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We aimed to evaluate the variation in the age structure of Cornops aquaticum (Bruner) population and its relation to the host plant biomass and the feeding of the different age classes of this grasshopper on the water hyacinth Eichhornia crassipes along 2 years, in a Paraná River floodplain lake (Chaco, Argentina). Individuals of C. aquaticum were captured with a 70-cm diameter sweep and separated in nymphs A (instars I and II), nymphs B (instars III to VI), adult females, and adult males; host plant biomass was sampled using a ring with a 0.30-m(2) diameter. Relative daily feeding of C. aquaticum population was calculated by multiplying the number of individuals captured per minute by the daily consumption by individual obtained in each age classes. We found that the age structure and the relative daily feeding of C. aquaticum varied between seasons and years. The highest values of grasshopper abundance, leaf biomass, and relative daily feeding of C. aquaticum population were observed in summer 2006. Plant biomass was directly correlated with nymph abundance and not correlated with adult abundance. Plant biomass available as refuge (leaves), food (laminas), and oviposition site (petioles) to C. aquaticum represented up to 62% of the total plant biomass. The results obtained in C. aquaticum show the importance of considering total plant biomass and plant biomass available for herbivores separately. Our study highlights the need to find an adequate method to estimate the density of C. aquaticum and other semiaquatic grasshoppers in the Paraná River floodplain involving different seasons, years, and water phases (rising and falling).

The effects of free-floating mats of the water hyacinth Eichhornia crassipes (Mart.) Solms on the limnological characteristics of two lakes located on the fringing floodplain of the lower Paraná River (27˚27'S, 58˚55'W) were studied monthly between April 1998 and April 1999. The mobility of the free-floating vegetation at both locations prevented continual observation. Therefore, to carry out the intensive sampling needed for this study, two experimental ponds (A and B) were filled with water from the Paraná River in 1996. Pond A was designed to reproduce conditions similar to those of lakes vegetated with water hyacinth. Pond B was designed to recreate the limnological conditions of these lakes, such as a high concentration of dissolved and fine particulate organic matter and a peat layer on the bottom; however, pond B lacked macrophytes. Natural lakes covered with free-floating vegetation were very similar to pond A, but these water bodies showed a lower temperature, dissolved oxygen level and pH and a higher conductivity than the non-vegetated pond. Our results indicated that water hyacinth has a strong local influence on the limnological conditions in subtropical shallow lakes. Our results may assist in developing causeconsequence models by demonstrating the relevance of the effect of thermal damping produced by floating meadows.

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.