Stable isotope compositions were studied in particulate organic matter (POM), zooplankton and different trophic groups of teleosts to compare food chains based on plankton at two sites (lagoon and outer slope) in a New Caledonian coral reef. For each trophic compartment, δ13C values were always lower in the outer slope than in the lagoon. This result may be explained by potential differences in POM composition between the two environments, suggesting that the two food chains are based on different primary sources of carbon. In contrast, δ15N values did not vary between the lagoon and the outer slope, indicating that these two food chains presented similar length and trophic levels, despite being distinguishable.

Nitrogen stable isotope composition (δ15N) of particulate organic matter (POM) has been used to infer dominant nitrogen cycling processes in lakes. However, very few studies have compared the isotope variations in lakes along trophic state and other biogeochemical gradients. Here we report an analysis of δ15NPOM and selected environmental variables from 96 subtropical lakes to assess the patterns and controls of isotope variations. Results indicate that δ15NPOM values varied from −2.8 to 13.2‰ and were not significantly correlated with total phosphorus (TP), total nitrogen (TN) and chlorophyll a (Chl a). The 15N depletion in POM was found across the entire trophic gradient and likely reflected contributions from planktonic nitrogen fixation. The 15N enrichment was attributed to high primary production and the contributions of anthropogenic wastes in eutrophic lakes, and the presence of microzooplankton in the water samples of the oligotrophic lakes. The δ15NPOM was negatively correlated with water color and positively correlated with pH. Because water color is indicative of light availability which affects phytoplankton growth and pH is significantly correlated with Chl a concentration in the study lakes, the close relationship between water color, pH and δ15NPOM therefore suggests primary productivity-driven isotope fractionation. Results from this study did not reveal the importance of trophic state, nitrogen concentration and surface area to the variations of δ15NPOM and points to the complex interactions of nitrogen cycling processes in lakes with diverse biogeochemical features.

Particulate organic matter was collected in the coastal waters of Terra Nova Bay during the Oceanographic Campaign of the Italian Antarctic Research Programme in summer (January–February) 1990. Particulate matter composition was analysed for organic carbon and nitrogen, carbohydrates, proteins, lipids, nucleic acids (DNA and RNA) and ATP. A vertical stratification was evident in the study area, and resulted from prior melting of the pack ice. Suspended organic matter in the mixed layer and below the mixed layer differed in quantitative and qualitative composition. Except for ATP, all the biochemical components showed higher concentrations in the mixed layer than in the deeper waters. The particulate organic matter in Terra Nova Bay was mostly detrital and of algal origin.

As part of the Ross Sea Marginal Ice Zone Ecology Project (ROSSMIZE) the summer organic matter flux through the water column was measured at 40 m depth in Terra Nova Bay. Water samples and material from a sediment trap on the sea-bottom were analysed for their biochemical composition. A close coupling between biochemical composition of the organic matter in the water column and the material collected in the sediment trap was found, resulting from complex interactions between physical and biological processes. The physical processes are related to break-up and melting of the ice cover and occur mainly in the early summer season, whilst biological processes play a key role in mid summer and, from the evidence from faecal pellets, are related to the primary production and consumption processes.

Les premières données relatives au seston des lacs du Jura sont présentées. 9 lacs ont fait l'objet de prélèvements en fin de période estivale, par pompage intégré à la verticale du point de profondeur maximale. La matière en suspension totale (MEST) varie selon les systèmes de 0,89 mg/l à 5,51 mg/l. Trois cas de figure se distinguent dans le seston des lacs étudiés : seston à prolifération planctonique, secton à trypton dominant, seston « équilibré ». Sur la moyenne des échantillons, la fraction détritique représente 30 % du nombre des particules en suspension. L'analyse microparticulaire a conduit à l'identification et à la définition de diverses catégories de particules organiques regroupées en deux grands ensembles : les éléments figurés et les associations organo-minérales composites. Dans les échantillons à prolifération planctonique ou à trypton dominant, les macrophytoclastes et les associations organominérales composites présentent leurs abondances maximales. Une interprétation est proposée relative à la formation des associations organo-minérales en rapport avec différents facteurs écologiques : importance de la frange littorale, couple oxygène/température, prolifération planctonique. Les résultats sont discutés en relation avec le mode de fonctionnement des systèmes.