The aim of this paper is to report the colonization process of three headwater rivers of two large trans-national Pacific riverbasins shared by Chile and Argentina, by exotic Chinook salmon (Oncorhynchus tshawytscha). To characterize the populations atthe sites, fish were analyzed in terms of length, weight, age, and stream residency patterns. Fishermen familiar with the area wereinterviewed to establish the seasonal occurrence of fish and to reconstruct the history of the colonization process. Records ofsalmon production in Chile by regions were examined to evaluate the potential origin of fish in coastal net pen aquaculture. Theregularity of spawning runs in our study sites indicates that Chinook salmon have established spawning populations in theserivers. While spawning was verified in all three rivers, only one seems to serve as a holding area for pre-spawning salmon early inspring. The correlation in the historical occurrence of Chinook salmon in the three rivers suggests that they have been colonizedas part of the same introduction event. Moreover, the temporal correlation with the development of ocean net pen production forthis species in the Chiloé region suggests that the fish may originate from escapes from these enterprises. As in all previousreports of exotic Chinook salmon in Patagonia and New Zealand, populations in the three rivers have a combination of streamand ocean ecotypes. We discuss the importance of plasticity in critical life history characteristics for the success of Chinooksalmon in Patagonia, compared to other species of anadromous salmonids.

Des investigations portant sur le genre Diadesmis (Bacillariophycées), observé à l'aide du microscope photonique et du microscope électronique à balayage, ont été réalisées dans différents biotopes des archipels de Crozet et de Kerguelen : lacs, mares temporaires, mousses et sols. Dix espèces ont été recensées. Diadesmis arcuata Heiden, décrite à l'origine dans l'archipel de Kerguelen, est réexaminée et typifiée. Cinq espèces nouvelles pour la science sont décrites : Diadesmis comperei, Diadesmis costei, Diadesmis crozetikerguelensis, Diadesmis langebertalotii et Diadesmis subantarctica. Les espèces appartenant au sous-genre Paradiadesmis ont une structure particulière : les interstries sont constituées de tubes creux quadrangulaires.

Two sediment cores from acid lake Store Hovvatn were analysed for chironomid head capsule remains over a sequence spanning from about 1730 A.D. to the late 1970s. Up to 30 taxa were identified from one (0.5 cm) level in the cores. Principal component analysis of taxon relative abundances showed a gradient corresponding more or less to the sediment sample chronology. Redundancy analysis with some chemostratigraphical variables, and with pH values inferred from diatoms, indicated that chironomid community change could be explained as a response to acidification. However, most of the taxa that declined or disappearedup-core are known to occur in acid lakes and pH sensitivity per se does not seem to explain the response of individualspecies. The relative abundance of Orthocladiinae increased up-core and the long term change may be conceived in terms of a retarded productivity or oligotrophication. The most conspicuous change in the chironomid community appears to coincide with decreasing aluminium concentrations in the sediments and may indicate toxic effects from labile aluminium in the water column.

Cyanobacteria are a worldwide group of photosynthetic prokaryotes that can cause nuisance blooms in eutrophic waters. It is generally accepted that their resting cells, akinetes, play an important role in the dispersal, recruitment, initiation of blooms and survival under unfavourable conditions, therefore information on the germination, distribution and abundance of akinetes in natural sediments is essential for understanding the ecology and bloom dynamics of N2-fixing cyanobacteria. The present study describes the effect of irradiance on the germination and subsequent growth of N2-fixing filamentous cyanobacteria developed from natural akinete stock in sediment of Lake Balaton (Hungary) with varying phosphorous supply. The research focuses on the invasive Cylindrospermopsis raciborskii and Aphanizomenon flos-aquae the most abundant species of this lake. In the experiments, the germination of ten filamentous N2-fixing cyanobacteria species was observed. The species assemblages of the germinated cyanobacteria populations showed strong light and phosphorus dependence. Anabaena and Anabaenopsis species became dominant in phosphorous-rich conditions, while in phosphorus-deficient environments Aphanizomenon species and C. raciborskii dominated. Among the germinated filaments we have detected Anabaenopsis cunningtonii and Anabaena compacta, which have not been observed in Lake Balaton previously. Our results suggest that among the filamentous heterocytic cyanobacteria of this shallow lake the invasive C. raciborskii was the best competitor when phosphorus supply and irradiance were low.

Lake Vela is a shallow lake facing eutrophication, like many other freshwater reservoirs. A reduction in biodiversity has been observed and cyanobacterial blooms are frequent, occasionally leading to large fish kills. An annual sampling cycle was carried out in the present study, with the aim of understanding the present seasonal variation of Lake Vela’s physico-chemistry and cladoceran assemblage. The present results were compared with those obtained in previous studies. Presently, Lake Vela is facing higher nutrient and Chl a levels than in past years. The cladoceran assemblage is now dominated by small-bodied forms, such as Chydoridae and Bosmina longirostris, except in the spring peak of Daphnia longispina. Still, the seasonal pattern of the cladoceran community is more or less coherent with the general model observed in other temperate lakes. The results also suggest that this was a rather unusual year, with a Daphnia summer peak, probably because of a large fish kill that occurred in late spring.

The prediction of the composition and structure of fish communities in reservoirs is essential in the management of the fisheries,an activity fundamental in emerging countries where human riverine populations depend on freshwater fish for their animalprotein supply. The transformation of the river in a reservoir constitutes an environmental filter for the riverine fish specieswhich, before anything else, must find food resources in the reservoir for their individual maintenance. After the closure of Petit-Saut Dam in 1994, fish were sampled by gillnets in the Sinnamary River in the period 1994-2000 and in the Petit-Saut Reservoirbetween 1998 and 2000. The stomach contents of the main species were analysed for 6 main food items. Analyses of similaritywere conducted in order to compare river and reservoir diets of individual species on one hand and the trophic structure offish assemblages on the other. Twelve species out of 23 analysed showed significant modifications in their feeding habits. Thechanges affected species from every feeding guild as established from the river stomach contents data. As a consequence thetrophic structure was deeply modified in the reservoir. The abundance of piscivores and herbivores decreased while the relativeimportance of omnivores and invertivores increased. There was a general tendency for species to increase their food niche breath.The ability of species to modify their feeding habits is shown to be of fundamental importance for the trophic organizationof the fish community in the Petit-Saut Reservoir.

To determine the relationship between biofilm characteristics and organic nutrient content, we investigated the development of hyporheic biofilms on artificial substrata (glass slides) in the early phase of bacterial colonization (one week) in contrasting river habitats with significantly different concentrations of dissolved organic carbon (DOC) and biodegradable and refractory DOC (BDOC and RDOC). Results suggest that the quality of DOC (i.e. BDOC versus RDOC content) controlled electron -transport system (ETS)-active bacterial abundances early in colonization, while DOC concentrations limit biofilm development onlyin the latter phase of colonization.

In 1996, a compound gauging weir was built in Villaveta Township (Navarra, North of Spain) in the river Erro (Ebro RiverBasin). Before and after its construction, fish and physical and chemical variables were examined to determine weir’s impact onthe reach. Fish captured by electrofishing were measured (total length), and population density and length distribution of thespecies were estimated. Species diversity (Shannon’s diversity index), dominance (Simpson’s dominance index) and evennesswere also calculated. The results revealed an increase in fish population density, especially in smaller specimens, but a decreasein the larger fishes. Substrate and shelter alterations could explain the observed variations in the fish community. In order toimprove the consolidation of well-developed fish populations, the placement of great boulders and the restoration of the riparianvegetation are suggested.

Particulate organic matter (POM), dissolved organic matter (DOM), bacteria and cladoceran were sampled seasonally atZhihugang Estuary and Lake Center in Taihu Lake. The δ13C of the four organic matter fractions showed consistent temporalvariation, with heaviest values in summer and lower at other times of the year. The cladoceran δ13C showed a significantcorrelation with that of POM, reflecting a heavy dietary dependence on POM during the study period. The bacteria becameenriched in 13C compared with that of DOM throughout the sampling dates, although no significant relationship was foundbetween the two fractions. δ13C values of POM, cladoceran and bacteria were all negative significantly correlated with oxidationand reduction potential (ORP), and specific conductivity (SpCond). As for δ15N, the seasonal pattern of food web componentswas variable. The POM δ15N signature exhibited the most enriched isotope ratios during the summer months when dissolvedinorganic nitrogen (DIN) nutrients were at their lowest concentrations. The consumption of DIN in summer can explain in partthe progressive accumulation of heavy nitrogen isotopes during this period. Spatially, δ13C and δ15N of the food web componentswere all slightly depleted at Estuary than that at Lake Center during the study period, possibly due to large allochthonous inputsat Zhihugang Estuary. Relatively wide ranges of stable isotopic values from both sites suggest that seasonality should beconsidered when attempting to establish food web structures in a eutrophic lake.

The description of new taxa of small minnow mayflies (Insecta : Ephemeroptera : Baetidae) includes the following. Cryptonymphacopiosa n. gen. and n.sp., from Brazil, is distinguished by long, robust, pilose setae on the medial margin of the prosthecae; apically narrow glossae and paraglossae ; absence of hindwingpads ; and long, basally narrow gill 1. Rivudiva n. gen., includingR. minantenna n.sp., from Brazil, and R. trichobasis n.sp., from Brazil and Paraguay, is distinguished by a broad linguaand relatively narrow superlinguae ; broad glossae with a distomedial row of long, robust, falcate, simple setae ; and tarsal clawswith minute denticles arranged in an ellipse. Spiritiops silvudus n.gen. and n. sp., from Brazil and French Guyana, is distinguishedby a long, robust, pilose seta arising from the medial margin of the right prostheca and tarsal claws with two rows of denticlesand lateral long, fine, simple seta. Varipes lasiobrachius n. gen. and n. sp., from Colombia and Ecuador, is distinguished bythe reduced right prostheca and U-shape arrangement of denticles on the tarsal claws. Zelusia principalis n. gen. and n. sp., fromBrazil and Colombia, is distinguished by a medially raised labrum, bifid right prostheca, and long, slender maxillary palps.

Growth of colonial Microcystis aeruginosa was investigated by performing an incubation experiment. Colonies of M. aeruginosa were separated based on size (colony diameter <100 μm, 100–200 μm and >200 μm) by filtration. Additionally, the cells around the surface of the colonies were separated from those inside the colonies by short-term ultrasonic treatment followed by filtration. Experimental results indicate that M. aeruginosa grew continuously throughout a 35-day incubation period in a nutrient-rich medium at specific growth rates between 0.045 and 0.310 d−1. On day 14, larger colonies exhibited insignificantly higher specific growth rates. However, on day 35, the specific growth rates of colonies with diameters less than 100 μm insignificantly exceeded those of larger colonies. Internal cells of the colonies tended to grow faster than peripheral cells. Furthermore, the specific growth rates of the cells that comprised colonies with diameters of below 200 μm exceeded those of peripheral cells. These results suggest a potential growth strategy of M. aeruginosa in maintaining a high growth rate, eventually leading to the dominance of large colonies, which have notable ecological advantages over smaller ones.

Knowledge on ecological impacts of forestry practices on aquatic ecosystems relies almost exclusively on data from large-scale forest harvesting, often involving clearfelling of whole stream catchments. To determine effects associated with less intensive and widespread forest management, we examined the responses of headwater streams to small-scale forest harvesting, including riparian zones adjacent to study reaches but corresponding to less than 5% of the catchment areas. Stream reaches running through recently (2–4 years) harvested forest patches were paired with and compared with adjacent reaches bordered by mature broadleaf forest. We determined abiotic stream characteristics, invertebrate community structures and abundances, trout size and population densities, and leaf litter breakdown rates in each of these pairs. Harvested reaches were found to have different channel cross-section morphology and greater invertebrate abundances in leaf packs than mature forest reaches. Shifts in the abundance of common invertebrate predators were also attributed to riparian forest harvesting. Litter breakdown rates and brown trout densities did not show any significant difference between harvested and mature forest reaches across the four site pairs, possibly because of nonlinear responses to post-harvest riparian canopy openness. Managers must be aware that small-scale forest harvesting in stream riparian areas is not without consequences for aquatic ecosystems. Whether natural riparian forest openings, such as caused by tree death and blow-down, have similar effects on stream ecosystems is an important question to address if we are to confirm the usefulness of small-scale forestry and improve forest and stream management schemes.

The lichen flora of freshwater habitats in the Italian Alps is reviewed: 20 genera and 43 species are known, of which crustoselichens with sexual reproduction prevail. Freshwater lichen richness is highest near the treeline on siliceous substrata in exposedand well-lit sites. The importance of several ecological factors on freshwater lichen distribution and ecology is discussed.

The effect of substrate texture on colonization by stream macroinvertebrates was studied in a mountain stream. Experimental patches made of natural substrate, with two levels of cobble and gravel texture (rough and smooth), were subjected to two colonization periods (two hours and one week) within a riffle. Cobble texture affected the number of individuals and taxa colonizing substrate patches, although these relationships depended on time of colonization and some local environmental factors. Texture influence was different for different taxa, and only affected the scraper functional feeding group after one week of colonization, probably in relation to the development of an epilithic layer. These results suggest a significant effect of the surface texture of substrate on colonization by macroinvertebrates, although the effect is complex and needs more careful examination.

The habitats of 356 phytocoenoses dominated by 15 species of aquatic plants were investigated. Among the properties of water, Na+, pH, total Fe, Ca2+, NO3−, SO42−, hardness and COD-KMnO4 were the most important in differentiating the habitats of the phytocoenoses studied. Considerable physicochemical differences were found between the waters of the phytocoenoses of Ranunculus circinatus (hard waters rich in Na+), Nuphar pumila (soft waters with high levels of total Fe), Elodea canadensis, Ceratophyllum demersum (alkaline waters) and Stratiotes aloides (waters containing high concentrations of Ca2+ and SO42−). The substrate properties that best differentiated the habitats compared were PO43−, hydration, organic matter content, total N, dissolved SiO2, Ca2+, Na+ and pH. Patches of R. circinatus inhabited substrates containing the highest levels of PO43−. Those of S. aloides were associated with substrates poor in PO43−. Patches of C. demersum were confined to substrates rich in total N. Phytocoenoses of R. circinatus were associated with large lakes, whereas those of N. pumila were restricted mostly to smaller lakes. The deepest parts of lakes were inhabited by Potamogeton lucens. Phytocoenoses of S. aloides occurred mostly in shallow waters, on thick sediments. The investigated phytocoenoses also represent different stages of succession. The phytocoenoses of Potamogeton perfoliatus, R. circinatus, Myriophyllum spicatum and Polygonum amphibium, which develop on mineral substrates, initiate the process of succession, whereas those of Myriophyllum verticillatum, Nymphaea candida, Nymphaea alba, N. pumila and S. aloides, which inhabit highly hydrated organic substrates, are characteristic of the late stages of succession.

Phytoplankton was sampled over 10 months at five stations along the north-eastern coast of Lake Tanganyika. The biomass is composed chiefly of diatoms together with blue-green algae and, occasionally, green algae and Euglenophytes. Seasonal changes in phytoplankton composition and densities are discussed in relation with abiotic factors.

Podonomus Philippi was recently found in Brazilian inselbergs. In this study, we used sequences from the barcode region of cytochrome c oxidase I (COI) to assess Podonomus molecular diversity from mountains in Southeastern Brazil. Our results revealed the adult female and the larvae of Podonomus pepinellii Roque and Trivinho-Strixino, 2004 and extended the known geographical distribution of the species. Moreover, we found high molecular divergence in two populations located in the Serra da Mantiqueira and Serra do Espinhaço mountain range when compared with the only species recorded to Brazil, indicating the possibility of two more species to be described.

Difficulties to simply transfer trophic cascade theory from temperate to tropical lakes are now well recognized. Many mechanisms trying to explain top-down divergences between these systems have been proposed, such as lack of key species of herbivorous zooplankton, absence of seasonality in fish reproduction, cyanobacteria development, or differences in fish foraging behaviour. Very few studies have considered bottom-up mechanisms, in particular differences in nutrient recycling and nutrient limitation between the two types of ecosystems. According to the ecological stoichiometry theory, fish-induced alterations of food-web structure could modify the efficiency of consumer-driven nutrient recycling by changing the relative biomass contribution of species in food webs. Consequently, by mostly considering top-down processes, one could underestimate consequences on nutrient availability for phytoplankton growth. In this paper, we compared the results of two mesocosm experiments carried out in temperate and tropical areas, each manipulating food-web structure via the presence or absence of fish. We found trophic cascades in both experiments, but differences between fishless and fish treatments were greater in temperate than in tropical systems. In the tropical experiment, the observed effects could not be supported by classical zooplankton community alteration or by cyanobacteria prevalence. Our results suggest a key contribution of fish nitrogen-excretion to phytoplankton growth in mostly nitrogen-limited tropical systems. Differences in stoichiometric response to food-web structure alteration between temperate and tropical lakes could thus represent a major difference between the two systems. Our study stresses the need for further studies that would allow robust generalization on the functioning of freshwater temperate and tropical ecosystems.

Le but de cette étude est de mieux connaître la qualité de l'eau et la macrofaune benthique des cours d'eau marocains duMoyen-Atlas. L'Oued Tizguit a été choisi pour son accessibilité et la variété de son environnement. Onze stations ont été suiviesen mesurant 8 paramètres physico-chimiques et en faisant l'inventaire faunistique pendant un cycle hydrologique.Ce travail montre que la qualité de l'eau évolue dans le temps selon un gradient amont-aval.Des prélèvements qualitatifs ont permis de dénombrer 167 taxons appartenant à différents groupes où les Coléoptères sontmajoritaires. La distribution de cette macrofaune semble déterminée par la variation longitudinale des différents paramètres dumilieu, et sa composition doit son originalité, en partie, à la qualité de l'eau du milieu.

Dans cette note, nous décrivons une méthode simple, permettant l'ultracentrifugation de particules virales planctoniques et leur observation en microscopie électronique à transmission. Une description de différents morphotypes de particules assimilables à des virus est faite à partir d'échantillons prélevés dans la couche euphotique d'un lac eutrophe (Aydat), au cours des périodes printanière et estivale. Ces particules présentent chacune une capside icosaédrique régulière à 6 faces et une queue, dont les dimensions sont similaires à celles des phages décrits en milieu marin. La diversité des formes et le nombre de virions (10 fois supérieur à celui des bactéries), permettent d'émettre l'hypothèse selon laquelle les virus interviennent de façon significative dans le contrôle des abondances des communautés bactériennes en milieu lacustre.