Global change drivers including eutrophication, hydrological disturbance, climate change, chemical pollution, overexploitation, invasive species, and land-use change are affecting the function and structure of freshwater ecosystems. South American freshwater ecosystems are especially threatened by the combination of rising human pressures on natural resources (i.e., water use, intensive agriculture, mining, deforestation, and afforestation) and the lack of adequate legislation and economic resources for environmental protection and restoration. We assess the state of freshwater ecosystems in Argentina, Brazil, Chile, Colombia, Ecuador, Uruguay, and Venezuela, focusing on broad categories of human-induced threats grouped into 5 categories: climate change, watershed stressors, hydrological alteration, channel modification, and biological stressors. For most countries, the most severe threats are related to land uses such as mining, agriculture, and urban expansion. Other threats relate to the abstraction or wasteful use of water and the intense regulation of flows, including the effects of large hydraulic infrastructure. There is also an increasing need to empower public organisations that focus on environmental protection, to update or develop an adequate regulatory and legal framework, to provide adequate funding for the implementation of environmental legislation, and not least to implement ecological rehabilitation. Implementation of these steps would reduce the threats to South American aquatic ecosystems and allow progress toward the sustainable development of this region in future decades.

The Cinaruco River, a lowland floodplain river in the Venezuelan llanos, has a strongly seasonal hydrology, low nutrient concentrations, and high fish diversity and abundance. Fish exclosure/enclosure experiments were conducted in the littoral zone of the river channel and connected lagoons to examine seasonal variation in the magnitude of fish effects on benthic organic matter and algal biomass. During the dry season, large-fish exclosures in the channel accrued significantly more sediment, organic material, and chlorophyll than control cages after 20 d. Grazing scars suggested the bocachico, Semaprochiloduskneri, was a major consumer of organic-rich sediments. Further experiments were conducted to test the hypothesis that the relative strength of top−down (grazer) control of organic matter in sediments varies according to species, hydrologic period, and habitat. At flooding onset (May), S. kneri migrate to the Orinoco River to reproduce and feed. Thus, their densities are extremely low in the Ci...

We examined primary production, respiration, and nutrient dynamics in littoral areas of the mainstem and lagoons of the Cinaruco, a nutrient-poor river in the Venezuelan llanos. Gross primary productivity (GPP) was relatively high, given the poor nutrient conditions in this river. Seasonal variability in net ecosystem production (NEP) was also high, with highest values occurring in the dry season (March–April) when fish biomass and chlorophyll a (both water-column and benthic) levels were greatest (dry-season NEP = 542 mg C m−2 d−1, wet-season NEP = 303 mg C m−2 d−1). NEP and algal biomass (measured as chlorophyll a) were higher in lagoons than at river sites, with more pronounced differences between these 2 habitats during the dry season. Strong N limitation was evident. Dissolved inorganic N (DIN) concentrations always were <2 μM and typically were <0.5 μM. Molar ratios of DIN:SRP (soluble reactive P) varied little seasonally and were always <8. As in other Neotropical aquatic systems, water-col...

Our study evaluated spatiotemporal variation of physicochemical parameters and benthic and sestonic algal biomass in littoral areas of the main channel and floodplain lakes of the Cinaruco River, a lowland tributary of the Orinoco River in Venezuela. The Cinaruco is characterized by a highly predictable annual flood pulse, high transparency, and extremely low conductivity and suspended sediment load. During 2002 and 2003, 10 sites in the main channel and floodplain lagoons were sampled on 12 occasions. Correspondence analysis based on water physicochemical variables and algal biomass showed that ˜80% of the variation in the multivariate space among sites and sampling dates was explained by 2 axes. During the high-water periods, river and lagoon sites showed high similarity in water variables and algal biomass. Conductivity, SiO3, and chlorophyll a concentrations (both benthic and sestonic) were low during the high-water phase. On the other hand, water variables and algal biomass differed between r...