2 years after variable watershed harvesting (harvesting mean 15%, range 0-35%). After harvesting, variations in annual precipitation resulted in lake water residence times that differed by an order of magnitude from one year to the next. During the first posttreatment year, total phosphorus concentrations increased (overall 40%) in most lakes; however, response was most consistent in lakes that were shallow and the water column mixed or weakly thermally stratified. Chlorophyll a, cyanobacteria (Aphanizomenon-Anabaena), and cyanotoxins (microcystin-LR) increased after harvesting, primarily in shallow lakes. Zooplankton abundance and biomass decreased after harvesting, particularly in stratified lakes where edible phytoplankton biomass declined. In the weakly or nonstratified lakes, declines in zooplankton biomass were associated with higher cyanobacterial biomass and cyanotoxins. Posttreatment change in total phosphorus concentration was strongly related to weather (greatest response in a wet year) and relative drainage basin size (drainage basin area to lake volume, r 2 = 0.78, P << 0.01). There was no evidence that buffer strip width (20, 100, and 200 m) influenced lake response. These results suggest that activities within the entire watershed should be the focus of catchment-lake interactions.Résumé : Nous avons fait le suivi des nutriments et du plancton dans onze lacs situés en tête de bassin hydrographique dans la Plaine boréale de l'Alberta 2 ans avant et 2 ans après des récoltes forestières variables (en moyenne 15% de la surface du bassin hydrographique, étendue de 0 à 35%). Après la récolte, les changements dans les précipitations annuelles ont entraîné des variations dans la durée du séjour de l'eau dans les lacs, par un facteur de dix, d'une année à une autre. Durant la première année après la récolte, les concentrations totales de phosphore ont augmenté (40% en tout) dans la plupart des lacs; cet effet a cependant été plus uniforme dans les lacs peu profonds où le brassage de la colonne d'eau était complet ou chez lesquels la stratification thermique était faible. Les concentrations de chlorophylle a et de cyanotoxines (microcystin-LR) et les densités de cyanobactéries (Aphanisomenon-Anabaena) ont crû après la récolte, particulièrement dans les lacs peu profonds. En revanche, l'abondance et la biomasse du zooplancton ont décliné après la récolte, surtout dans les lacs stratifiés où la biomasse de phytoplancton comestible avait diminué. Dans les lacs peu ou pas stratifiés, le déclin de la biomasse du zooplancton était associé à une augmentation de la biomasse des cyanobactéries et de la concentration des cyanotoxines. Les changements dans les concentrations totales de phosphore après la récolte étaient fortement reliés aux conditions météorologiques (avec les effets les plus marqués au cours de l'année pluvieuse) et à la taille relative du bassin hydrographique (surface du bassin de drainage p...

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The recent discovery that N2 fixation rates by the feather moss carpet of boreal Scandinavian forests increases with stand maturity has put into question the paradigm that N2 fixation is negligible in mature boreal forest. The N2 fixation was attributed to a previously unknown association between Nostoc sp., a N2-fixing cyanobacteium and Pleurozium schreberi (Brid.) Mitt., a feather moss that is abundant worldwide in the boreal forest. Here we report for the first time that this association also exists in the Canadian boreal forest. We discovered, however, that Nostoc was found growing not only on Pleurozium but also on two other moss species ( Hylocomnium splendens (Hedw.) Br. Eur. and Ptillium crista-castrensis (Hedw.) De Not.). In addition, the N2-fixing cyanobacterium Stigonema sp. was observed on the three moss species mentioned above, indicating the existence of six different associations. At least one of the six associations was found at 9 of 13 sites that are representative of a large area of the Quebec boreal forest. These findings suggest possibilities for further research, aimed at measuring the unaccounted for N2-fixing potential of the feather moss carpet in Canadian boreal forests.

HighlightsPhosphate increased glyphosate uptake and decreased its toxicity in willows PO 4 3-concentrations ≥ 200 mg l -1 doubled glyphosate uptake by willow roots PO 4 3-concentrations ≥ 200 mg l -1 increased antioxidant system activity PO 4 3-maintained photosynthesis rates by inducing reactive oxygen species scavenging 3 Abstract Phosphate (PO 4 3-) has been shown to increase glyphosate uptake by willow, a plant species known for its phytoremediation potential. However, it remains unclear if this stimulation of glyphosate uptake can result in an elevated glyphosate toxicity to plants (which could prevent the use of willows in glyphosate-remediation programs). Consequently, we studied the effects of PO 4 3-on glyphosate uptake and toxicity in a fast growing willow cultivar (Salix miyabeana SX64). Plants were grown in hydroponic solution with a combination of glyphosate (0, 0.001, 0.065 and 1 mg l -1 ) and PO 4 3-(0, 200 and 400 mg l -1 ). We demonstrated that PO 4 3-fertilization greatly increased glyphosate uptake by roots and its translocation to leaves, which resulted in increased shikimate concentration in leaves. In addition to its deleterious effects in photosynthesis, glyphosate induced oxidative stress through hydrogen peroxide accumulation.Although it has increased glyphosate accumulation, PO 4 3-fertilization attenuated the herbicide's deleterious effects by increasing the activity of antioxidant systems and alleviating glyphosateinduced oxidative stress. Our results indicate that in addition to the glyphosate uptake, PO 4 3-is involved in glyphosate toxicity in willow by preventing glyphosate induced oxidative stress.

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Longitudinal variations of phytoplankton biomass and composition were assessed in a 250 km-long section of the St.Lawrence River, which alternately runs through narrow (< 2 km) river cross sections and wide (up to 10 km) fluvial lakes . In the main river stem, concentrations of suspended matter and total phosphorus increased with distance downstream, whereas light penetration decreased . Seasonal changes in plankton composition and biomass were more important than those resulting from differences in water mass (tributary) of origin . Sampling at three cross river sections and in two fluvial lakes showed a progressive downstream decrease in phytoplankton biomass and changes in size structure and taxonomic composition. River plankton was primarily composed of small (< 10 µm equivalent spherical diameter), truly planktonic cells belonging to Cryptophyceae and diatoms, with Chlorophyceae in summer. Plankton sampled in summer among rooted macrophytes in fluvial lakes exhibited a higher biomass of resuspended periphytic algae than in the main river stem, which contributed slightly to downstream phytoplankton biomass.Successive river cross sections always shared about 50% of their taxa, indicating a rapid downstream transport of algae within the main water mass . However, the proportion of species common to all cross sections was highest during the spring freshet, and lowest during summer low discharge, likely resulting from the development of a distinct flora in fluvial lakes during summer . Conversely, about 30% of the identified taxa were exclusive to a cross section and were replaced by others occurring downstream . Overall, phytoplankton composition along the St .Lawrence River is primarily controlled by advective forces, which result in a homogeneous flora in the main river stem, with a local contribution of resuspended periphyton from fluvial lakes .

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