Dreissenid mussels have been exceptionally successful invaders in North American lakes and rivers, especially in the lower Laurentian Great Lakes. As benthic filter feeders capable of attaching to hard substrates, the magnitudes of their biomass and filtering activity in nearshore waters are without precedent. The dreissenid colonization has implications for the removal and fate of materials filtered from the water by the mussels and for the longer-term development of the nearshore benthic community and lake ecosystem. A conceptual model, the nearshore shunt, seeks to describe a fundamental redirection of nutrient and energy flow consequent to dreissenid establishment. The model explains some emergent problems in the Great Lakes, such as reemergence of Cladophora in some coastal zones while offshore P concentrations remain low, and highlights areas in need of more research. The source of particulate nutrient inputs to dreissenids and the fate of materials exported from the benthic community are critical to understanding the role of dreissenids in the lakes and assessing the applicability of current models for managing nutrients and fisheries. The nearshore shunt would require even more stringent P management for lakes strongly impacted by dreissenids to maintain nearshore water quality.Résumé : Les moules dreissenidées ont envahi avec un succès exceptionnel les lacs et rivières d'Amérique du Nord, et en particulier les Grands Lacs inférieurs. Des animaux benthiques se nourrissant par filtration et capables de se fixer à des substrats durs, ils ont atteint des biomasses et des niveaux d'activité de filtration sans précédent dans les eaux côtières. La colonisation des dreissenidés a des conséquences sur le retrait et le sort des matériaux filtrés de l'eau par les moules, ainsi que sur le développement à plus long terme de la communauté benthique près du rivage et sur l'écosystème lacustre. Un modèle conceptuel, la « déviation côtière », cherche à décrire l'altération fondamentale du cheminement des nutriments et du flux d'énergie depuis l'établissement des dreissenidés. Le modèle explique certains des problèmes qui surgissent dans les Grands Lacs, comme la ré-emergence de Cladophora dans certaines zones côtiè-res, bien que les concentrations de phosphore au large restent faibles et il identifie des domaines qui nécessitent des recherches supplémentaires. Une connaissance des sources des apports de particules nutritives pour les dreissenidés et du sort des matériaux exportés de la communauté benthique est essentielle pour comprendre le rôle des dreissenidés dans les lacs et pour évaluer l'applicabilité des modèles courants de gestion des nutriments et des pêches. La « dévia-tion côtière » exigerait une gestion encore plus rigoureuse du phosphore dans les lacs fortement affectés par les dreissenidés afin de maintenir la qualité de l'eau près du rivage.[Traduit par la Rédaction] Hecky et al. 1293

Sleep has tremendous importance to organizations because of its relationship with employee performance, safety, health, and attitudes. Moreover, sleep is a malleable behavior that may be improved by individual and organizational changes. Despite the consequential and modifiable nature of sleep, little consensus exists regarding its conceptualization, and how the choice of conceptualization may impact relationships with organizational antecedents and outcomes. To offer a stronger foundation for future theory and research about employee sleep, this study calculated meta-analytic correlations of sleep quality and sleep quantity from 152 primary studies of sleep among workers in organizations. Analyses revealed that both sleep quality and sleep quantity associated negatively with workload and a number of health, attitudinal, and affective outcomes. Despite their conceptual similarity, notable differences existed in sleep quality and sleep quantity in terms of their relationships to many different correlates. Generally, the relationships between sleep quality and the examined correlates were stronger for variables that reflected perceptions. Moderator analyses showed that relationships between sleep quality and quantity may be affected by measurement method and the number of self-report items used, while there is little evidence of the effect of measurement time frame. Findings from this first meta-analytic investigation of the occupational sleep literature have implications for the development of theory about relationships between sleep and work, the measurement of sleep, the identification of organizational correlates of sleep, and the design of interventions intended to improve employee sleep. (PsycINFO Database Record

Atmospheric photodissociation rate coefficients and photodissociation lifetimes for nitromethane, methyl nitrite, and methyl nitrate were calculated as a function of altitude from their measured visible and near ultraviolet photoabsorption cross sections at 298 K. The lifetime of methyl nitrite is nearly independent of altitude and is approximately 2 min. From 0 to 50 km the lifetime of nitromethane varies from 10 to 0.5 hr, while that of methyl nitrate changes from 5.3 to 0.09 days, respectively.

Phosphate is an important nutrient that restricts microbial production in many freshwater and marine environments. The actual concentration of phosphate in phosphorus-limited waters is largely unknown because commonly used chemical and radiochemical techniques overestimate the concentration. Here, using a new steady-state radiobioassay to survey a diverse set of lakes, we report phosphate concentrations in lakes that are orders of magnitude lower than estimates made spectrophotometrically or with the frequently used Rigler radiobioassay. Our results, combined with those from the literature, indicate that microbes can achieve rapid turnover rates at picomolar nutrient concentrations. This occurs even though these concentrations are about two orders of magnitude below the level where phosphate uptake is estimated to be half the saturation level for the pico-plankton community. Also, while phosphate concentration increased with the concentration of total phosphorus and soluble reactive phosphorus in the lakes we sampled, the proportion of phosphate in the total phosphorus pool decreased from oligotrophic to eutrophic lakes. Such information, as revealed by the phosphate assay that we use here, should allow us to address hypotheses concerning the concentration of phosphate available to planktonic microorganisms in aquatic systems.

Electron spin resonance was used to measure the diffusion of a small (Mr 170) spin label in the aqueous cytoplasm of mammalian cells. Translational and rotational motion were determined from the same spectra. Based on measurements made in model systems, it was hypothesized that calculations of the apparent viscosity from either rotational or translational motion would distinguish between the effects of cytoplasmic viscosity or cytoplasmic structure on diffusion. The diffusion coefficient calculated from spin label collision frequency, averaged 3.3 x 10-6 cm2/sec in several cell lines. It was greater in growing cells and in cells treated with cytochalasin B than in quiescent cells. The viscosity of the cytoplasm calculated from the translational diffusion coefficient or the rotational correlation time was 2.0-3.0 centipoise (1 P = 0.1 Pasec), about 2-3 times that of the spin label in water. Therefore, over the dimensions measured by the technique, 50-100 A, solvent viscosity appears to be the major determinant of particle movement in cells under physiological conditions. However, when cells were subjected to hypertonic conditions, the translational motion decreased by 67%, while the rotational motion changed less than 20%. These data suggested that the decrease in cell volume under hypertonic conditions was accompanied by an increase in cytoplasmic barriers and a decrease in the spacing between existing components. In addition, a comparison of reported values for diffusion of a variety of molecules in water and in cells indicates that cytoplasmic structure plays an important role in the diffusion of proteins such as bovine serum albumin.Movement in cell cytoplasm has been a topic of investigation since early observations of cytoplasmic streaming. The study of movement has been intimately associated with a study of the structure of the cytoplasm. As the use of transmission and high-voltage electron microscopes revealed a complex and ordered structure, theories about how mole--cules move within the cytoplasm have had to be modified to take this structure into account.Over the years various approaches have been taken to measure the movement of large probes placed into cells by phagocytosis or by microinjection. For example, Crick and Hughes (1) examined the cytoplasm of fibroblasts that had phagocytized iron filings. Others have used radiolabeled molecules to measure diffusion in oocytes, muscle fibers, or axons (2-6). More recently, the techniques of microinjection and of fluorescence recovery after photobleaching have been combined to estimate the translational diffusion of proteins such as bovine serum albumin in cell cytoplasm (7,8).ESR techniques have been used to measure the rotational diffusion of small probes in cytoplasm (9). The rotational correlation time, Tc, of the spin label is closely related to local cytoplasmic viscosity. However, the translational motion also can be calculated from the same spectra when an appropriate concentration range of spin label is used. In this study we used ESR to c...

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The epilimnion depth of lakes is related both to lake size, which affects wind‐induced mixing, and to water clarity, which affects the depth over which solar radiation heats the water. Here we attempt to isolate the relative importance of these two variables by examining lakes that have changed in water clarity with time (between years), by examining nearby lakes of comparable size, and by partitioning a large number of lakes into subsets with a restricted range in size or Secchi depth. Overall, results indicate that both lake size and water clarity are important determinants of epilimnion depth, but the absolute effect (indicated by slope) of Secchi depth is approximately constant in small (<12.5 km2) as well as large lakes and the Laurentian Great Lakes, while its relative importance (indicated by r2) appears to be restricted to the small lakes.