ABSTRACT1. Lake Rodó is a turbid system, a condition attributed to algal biomass. The proximal source of the eutrophication was stormwater discharges from an ill-defined urban area. This paper describes an attempt to restore the water quality of Lake Rodó , the first time this has been done in Uruguay. In spring 1996 it was drained, sediments were removed and stream inputs were diverted. Groundwater was used to re-fill the lake. Due to its high nutrient concentration a re-circulation system was designed, pumping water from associated pools covered with free-floating plants.2. After the lake was refilled, the system was characterized by oxygen saturation or over-saturation, neutral to basic pH, and high phosphorus, nitrogen and silicate concentrations. Ratios of total nitrogen (TN):total phosphorus (TP) and chlorophyll a (Chl a):TP indicated that phosphorus was the primary limiting nutrient during the period of groundwater supply. Once groundwater pumping had ceased, there was a decrease in TN:TP and Chl a:TP ratios, suggesting N-limiting conditions prevailed in some periods.3. Before restoration, the phytoplankton community was dominated year-round by Planktothrix agardhii; since restoration the community has been more diverse. This change has favoured grazing by mesozooplankton, and the onset of clear-water phases in spring.4. Abundant populations of small omnivorous fish maintained a high predation pressure on zooplankton, restricting the abundance of large-bodied herbivores, which, in turn, allowed an increase in phytoplanton biomass and a decrease in water transparency. Based on this observation, together with the phosphorus concentration and the low abundance of filamentous cyanobacteria compared with previous studies, we suggest that top-down control has played a key role in increasing transparency in Lake Rodó.5. A nutrient reduction programme, by the mechanical harvest of floating plants, and a removal of small omnivorous fishes and stocking strictly with piscivores, could be key factors in the achievement of a stable clear-water phase. However, if blooms of Microcystis or other similar genera occur in summer, additional measures (e.g. reduction of the hydraulic residence time) will be needed to improve water transparency.

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Lake Rodó (Montevideo, Uruguay) is a small, urban, hypertrophic lake undergoing restoration. In this study, we evaluated the nutrient removal efficiency and water quality improvement attributable to a water recirculation system, consisting of the lake and three connected pools converted to artificial wetlands dominated by free-floating hydrophytes. Eichhornia crassipes and Spirodela intermedia dominated the hydrophyte community during summer and winter, respectively, with the biomass production being maintained throughout the year. The maximum production values of E. crassipes were 11.3 and 5.6 g DW m -2 d -1 in the summers of 1998 and 2000, respectively, while those of S. intermedia were 2.7 and 0.8 g DW m -2 d -1 in the summers of 1999 and 2000, respectively. The aquatic plant community reduced the concentration of nutrients in the water column but did not significantly affect the sediment concentrations. Harvesting the hydrophytes removed the equivalent of 58-88% and 39-78% of the nitrogen (N) and phosphorus (P) load associated with the water column, respectively. In contrast, the harvests accounted for only 1-2% of the N and P load associated with the sediments. In the pools, the combination of water recirculation and hydrophytes generally diminished the algal biomass and the associated N and P, compared to that observed for the lake. The combined use of adequate aquatic plant harvests and hydraulic management increased the efficiency of the system and, therefore, seems to be a useful tool for restoring small, shallow lakes in tropical and subtropical regions.

Saito Grande (mean flow 4640 m'/s; surface 757 km') is a subtropical polymictic reservoir built in 1979 for power generation by damming the Uruguay River. Relevant Iirnnological differences be tween the main channel and lateral arms of the reservoir were registered due to different circulation patterns. The main force driving the system was assumed to be the sharp seasonality of the hydrolog ical cycle. Due to the dendritic shape and complex morphometry. a lacustri ne-type reservoir devel ops during the summer. whilst a ri ver-like system occurs in winter. Runoff from the northern area of the basin is the main source of phosphorus entering the reservoir. whereas nitrogen and silicate enter the lacustrine area of the reservoir through lateral arms receiving local runoff.

We studied the succession patterns of the benthic community following a whole-lake restoration experiment in a subtropical hypertrophic lake (Lake Rodo´, 34°55¢ S 56°10¢ W, Montevideo, Uruguay). The restoration measures involved diversion of the main inlet and removal of upper 1-m sediment and biomanipulation of the fish community. Between January 1997 and November 1999, we sampled sediments seasonally to analyse changes in benthos in relation to other abiotic and biotic characteristics of the system. The benthic community of the lake was composed of three families and nine genera. The maximum density (646 ind m )2 ), as well as the maximum taxonomic richness (six), were observed 1 month after the lake was refilled. Since 1998, the benthic abundance decreased considerably and continuously and a total absence of benthic organisms was registered by the end of the year. The low abundance of macroinvertebrates during 1997 could be explained by the food preferences of the dominant fish species, and the high fish biomass at the beginning of the biomanipulation process. However, the most relevant physico-chemical temporal patterns were the increase of organic matter and nutrients in the sediment and the fluctuations of oxygen and nitrate in the deepest layer of the water column. The disappearance of benthos was related to these temporal changes. These results stress the importance of the increase of organic matter for the changes in the physicochemical environment, and its importance in the benthic succession and possible collapse. We suggest that in hypertrophic lakes, the effects of organic matter enrichment in the sediment can be even more relevant than fish predation in shaping the zoobenthos.

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