Mercedes Marchese

Benthos of the lotic environments in the middle Paraná River system : transverse zonation

This study synthesizes the results from ten years (1978–1988) of research in lotic environments of the middle Paraná River. We elucidate the important environmental factors that determine differences in quality and structure of the benthos among the main channel, secondary channels and tributaries, and we determine species or descriptive associations of the different types of environment.The results of a principal components analysis reveal a transverse gradient from the main channel through the secondary channels of progressively decreasing size. The gradient shows decreasing depth, current velocity, discharge, sand, oxygen, and density of the benthos, and increasing transparency, pH, conductivity, temperature, silt, clay, sediment, organic matter and species richness and diversity of the benthos.The centre of the river channels was found to be the zone of greatest descriptive and representative value for the characterization of the benthos.In relation to the transverse gradient, the following succession of benthic fauna associations was observed in the center of channels from the main channel to the anabranches:1-Narapa bonettoi + Haplotaxis aedeochaeta + Microstomum? sp.2-N. bonettoi + H. aedeochaeta + Microstomum? sp. + Parachironomus sp.3-Parachironomus sp. + Xenochironomus sp. + Cryptochironomus sp. + Coelotanypus sp. + Limnodrilus hoffmeisteri + Paranadrilus descolei + Campsurus sp. + Pisidium sp.

Benthos of a large neotropical river: spatial patterns and species assemblages in the Lower Paraguay and its floodplains

We studied the composition and spatial structure of benthic invertebrate assemblages and their physical and chemical environment on a 331 km long section of the Lower Paraguay River between Asuncion (Paraguay) and the rivers confluence with the Parana River near Corrientes (Argentina). Applying the same methods used in former studies in the Upper Paraguay and Parana Rivers, we sampled bottom fauna, bed sediments and water at ten riverine stations, two tributaries, and six floodplain lake sites. Seventy-six taxa, mostly oligochaetes (18 species) and chironomids (17 species), were identified. There was a clear distinction between central sites, river banks, and floodplain habitats. Highly significant regressions between sediment composition and biotic parameters were obtained (clay/diversity, R = 0.60, p < 0.0001; silt/diversity, R = 0.61, p < 0.0001; organic matter/diversity, R = 0.63, p <0.0001; coarse-middle sand/density, R = 0.75, p = 0.001). Central channel sites were dominated by the psammophilous oligochaete Narapa bonettoi, which attained densities of 10-3,500 ind. m -2 . The river banks had higher diversity indices, but lower average densities than the main channel center. Some sites with solid substrates were densely colonized by an invading mollusk species, Limnoperna fortunei, which had maximum densities of 13,500 ind. m -2 . Diversity was highest in the floodplain lakes (Shannons H = 2.4 ± 0.67) compared with banks (H = 1.79 ± 0.77) and central sites above the confluence of the Bermejo River (H = 0.89 ± 0.35). This tributary carries large amounts of fine sediments and organic matter into the Paraguay River, thereby reducing biodiversity and abundance of Narapa bonettoi. The faunal composition of the Lower Paraguay was more similar to that of the Middle Parana than to that of the Upper Paraguay. From this and former studies, two general patterns of river ecology become apparent: 1) naturally flowing sand-bed rivers can have continuous species assemblages over very long distances, characterized by few, well-adapted species that occur in high densities, but contribute relatively little biomass to the riverine foodweb; and 2) the biodiversity in the floodplain water bodies is higher than in the river channel as a result of the greater habitat diversity.

The ecology of some Benthic Oligochaeta from the Paraná River, Argentina

Simple and multiple correlations among physical and chemical parameters and densities of the dominant Oligochaete species were calculated for various channels of the middle Parana valley and its tributaries. Paranadrilus descolei Gavrilov, 1955, was observed in rivers with average depth and current velocity, mud-clay sediments, and low conductivity. Tubifex tubifex f. blanchardi Vejd., 1891, was found in channels of low depth, current velocity and discharge but with high conductivity. Limnodrilus hoffmeisteri Claparede, 1862, and Aulodrilus pigueti Kowalewski, 1914, were ubiquitous species. Narapa bonettoi Righi and Varela, 1983, was typical of rivers highest in depth, current velocity, and discharge with sandy sediments and low conductivity.

Patterns in macroinvertebrate assemblages at different spatial scales. Implications of hydrological connectivity in a large floodplain river

The habitat heterogeneity generated and sustained by the connectivity of floodplain habitats, the seasonal flood pulse, and the variability of the physical structures typically found in floodplains of large rivers results in a variable space–time mosaic of water sources that results in a high biodiversity of the river-floodplain system. In order to assess the implications of natural connectivity and the heterogeneity on the patterns of macroinvertebrate assemblages at different spatial scales, monthly samplings in six different mesohabitats (lakes with different hydrological connection and secondary channels with permanently and intermittent flow) of the Paraná River floodplain were performed from April 2005 to March 2006. The mesohabitats had different granulometry and detritus composition of their bottom sediments. They also had different conductivity, transparency, and depth in relation to the different connectivity degrees. Mesohabitats differed in the abundance of macroinvertebrates of different taxonomic groups and diversity. The environmental variables were correlated to the patterns of macroinvertebrate abundance, with dominance of different species of annelids and mollusks at the patch, mesohabitats, and island scales. An alpha diversity gradient from the isolated lake (65 taxonomic units) to the secondary channels (25 taxonomic units) was obtained. The analyzed mesohabitats showed a high taxa turnover, with high values not only among the mesohabitats located in the different islands, but also among the mesohabitats in relation to different connectivity degrees. The mesohabitats showed negative co-occurrence of macroinvertebrate assemblages. The spatial heterogeneity, sustained by the connectivity degree, played a key role in structuring benthic assemblages at different scales, positively influencing the regional diversity.

Accumulation and Elimination of Chromium by Freshwater Species Exposed to Spiked Sediments

The bioaccumulation and elimination capacity of chromium were examined in four freshwater species: the submersed aquatic plant Ceratophyllum demersum (Ceratophyllaceae), the oligochaete Limnodrilus udekemianus (Tubificidae), the crab Zilchiopsis collastinensis (Decapoda), and the fish Cnesterodon decemmaculatus (Poeciliidae). All of the species were exposed simultaneously to sediments spiked with Cr (K2Cr2O7) at different concentrations for 28 days, followed by 7 days without Cr to evaluate the concentration of residual Cr. We found that Cr accumulated in the tissues of all four species. The highest bioconcentration factor obtained for each species is as follows: C. demersum, 718.66 (±272.91); L. udekemianus, 172.55 (±80.8), Z. collastinensis, 67.72 (±35.4); C. decemmaculatus, 23.11 (±12.82), all at 28 days of exposure.

Diet and functional feeding groups of Chironomidae (Diptera) in the Middle Paraná River floodplain (Argentina)

The gut contents of nine genera of benthic Chironominae and Tanypodinae from the Middle Parana River floodplain habitats (a lake and a secondary channel) were analyzed to determine their feeding patterns and functional feeding groups. Amorphous detritus, animal and vegetal tissues, and mineral materials (predominantly sand) were observed in the larval guts. Amorphous detritus were the main food item found for Polypedilum (Tripodura) sp., Chironomus gr. decorus sp., Endotribelos sp., Phaenopsectra sp., Cladopelma sp., and Pelomus sp. (Chironominae), while animal tissues (mainly oligochaetes) were the most important food item found for Ablabesmyia (Karelia) sp., Coelotanypus sp., and Procladius sp. (Tanypodinae). Dietary overlap was calculated for all pairs of genera. Within predators, the highest overlap was obtained between Coelotanypus sp. and Ablabesmyia (Karelia) sp., while within detritivores the highest niche overlap was obtained between Endotribelos sp. and Phaenopsectra sp.

Population dynamics of Narapa bonettoi Righi and Varela, 1983 (Oligochaeta: Narapidae) from the main channel of the Middle Paraná River, Argentina

The aim of the present study was to determine the life-cycle of Narapa bonettoi in the main channel of the Middle Parana River. The average density varied between 17 and 624 ind. sample−1. The spatial distribution was contagious with a higher degree of aggregation during the first months of the observed cycle, than during the last months. Mature specimens were more abundant in September 1985–86, resulting in greater breeding activity in early summer, when there was a large population increase. Two reproductive strategies were observed: asexual and sexual. After breeding, worms undergo a resorption process of their reproductive organs, returning to a non-sexual condition. Narapa bonettoi probably uses asexual reproduction as an adaptative strategy, because cocoon development will be affected by environmental factors such as the drift in the Parana River which is highly erodable. Population growth was rapid during the first four months and then stabilized.

The systematic position of Opistocystidae (Annelida, Clitellata) revealed by DNA data.

Opistocystidae Cernosvitov, 1936 is a largely Neotropical oligochaete taxon containing seven species. Its familial status has never been formally challenged, although possible close relationships with Naididae and Phreodrilidae have been noted. Mitochondrial 12S and 16S rDNA, and nuclear 18S rDNA, of a range of aquatic oligochaete taxa, including Trieminentia corderoi (Opistocystidae), were analysed by Bayesian inference. This showed that T. corderoi is a derived lineage within Naididae, closely related to Pristina and its monotypic subfamily Pristininae. Opistocystidae as a whole (with its three genera, Opistocysta, Trieminentia, and Crustipellis) is thus likely to be a group within Naididae.

Cyst formation in tubificidae (Naidinae) and opistocystidae (Annelida, Oligochaeta) as an adaptive strategy for drought tolerance in fluvial wetlands of the Paraná River, Argentina

During a broad study of invertebrate colonization in marginal fluvial wetlands of the Middle Paraná River in Argentina, cyst formation was observed in two of the 26 species of aquatic oligochaetes identified from the study—Dero multibranchiata Steiren, 1892 (Tubificidae, Naidinae), andTrieminentia corderoi (Harman, 1969) (Opistocystidae). All specimens of the two species in which cyst formation was observed had been collected after drying conditions ranging from 14 to 28 days. A clitellum was not observed in any of the specimens collected. Lengths of specimens ranged from 0.88 mm to 2.84 mm inDero multibranchiata and 2.25 mm to 2.40 mm inTrieminentia corderoi. The development of protective cysts byDero multibranchiata andTrieminentia corderoi, and perhaps many other species of aquatic oligochaetes, appears to be an adaptive strategy enabling them to survive drought in temporary wetlands, recolonize freshwater habitats upon inundation, and disperse both laterally and downstream with rising water levels in wetlands and streams.

A comparison of two tubificid oligochaete species as candidates for sublethal bioassay tests relevant to subtropical and tropical regions

Two tubificid oligochaetes, Branchiura sowerbyi and Tubifex tubifex, were evaluated as potential test organisms for sediment bioassays. We attempt to reproduce the sediment bioassay proposed by Reynoldson et al. (1991) using his strain of Tubifex tubifex and his reference sediment and to compare this technique using Branchiura sowerbyi. This species was chosen because it is more common and dominant in tropical and subtropical environments than Tubifex tubifex. Data on survival and reproduction were obtained for both species, and growth estimates were obtained for B. sowerbyi. The sublethal bioassay with T. tubifex confirmed earlier estimates of a test duration of 4 weeks at 22.5 °C. B. sowerbyi cultures can produce usable estimates in 21 days at 30 °C.