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The objective of this study is to verify whether low amounts of fine sand added to a muddy sediment matrix can be detected and quantified with accuracy using a Mastersizer 3000 (Malvern Panalytical) laser diffraction particle-size analyzer equipped with a Hydro LV large volume liquid dispersion module. To achieve this goal, a postglacial sediment sample was sieved to recover naturally co-occurring sand and clay-silt fractions. Sand in the range of 1%-7% by weight was added to the clay-silt at three concentrations (88, 132, and 276 mg dry weight) and each sample was duplicated. A very strong linear relationship was found between the measured % volume of sand added and the actual weight of sand added to the mud. Sand representing as low as 1% by weight could be detected. On average, there was only a 0.7% absolute difference between the measured and actual % sand values (range 0.02%-1.65%). Sample concentration had a negligible impact on the measured % sand. A range of plausible values for the refractive and absorption indexes, essential parameters for the Mie theory calculation of the size distribution from the measured light scattering, also had very small impact on the measured % sand. The demonstrated possibility of detecting a small input of fine sand to muddy sediment provides a basis for studies using grain-size data to reconstruct past and modern detrital inputs and sediment transport variations. Key words: laser diffraction, particle-size analysis, grain size, sediments L'objectif de l'etude est de verifier si de faibles quantites de sable fin ajoutees a une matrice de sediments boueux peuvent etre detectees et quantifies avec exactitude a l'aide du granulometre a diffraction laser Mastersizer 3000 (Malvern Panalytical) equipe d'un module de dispersion en voie liquide a large volume Hydro LV. A cette fin, un echantillon de sediment postglaciaire a ete tamise pour en recuperer les fractions de sable et d'argile-silt s'y trouvant a l'etat naturel. Du sable dans une fourchette de 1-7 % en poids a ete ajoute a trois concentrations d'argile-silt (88, 132 et 276 mg en poids sec), et chaque echantillon a ete repete. Une relation lineaire tres forte est relevee entre le pourcentage volumique mesure de sable ajoute et le poids reel de sable ajoute a la boue. Du sable representant aussi peu que 1 % du poids a pu etre detecte. En moyenne, la difference absolue entre les pourcentages mesures et reels de sable (fourchette de 0,02-1,65 %) n'est que de 0,7 %. L'incidence de la concentration de l'echantillon sur le pourcentage de sable mesure est negligeable. Une fourchette de valeurs plausibles des indices de refraction et d'absorption, des parametres essentiels pour le calcul selon la theorie de Mie de la granulometrie a partir de la diffusion de la lumiere mesuree, a aussi tres peu d'incidence sur le pourcentage de sable mesure. La possibilite demontree de detecter un petit apport de sable fin dans des sediments boueux fournit la base pour des etudes utilisant des donnees granulometriques pour reconstituer les apports detritiques et les variations du transport de sediments passes et actuels. [Traduit par la Redaction] Mots-cles : diffraction laser, analyse granulometrique, granulometrie, sediments, Introduction An accurate characterization of sediment particle-size distribution is essential to understand the genesis of sediments, untangle transport mechanisms, infer past hydrologic and climatic conditions, track sea-level variations, map and [...]

Abstract: We profiled five ice-covered lakes and two ice-covered fiords of Ellesmere Island at the northern limit of High Arctic Canada to examine their environmental characteristics, and to evaluate the [...]

Thick ice cover is a feature of cold-temperate, polar, and alpine lakes and rivers throughout much of the year. Our observations from Canadian lakes and rivers across the latitudinal gradient 46-80[degrees]N show that their overlying ice contains low concentrations of dissolved organic carbon (DOC) and colored dissolved organic matter (CDOM) relative to the underlying waters. The CDOM exclusion factor (water/ice) ranged from 1.4 to 114 and was typically greater than twice the exclusion factor for inorganic solutes. Application of synchronous fluorescence analysis to lake ice samples and experimentally frozen lakewater indicated that only less complex, lower molecular weight molecules were retained within the ice. Consistent with this analysis, DOC-specific absorption showed that the DOC in the ice was generally less colored than that in the underlying waters. The reduced CDOM absorption within the ice allowed relatively high ultraviolet (UV) transmission despite the elevated scattering within the ice and resulted in UV diffuse attenuation coefficients up to eight times lower in the ice than in the underlying waters. This relatively low attenuation by the ice would cause organisms trapped near the surface by inverse stratification to experience high UV exposure prior to ice breakup. The ice exclusion effect gives rise to a concentrated zone of CDOM and DOC that is likely to favor heterotrophic and mixotrophic processes and influence biogeochemical interactions.

Underwater ultraviolet (UV) attenuation is typically modeled using correlative relationships between UV attenuation and dissolved organic carbon (DOC) concentration or colored dissolved organic matter (CDOM) absorption. Our objective was to validate and extend a biogeooptical model of UV penetration that would take into account total absorption as well as scattering by all components of the water column. Diffuse attenuation coefficients, [K.sub.d]([lambda]), for photosynthetically available radiation (PAR) and UV irradiance were measured using a PUV500 radiometer at 13 stations in Lake Biwa, Japan, in late summer. Absorption, a, and scattering, b, were measured at nine PAR wavelengths using an AC-9 absorption-attenuation meter. The average [K.sub.d](PAR) through the euphotic zone, [K.sub.dav](PAR), was estimated from the measured inherent optical properties using Kirk's equations derived from Monte Carlo simulations. The modeled [K.sub.dav](PAR) was strongly correlated with measured [K.sub.d](PAR) values ([r.sup.2] = 0.998), although the model systematically underestimated measured values by [greater than or equal to] 15%. The relative contribution of absorption and scattering to UV attenuation was evaluated from measurements of the absorption coefficients, an extrapolation of b into the UV, and Kirk's equations. The UV-absorption coefficients for CDOM and particulate matter were measured spectrophotometrically. There was a close agreement between modeled and measured [K.sub.d](UV) values ([r.sup.2] [greater than or equal to] 0.990). The Lake Biwa data showed that particulate absorption and scattering play significant roles in UV attenuation; on average, absorption by water, CDOM, and particles contributed 1, 66, and 21% respectively to [K.sub.dav](340), while scattering by particles contributed 11%. At a turbid site (NS9) absorption by water, CDOM, and particles contributed 0.3, 40, and 36% respectively to [K.sub.dav] (340), while scattering contributed the remaining 24%. The model described here integrates the effects of all absorption as well as scattering components and provides a more accurate estimate of underwater UV exposure than previous approaches based on CDOM absorption only.

A mesocosm experimental test has been made on the effect of ultraviolet-B (UVB) radiation in a planktonic community and findings indicate enhanced UVB radiation can change dynamics and structure of the pelagic food web. Dynamics of several populations at different trophic levels were looked at in the experiment. Severe effects of UVB seen on ciliates and large phytoplankton communities would severely limit upward transfer of energy and mass. If UVB is enhanced at the community level, the ecosystem moves toward a more microbial food web.

Vertical profiles of ultraviolet radiation (UVR) and photosynthetically available radiation (PAR) were measured under five large ice floes located in the North Water Polynya, northern Baffin Bay, in June 1998. Together with incident irradiance measurements, these profiles were used to assess the irradiance attenuation by the ice and its constituents. We also measured vertical distribution of absorption by colored dissolved organic matter (CDOM) and particulate organic matter (POM) in three melted ice cores. The ice thickness and snow depth varied from 0.5 to 1.3 m and from 1 to 9 cm, respectively. The ice-snow interface was infiltrated by meltwater. About 2-13% of incident UV-B irradiance was transmitted through the snow, ice, and ice algae biomass; transmittance increased to 5-19% for UV-A and to 5-12% for PAR. CDOM and POM contributed significantly to the attenuation of irradiance within the ice. The relatively high UVR transparency found in this study coincided with the seasonal maximum of incident UV irradiance. Hence, the resulting very high UVR:PAR ratio could affect the communities in the sea ice, at the ice-water interface, and in the surface waters underneath the ice cover. In addition, the strong absorption by CDOM found in this high-UVR environment indicates that significant photochemical reactions could occur.