Introduction: Eichhornia crassipes is a dominant aquatic plant of neotropical-river floodplains and is invasive in warm waters of different continents. Plants provide food and habitat for fish, especially Prochilodus lineatus and Gymnotus omarorum, which are relevant to subsistence fishing by local indigenous communities. Objective: In this study we describe the main traits of E. crassipes floating meadows, and analyze the abundance and composition of macroinvertebrate assemblages associated with their roots in two floodplain wetlands of the Paraná River (within the Ramsar site Wetlands Chaco) and of the Pilcomayo River (within the Pilcomayo River National Park). Methods: During spring and summer, the macroinvertebrates were collected in monospecific stands of E. crassipes with a net of 962 cm2 area and 500 µm mesh size. Leaf density, the biomass of leaves and roots, the length of leaves and roots, and the water quality were measured simultaneously. Results: Temperature, oxygen concentration, and nutrient content were significantly higher and electrical conductivity was lower in the Pilcomayo than in the Paraná floodplain. E. crassipes growing in the Pilcomayo floodplain had longer leaves and less root biomass than those found in the Paraná floodplain. The number of macroinvertebrates per 1 000 g root dry weight and per m2 was significantly different between both floodplains, but the taxon richness was similar. Non Metric Multidimensional Scaling analysis differentiated the abundance of 14 selected taxa between both floodplains, and showed a high correlation between the environmental variables and macroinvertebrate abundance. The most abundant taxonomic groups in the Paraná River floodplain were oligochaetes (Naididae), ostracods mainly Cytheridella ilosvayi and larvae of non-biting midges (Chironomidae). Two families, Hydrobiidae (Heleobia parchappii) and Chironomidae accounted for 49.3 % of the total abundance in the Pilcomayo floodplain. The prawns Macrobrachium jelskii and M. amazonicum, frequent in the Pilcomayo floodplain, were not found in the Paraná floodplain. Conclusions: The different environmental conditions in both floodplains affect the abundance, composition, and the dominance of macroinvertebrate assemblages; however, the total taxa richness was similar. The area occupied by E. crassipes in the floodplains of these rivers provides the habitat that contributes most to overall biotic diversity, which must be considered in management strategies.

We studied the breakdown rates and the invertebrate abundance and biomass for the litter of five native aquatic plants (Eichhornia crassipes, Eichhornia azurea, Thalia multiflora, Oxycaryum cubense and Hydrocotyle ranunculoides) in a shallow rain-fed lake using litter bags. The diets of the prawn Pseudopalaemon bouvieri and the amphipod Hyalella curvispina were determined and classified into five food items. Litter breakdown was fast (>0.010 day −1 ) for all species studied, although the breakdown rates were significantly affected by the litter species. The abundance of invertebrates colonizing the litter was significantly different among the species, but the biomass did not differ. The invertebrate taxa that colonized the litter of the different species were broadly similar, consisting primarily of oligochaetes, amphipods, prawns, ostracods, gastropods, water mites and several types of insect larvae (chironomids and mayflies). In terms of the number of individuals, naidid oligochaetes dominated the assemblages. In terms of biomass, P. bouvieri and H. curvispina reached 67.6 and 18.2 % of the total, respectively. Our results indicate that macroconsumers are involved in the breakdown process, since these species consume plant remains and detritus and highlight the importance of leaf litter composition on the abundance of invertebrates that colonize the litter.

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Does the effect of aquatic plant types on invertebrate assemblages change across seasons in a subtropical wetland?Habitat complexity is one of the fundamental factors determining the distribution of invertebrate assemblages due to the different architectures and growth forms of aquatic plants, which affect their density, species richness, biomass, and trophic structure. In this paper, we compared the density and richness of macroinvertebrates present in two species of aquatic plants with different architectures (Salvinia biloba and Egeria najas) across seasons. Samples of aquatic plants and associated invertebrates were collected seasonally in a wetland located in Corrientes (Argentina) with a net area of 962 cm 2 . To characterize the different habitat types, we selected 12 traits of aquatic plants, including fractal complexity. A total of 15 225 invertebrates from 13 major groups were recorded from both habitat types (41 families and 24 genera in S. biloba and 24 families and 22 genera in E. najas). Significant differences in the total density (number of invertebrates per 1000 g plant dry weight) and taxa richness at family level were found between habitat types. Non-metric multidimensional scaling reflected differences in the seasonal patterns of 16 macroinvertebrate genera selected between both habitat types. A Similarity Analysis showed statistically significant differences between the habitat types and seasons. Structurally more complex habitat (S. biloba) supported a greater number of taxa and the highest number of individuals per plant dry weight. The differences in the density and richness of the invertebrate assemblages between S. biloba and E. najas were maintained throughout the seasons. The habitat structuring of aquatic macrophytes is a key factor in macroinvertebrate assemblages, and it is important to keep in mind when comparing wetlands with different aquatic vegetation.

Different architectures in aquatic plants with different levels of morphological complexity provide environmental heterogeneity in freshwater ecosystems, and consequently influence invertebrate assemblages. We investigated the relative importance of the structural complexity of macrophytes and environmental variables on the abundance and richness of the macroinvertebrate assemblages associated with aquatic plants across the Esteros del Iberá. This protected wetland system located in Corrientes (Argentina) is fed by rain. Macrophyte habitat complexity was quantified by measuring fractal geometry dimensions of area and perimeter and plant biomass. We sampled macroinvertebrates associated with five species of macrophyte (Egeria najas, Cabomba caroliniana, Potamogeton gayi, Eichhornia azurea and Salvinia biloba) in five shallow lakes during two different seasons (dry and rainy) between 2007 and 2008. Regression analyses revealed that macrophyte structural complexity was an important factor on macroinvertebrate assemblages, whereas explanatory power of environmental variables was low. In both seasons, the fractal dimension of area was the variable with the highest explanatory power on richness, and plant biomass was in the case of macroinvertebrate abundance. To conserve macroinvertebrate diversity in Esteros del Iberá, it would be necessary to maintain the natural heterogeneity indicated by the different structural complexities of the macrophytes across the wetland.

RESUMEN Las Podostemaceae son importantes componentes de las mallas tróficas de los ambientes reófilos pero su importancia como hábitat para los invertebrados es poco conocida. En este estudio se analizó la variación temporal de la abundancia y riqueza de taxa de los macroinvertebrados asociados a Podostemum distichum y Podostemum muelleri (Malpighiales: Podostemaceae) en dos arroyos de bajo orden durante diferentes estaciones del año. La abundancia total promedio difirió entre fechas de muestreo, pero no entre arroyos. Se registraron 14 familias de macroinvertebrados distribuidas en 9 órdenes en ambos arroyos. El Análisis de Escalamiento Multidimensional No Métrico (NMDS) reflejó también diferencias en el patrón estacional. Tanto el coeficiente de similitud como el de complementariedad, indican que ambos arroyos fueron muy similares en cuanto a la composición de las familias de macroinvertebrados. Las especies de Podostemaceae dominantes en cada arroyo constituyen hábitats de diversidad intermedia de acuerdo al índice de diversidad utilizado.

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This study assesses the effect of vegetation structure on the subtropical invertebrate communities in contrasting sampling dates of macrophyte populations in the RAMSAR site of Iberá wetlands, South America. Invertebrates associated with the submersed Egeria najas and the floating rooted Pontederia azurea were chosen to provide a model involving different microhabitat complexity. The results suggest that vegetation structure provided by the two macrophyte species supported significant differences in the density of animals, with invertebrate abundance of E. najas twice as high as on P. azurea. Abundance showed no significant differences in both contrasting sampling dates, growth and decline. Our result clearly showed invertebrates exclusively associated with each macrophyte species, as well different invertebrate taxa dominating in each sampling date (decline: Cladocera; growth: Aphididae, Belostomatidae and Planorbidae). We also show that working at the taxonomic levels of family could be a sensible trade-off between taxonomic identification effort versus reaching reliable and useful results for environmental monitoring and natural resource management in highly diverse subtropical wetlands. Our results emphasize the role of vegetation structure on invertebrate communities, as well suggest that the growth cycle of macrophyte populations could be a relevant variable influencing these animals in pristine subtropical wetlands.