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.
The mean body size of limnetic cladocerans decreases from cold temperate to tropical regions, in both the northern and the southern hemisphere. This size shift has been attributed to both direct (e.g. physiological) or indirect (especially increased predation) impacts. To provide further information on the role of predation, we compiled results from several studies of subtropical Uruguayan lakes using three different approaches: (i) field observations from two lakes with contrasting fish abundance, Lakes Rivera and Rodó, (ii) fish exclusion experiments conducted in in-lake mesocosms in three lakes, and (iii) analyses of the Daphnia egg bank in the surface sediment of eighteen lakes. When fish predation pressure was low due to fish kills in Lake Rivera, large-bodied Daphnia appeared. In contrast, small-sized cladocerans were abundant in Lake Rodó, which exhibited a typical high abundance of fish. Likewise, relatively large cladocerans (e.g. mesocosms after only 2 weeks, most likely hatched from resting egg banks stored in the surface sediment, but their abundance declined again after fish stocking. Moreover, field studies showed that 9 out of 18 Uruguayan shallow lakes had resting eggs of Daphnia in their surface sediment despite that this genus was only recorded in three of the lakes in summer water samples, indicating that Daphnia might be able to build up populations at low risk of predation. Our results show that medium and large-sized zooplankton can occur in subtropical lakes when fish predation is removed. The evidence provided here collectively confirms the hypothesis that predation, rather than high-temperature induced physiological constraints, is the key factor determining the dominance of smallsized zooplankton in warm lakes.
1. Shallow lakes and ponds contribute disproportionally to species richness relative to other aquatic ecosystems. In-lake conditions (e.g. presence of submerged plants) seem to play a key role in determining diversity, as has been demonstrated for temperate lakes. When water quality deteriorates and turbidity increases, conditions in such lakes are affected drastically resulting in a loss of diversity. However, it is not clear whether subtropical lakes show the same pattern and whether the richness of all groups reacts similarly to environmental changes. 2. Our aim was to analyse the main factors explaining patterns of species richness in plankton, fish and submerged macrophyte assemblages in both turbid and clear subtropical shallow lakes. We analysed abiotic and biotic features of 18 subtropical, small-to mediumsized, shallow lakes along the Uruguayan coast. We compared both turbid and clear ecosystem states and evaluated the relative variance explained by the factors measured. 3. Variables describing lake and catchment morphology, as well as the percentage of the water column occupied by submerged macrophytes (%PVI) and water turbidity, had strong effects on taxon richness. Interestingly, individual biotic groups had dissimilar richness patterns. Macrophyte %PVI decreased with increasing lake area, while fish species richness showed the opposite pattern. Phytoplankton species richness increased with macrophyte %PVI, while the zooplankton richness pattern varied depending on the taxonomic group considered. 4. Overall, our results indicate that, as found for temperate lakes, a greater submerged plant cover promotes higher species richness in several groups, and that this may overwhelm the otherwise expected positive effect of lake size on species richness. On the other hand, small-bodied zooplankton predominated in lakes with high plant abundance. Our findings concur with recent studies, indicating that refuge capacity of aquatic plants might be weaker in (sub)tropical than in temperate shallow lakes.
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.
Diet of two cichlid species, Cichlasoma facetum (Jenyns, 1842), and Gymnogeophagus rhabdotus Hensel, 1870, was studied in Rodó Lake, an urban hypertrophic lake in Uruguay. The stomach contents from 192 individuals of C. facetum and 202 of G. rhabdotus, obtained through seasonal sampling in the year 2000, were analyzed. The occurrence frequency and the alimentary importance index of each food item were calculated for each season and size class in both species. Cichlasoma facetum fed upon insects (mainly chironomid larvae and pupae), fish (Cnesterodon decemmaculatus Jenyns, 1842), and vegetals (algae, periphyton and macrophytes debris); large individuals also fed upon the freshwater shrimp Palaemonetes argentinus Nobili, 1901. Gymnogeophagus rhabdotus consumed zooplankton (mainly copepods), vegetals (algae and detritus) and Chironomidae larvae in a lesser extent.
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