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Planas, Do, L. Lapierre, G. Moreau, and M. Ailard. 1989. Structural organization and species composition of a lstic periphyton community in response to experimental acidification. Can. 8. Changes in species compositisn, richness, diversity, biomass (biovslume), and spatial community structure of a lstic periphyton community were observed under experimental acidification (mean pH 4.5) with and without aluminum, in troughs fed from an oligo-dystrophic creek (mean pH 6.7). In experimental and control troughs, the community was dominated by diatoms throughout the treatment. Achwanthes lineasis was dominant in all troughs before treatment as well as in the control during the experiment. Eunotia pectinalis, which was rare in all troughs before treatment, became dominant in the acidified troughs. After 3 ms of acidification, algal cell density and biomass were respectively 30 and 40-78 times higher in the treated troughs as compared with that in the control. The low diversity (H= 1.45-2.30) characteristic of the control trough, increased to 3.3 in the treated troughs after 1 mo of acidification, but after 3 rno of experiment the diversity was slightly Bower in experimental troughs. In the control trough, spatial organization of the community was characterized by monolayered arrangements during the whole experiment. In both acidified troughs (acid and acid plus aluminum) the community became multilayered (3-4 layers). t h e increased complexity of the community in the acidified troughs seemed to be related to a decrease in the abundance of grazers in the periphyt'rc community, mainly the collectorgatherers Ephemeroptera and Brthocladiinae.On a sbserv6 des modifications de la composition en especes, de la richesse, de la diversite, de la bismasse (biovolume), et de la structure spatiale des populations d'un periphytow htique ssumis une acidification exp6-rimentale (pH moyen de 4,5), avec et sans aluminium, dans des bassins aliment& par un ruisseau oligotropke (pH moyen de 6,7). Bans les bassins exp6rimentaux et t6moins, les diatomees ont predornine pendant toute la duree du traitement. Achnawthes Einearr's a domine dans tous les bassins avant le traitement ainsi qeJe chez les ternsins au csurs de I'expkrience. Eurpoeia pecelnalis, rare dans toars les bassins avant le traiternent, est devenue dorninante dans Oes bassins acidifies. AprGs 3 rno d'acidification, la densit6 des algues et la biomasse ktaient respectivernewt 38 et 40 3 78 fois plus 68ev4es dans les bassins trait& que dans les bassins ternsins. La faibte diversite ( H -1,45-2,381 du bassin de contrsle a augment6 2 3,3 daws les bassins trait& aprPs 1 rno d'acidification, mais au bout de 3 rno $'experience, la diversite etait Iegerernent plus iaible dans les bassins experitnewtaux. Dans le bassin t h o i n , les populations se sont reparties en monocouches pendant toaate la duree de I'experience. Dans les deux bassins acidifies (acide et acide plus aluminium), O e s populations se sont reparties en plusierrrs couches (3 3 4 couches). La complexit6 accrue des popul...

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Yellow perch (Perca flavescens) (n = 50) were collected from five geographic sectors between 1991 and 1992 to assess the spatial variability in trace metal and PCB (10 congeners) levels along the St. Lawrence River. Spatial differences among the five sectors were analyzed using two-way ANOVA and were significant only for PCBs. Both Hg and total congener concentrations were significantly and positively correlated with fish age, but not with mass or length. The relative proportions of the PCB congeners did not vary among sites. The bioaccumulation factor (BAF) of 3700 for total congeners compared favourably with previously reported results. Since 1975, Hg and PCB levels in St. Lawrence River yellow perch have decreased by factors of 2-3 and 30, respectively. Power analyses revealed that future monitoring studies may require large sample sizes to successfully detect the small spatial differences found for many contaminants. For example, to detect, at a power level of 90%, a 33% difference in Hg levels in yellow perch (i.e., the maximum difference found in this study) among the five sectors, 80 fish per sector would be required.

The effects of river diversion on phytoplankton primary production and biomass in the downstream part of two rivers were studied in relation to physical and chemical variables. These rivers, situated north of the 52nd parallel, are characteristic of oligotrophic systems with phytoplankton primary production less than 10.76 mg C m-2 h-l, chlorophyll -a lower than 3.0 mg m-3 and biomass between 118-1 007 mg m-3. The decrease in flow favored the establishment of an algal biomass approximately two times greater then that present before diversion. This increase in biomass was associated in one river with an increase of 2.5 times of the mean primary production. In the other river the primary production per unit of surface area remained stable but increased when expressed by unit volume, due to a great decrease in underwater light penetration, consequence of inorganic particular matter increase.