The effects of temperature on the life table, and of seston quality on the individual growth and reproduction of cladocerans from a tropical lake were tested in the laboratory. Life-table experiments were carried out at 17°C, 23°C, and 27°C. Growth bioassays tested the influence of natural seston fractions, separated by net filtration, on cladocerans. The treatments were: (1) total seston plus Scenedesmus spinosus (1 mg C.L−1), (2) seston ≤36 μm, and (3) seston >36 μm. Phytoplankton composition, density, and biomass were evaluated during growth experiments, together with sestonic carbon, nitrogen, and phosphorus concentrations. The intrinsic rates of natural increase were higher for Moina micrura and Daphnia ambigua at 27°C compared to 17°C. The age at first reproduction of both species was delayed at 17°C. Growth rates and fecundity of M. micrura were higher in the seston fraction ≤36 μm than in the fraction >36 μm. Higher growth rates and fecundity of Moina minuta were observed in the seston enriched with the green alga in comparison to the seston ≤36 μm and >36 μm. Bosmina longirostris was unable to reproduce at 17°C and to grow in the seston >36 μm in one experiment. High densities and/or biomass of large colonial and filamentous algae present in the larger seston fraction could have contributed to reduce growth and reproduction. Episodes of food-quantity limitation may occur, but there was no evidence of mineral limitation, although seston C∶P and C∶N ratios were always above the limiting values assumed for temperate water bodies. The C∶P and C∶N ratios are highly influenced by carbon that originates primarily from resuspended detritus from the lake.

We studied the effects of a waste water treatment plant on organic matter dynamics in Fosso Bagnatore, a Mediterraneanstream near Rome (Italy). Concentrations of dissolved organic carbon and seston, along with the standing crops of coarse benthicorganic matter and fine benthic organic matter, were monitored monthly during 2002 in two reaches situated upstream anddownstream of a waste water treatment plant effluent. Additionally, we examined two stream functional characteristics: wholestream metabolism, which was measured from diel changes in oxygen concentrations, and nutrient removal, which was studiedby experimental additions of NH4-N, NO3-N and PO4-P. The possible influence of WWTP organic matter alterations on ecosystemfunctioning was addressed by measuring epilithon characteristics (standing crop, C:N:P ratios and chlorophyll a content).The WWTP effluent caused an increase (several-fold) of DOC and seston concentrations, and FBOM standing crop, moreoverit modified their temporal variation. Downstream of the wastewater treatment plant we detected an higher heterotrophic microbialbiomass and a correspondent higher community respiration. The uptake of NH4-N, NO3-N and PO4-P was highly variableprobably as consequence of the high variability of the compartments involved (e.g. DOC, FBOM, community respiration). Weshowed that WWTP effluent altered the organic matter dynamics of this Mediterranean-type headwater stream and that theseeffects were reflected in its ecosystem functioning. Because of the linkage of headwaters to downstream ecosystems and due totheir significant role in landscape and catchment processes, the alterations of organic matter dynamics that we detected at a localscale may have important consequences on a broader scale.

Coupling between pelagic and benthic systems has been described in numerous shallow water communities. In Potter Cove, where pelagic primary production is low (not only during the Antarctic winter but also during the summer), the rich benthic community present there must depend on other food sources than phytoplankton. Over a year-round period we determined the abundance of the different seston particles which constituted the stomach contents of the Antarctic ascidian Cnemidocarpa verrucosa (Lesson, 1830) at Potter Cove. Stomach repletion was highest in November and lowest in June. Ascidians took in a wide range of particles from large detritus (macroalgal debris and faecal pellets) to minute particles < 5 μm. Large detritus and minute particles together represent the main percentage of contents throughout the year (mean 91%). Diatoms were a low percentage (mean 4.5%). Unidentified flagellates, dinoflagellates and coccolithophorids were scarce, with mean values lower than 4%. Among diatoms benthic species were more abundant in summer and pelagic ones prevailed from March to November. Resuspension of benthic material due to wind mixing and the input of allochthonous particles by currents are important mechanisms that ensure food for ascidians and the community of suspension feeders in Potter Cove.

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.