The trophic structure of pelagic communities in lakes of glaciated regions is highly variable due to restricted dispersal of glacial relict taxa and recent species introductions. Much of the enormous between-lake variability in PCB levels in lake trout flesh (15–10 000 ng/g) from the St. Lawrence system results from differences in the length of pelagic food chains. Ontario Ministry of the Environment data (1978–81) on PCB concentrations in lake trout flesh indicate that PCB concentrations increased with the length of the food chain and tissue lipid content, and decreased with distance north of urban-industrial centres. Each trophic level contributed about a 3.5-fold biomagnification factor to the PCB concentrations in the trout, and the lipid content of the trout flesh increased by a factor of 1.5 for each additional trophic level. An empirical model capable of predicting PCB levels in pelagic salmonids and forage fish (smelt and coregonids) indicated that biomagnification of small atmospheric inputs of persistent lipophilic contaminants can explain the frequent occurrence of high levels of contaminants in some biota from remote areas, and that species introductions that lengthen food chains will lead to significant increases in levels of atmospherically dispersed persistent organic contaminants in top predators.

Although many measurements have been made on radiocesium levels in water and aquatic biota, no agreement has been reached regarding the factors affecting bioaccumulation of these radionuclides. With monitoring data from countries that operate nuclear facilities and data from the primary literature, we explored the chemical and ecological factors that determine the bioaccumulation of radiocesium. Using log–linear regression we found that the bioaccumulation of 137Cs by fish was a negative function of both dissolved potassium and suspended sediment concentration, and a positive function of temperature. Important ecological factors were the trophic level of the fish (piscivores bioaccumulate more than plank-tivores and benthivores), and the length of the food chain as reflected by the ratio of piscivore yield relative to net primary production. Fish from softwater drainages, which make up a large portion of northern Europe and Canada, are more vulnerable to radiocesium contamination than fish from hardwater sedimentary drainages, because these waters are extremely low in potassium and suspended sediment, and their watersheds are less efficient in retaining radiocesium. High dissolved potassium, short food chains, and the much greater volume and mixing potential of the ocean make marine fish less vulnerable to releases of radiocesium.

The mud deposition boundary depth (mud DBD) is the depth in lakes at which the boundary occurs between high-energy erosive environments (coarse-grained noncohesive sediments) and low-energy depositional zones where fine-grained cohesive sediments accumulate. We have derived a model from the theory of waves and sediment thresholds that predicts the upper limit to the distribution of fine-grained sediments in lakes of any size. Our results suggest that the several biggest storms each year, rather than extremely rare events, are responsible for the upper limit to the distribution of mud. However, significant areas of coarse-grained sediments and many mud DBDs occur deeper than this upper limit, usually on slopes greater than 3%. For sediment at the mud DBD (23 μm), we have developed an empirical relationship between slope and maximum horizontal velocity that demonstrates the significant effect of slope on reducing either sediment threshold velocity or sediment stability.

FLAIR subarachnoid space hyperintensity may be encountered with both pathological conditions and artifacts. Knowledge of these conditions and appearances coupled with any associated findings may suggest the cause of the FLAIR subarachnoid space hyperintensity. A diffuse distribution and a lack of ancillary findings often remain nonspecific and may require clinical correlation and CSF analysis.

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While the flow of energy is understood to determine the growth of organisms and the productivity of ecosystems, little is known about the sublethal effect of pollutants on the energetic efficiency of wild populations. We used field estimates of fish growth coupled to in situ estimates of food consumption rates obtained from the mass balance of a globally dispersed, trophically transferred radiotracer ( 137 Cs) to demonstrate the bioenergetic impairment of yellow perch (Perca flavescens) from lakes polluted by heavy metals (Cd, Cu, and Zn). Annual growth increment relative to the total energy budget (conversion efficiency) was about three times lower in cortisol-impaired yellow perch from metal-polluted lakes relative to yellow perch from reference lakes (4.2% compared with 10.8%), suggesting that fish exposed to pollutants experienced greater total energetic costs. In addition, metal-polluted lakes were dominated by adult yellow perch populations and simplified prey bases, suggesting that effects are occurring at multiple levels of biological organization. Our in situ bioenergetic approach to toxicity assessment provides a measurable and ecologically relevant endpoint for assessing the sublethal effects of pollutants on fish communities.Résumé : Bien qu'on sache que le flux d'énergie détermine la croissance des organismes et la productivité des écosys-tèmes, on sait peu de choses de l'effet sublétal des polluants sur l'efficacité énergétique des populations sauvages. Nous avons utilisé des estimations de la croissance des poissons dans la nature couplées à des estimations des taux d'alimentation in situ obtenues à partir du bilan massique d'un radiotraceur présent partout sur la Terre et transmis par voie trophique ( 137 Cs) pour déterminer la perturbation bioénergétique de la perchaude (Perca flavescens) de lacs pollués par des métaux lourds (Cd, Cu et Zn). L'incrément annuel de croissance en rapport avec le bilan énergétique global (efficacité de conversion) était environ 3 fois plus bas chez les perchaudes des lacs pollués par les métaux, dont le mé-tabolisme du cortisol était perturbé, que chez les perchaudes de lacs témoins (4,2% comparativement à 10,8%), ce qui indique que les coûts énergétiques totaux sont plus élevés chez les poissons exposés aux polluants. De plus, dans les lacs pollués par les métaux, les populations de perchaudes adultes étaient dominantes et la diversité des proies était faible, ce qui laisse penser que les effets des polluants se font sentir à divers niveaux du biote. Notre approche bioéner-gétique in situ en matière d'évaluation de la toxicité offre un paramètre mesurable et écologiquement pertinent pour évaluer les effets sublétaux des polluants sur les communautés de poissons.[Traduit par la Rédaction] Sherwood et al. 450

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