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La agriculturización es un causante de transformación en los territorios. El actual modelo de agricultura predominante, basado en la aplicación de plagui- cidas para mantener el cultivo saludable y sin competencia, no solo afecta a la especie objetivo sino también a aquellas no blanco, impactando en la biodiversidad local y el funcionamiento de los ecosistemas. El objetivo del presente estudio fue analizar los cambios en los usos del suelo en la Cuenca Alta del Arroyo Chapaleofú y determinar la toxicidad del clorpirifos sobre una especie no blanco. Para esto, se compararon imágenes clasificadas supervisadas de los años 1989 y 2019 del partido de Tandil (Buenos Aires, Argentina) y se realizó la evaluación de la Concentración Letal 50 (CL50) del insecticida clorpirifos sobre el molusco Physa acuta, a 96 horas de exposición. Se detecta un 74.35% de avance de las tierras destinadas a actividades agrícolas, lo que evidencia el proceso de agriculturización. El molusco Physa acuta tiene una CL50 de 0.55 mg L−1, presentando al mismo tiempo efectos subletales, pese a tratarse de una especie tolerante al químico en cuestión. Debido a lo anterior, es necesario realizar más estudios que permitan determinar los efectos de los múltiples químicos utilizados en la actividad agrícola, considerando exposiciones más prolongadas y con especies más sensibles.

The present study shows the most important results in the sampling carried out in the Río de la Plata river from the year 1993. The main components of the zoobenthos in the studied area were the oligochaetes and nematods, particularly in sectors with high organic matter content and low dissolved oxygen. Tubificidae was very frequent, mainly Limnodrilus claparedeianus. A prevalence of Corbicula fluminea among the mollusks was observed in the coastal area in association with Limnoperma fortunei. This assemblage supports the presence of macroinvertebrates as Hirudinea, Temnocephala, Lumbriculidae, etc. Chironomidae showed an almost constant presence in the internal and intermediate areas of the river. The most frequent microcrustaceous were Copepoda, Cladocera, Ostracoda and their larval forms. In the offshore of the river the mollusks, annelids, crustaceans and nematods were present in the great majority of the sampling sites. Polychaeta were present in 90% of the sampling places being Nephtidae the most abundant genus. Corbicula fluminea dominated in the interior sector of the river and it was replaced by Erodona mactroides and Mactra isabelleana in the external area. Rapana venosa was also observed in this area. Different benthic assemblages were determined and their faunistic composition varied in function of the substrate, silt, size, coast, proximity, organic matter content and food resources.

ABSTRACT One of the most important effects derived from the intensive land use is the increase of nutrient concentration in the aquatic systems due to superficial drainage. Besides, the increment of precipitations in South America connected to the global climate change could intensify these anthropic impacts due to the changes in the runoff pattern and a greater discharge of water in the streams and rivers. The pampean streams are singular environments with high natural nutrient concentrations which could be increased even more if the predictions of global climate change for the area are met. In this context, the effect of experimental nutrient addition on macroinvertebrates in a lowland stream is studied. Samplings were carried out from March 2007 to February 2009 in two reaches (fertilized and unfertilized), upstream and downstream from the input of nutrients. The addition of nutrients caused an increase in the phosphorus concentration in the fertilized reach which was not observed for nitrogen concentration. From all macroinvertebrates studied only two taxa had significant differences in their abundance after fertilization: Corbicula fluminea and Ostracoda. Our results reveal that the disturbance caused by the increase of nutrients on the benthic community depends on basal nutrients concentration. The weak response of macroinvertebrates to fertilization in the pampean streams could be due to their tolerance to high concentrations of nutrients in relation to their evolutionary history in streams naturally enriched with nutrients. Further research concerning the thresholds of nutrients affecting macroinvertebrates and about the adaptive advantages of taxa in naturally eutrophic environments is still needed. This information will allow for a better understanding of the processes of nutrient cycling and for the construction of restoration measures in natural eutrophic ecosystems.

AIM: This study evaluated the use of oligochaetes at different levels of taxonomic resolution as environmental indicators in Argentine lowland streams affected by different land uses. METHODS: Sampling sites were grouped based on the physicochemical and habitat characteristics (low-, moderate-, and high-impact disturbance). Collection of the oligochaetes samples was carried out seasonally in sediment and vegetation habitats. RESULTS: The increases in nutrients and organic matter produced elevated densities of the Oligochaeta, but when the disturbance also involved changes in the physical habitat or enhancements in toxic substances, the abundance decreased significantly to values even lower than those of non-impacted environments. The responses of Naidinae and Tubificinae were similar. The density of the Pristininae decreased with increasing impact, but those of the Enchytraeidae and Rhyacodrilinae increased at the most highly impacted sites. The Opistocystidae were not recorded in high-impact sites. Species richness and diversity (H') were lower in high-impact sites and even lower in sediments. Some species presented no restrictions in the habitat type or with the contamination level: Limnodrilus hoffmeisteri, Dero furcatus, D. digitata, D. pectinata, Pristina longiseta, and P. aequiseta. Moreover, Trieminentia corderoi, Slavina appendiculata, and Aulodrilus pigueti exhibited the highest abundances at low-impact sites and were not registered in high-impact sites. CONCLUSIONS: The Oligochaeta show a relatively wide ecological valence through their extensive number of species. Although lower taxonomic levels can give information about environmental status, test-species' sensitivities to different types and degrees of contamination will be of utmost relevance to the evaluation of ecological quality.

Understanding the effect of habitat fragmentation is a fundamental yet complicated aim of many ecological studies. Beni savanna is a naturally fragmented forest habitat, where forest islands exhibit variation in resources and threats. To understand how the availability of resources and threats affect the use of forest islands by parrots, we applied occupancy modeling to quantify use and detection probabilities for 12 parrot species on 60 forest islands. The presence of urucuri (Attalea phalerata) and macaw (Acrocomia aculeata) palms, the number of tree cavities on the islands, and the presence of selective logging,and fire were included as covariates associated with availability of resources and threats. The model-selection analysis indicated that both resources and threats variables explained the use of forest islands by parrots. For most species, the best models confirmed predictions. The number of cavities was positively associated with use of forest islands by 11 species. The area of the island and the presence of macaw palm showed a positive association with the probability of use by seven and five species, respectively, while selective logging and fire showed a negative association with five and six species, respectively. The Blue-throated Macaw (Ara glaucogularis), the critically endangered parrot species endemic to our study area, was the only species that showed a negative association with both threats. Monitoring continues to be essential to evaluate conservation and management actions of parrot populations. Understanding of how species are using this natural fragmented habitat will help determine which fragments should be preserved and which conservation actions are needed.

We assessed the effects of nutrient enrichment on three stream ecosystems running through distinct biomes (Mediterranean, Pampean and Andean). We increased the concentrations of N and P in the stream water 1.6–4-fold following a before–after control–impact paired series (BACIPS) design in each stream, and evaluated changes in the biomass of bacteria, primary producers, invertebrates and fish in the enriched (E) versus control (C) reaches after nutrient addition through a predictive-BACIPS approach. The treatment produced variable biomass responses (2–77% of explained variance) among biological communities and streams. The greatest biomass response was observed for algae in the Andean stream (77% of the variance), although fish also showed important biomass responses (about 9–48%). The strongest biomass response to enrichment (77% in all biological compartments) was found in the Andean stream. The magnitude and seasonality of biomass responses to enrichment were highly site specific, often depending on the basal nutrient concentration and on windows of ecological opportunity (periods when environmental constraints other than nutrients do not limit biomass growth). The Pampean stream, with high basal nutrient concentrations, showed a weak response to enrichment (except for invertebrates), whereas the greater responses of Andean stream communities were presumably favored by wider windows of ecological opportunity in comparison to those from the Mediterranean stream. Despite variation among sites, enrichment globally stimulated the algal-based food webs (algae and invertebrate grazers) but not the detritus-based food webs (bacteria and invertebrate shredders). This study shows that nutrient enrichment tends to globally enhance the biomass of stream biological assemblages, but that its magnitude and extent within the food web are complex and are strongly determined by environmental factors and ecosystem structure.

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