Limnological changes in a sub-tropical shallow hypertrophic lake during its restoration: two years of a whole-lake experiment

1. Lake Rodo is a turbid system, a condition attributed to algal biomass. T he proximal source of the eutrophication was stormwater discharges from an ill-defined urban area. This paper describes an attempt to restore the wate r quality of Lake Rodo, the first time this has been done in Uruguay. In sp ring 1996 it was drained, sediments were removed and stream inputs were div erted. Groundwater was used to re-fill the lake. Due to its high nutrient c oncentration a re-circulation system was designed, pumping water from assoc iated pools covered with free-floating plants. 2. After the lake was refilled, the system was characterized by oxygen satu ration or over-saturation, neutral to basic pH, and high phosphorus, nitrog en and silicate concentrations. Ratios of total nitrogen (TN):total phospho rus (TP) and chlorophyll a (Chl a):TP indicated that phosphorus was the pri mary limiting nutrient during the period of groundwater supply. Once ground water 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 di verse. This change has favoured grazing by mesozooplankton, and the onset o f clear-water phases in spring. 4. Abundant populations of small omnivorous fish maintained a high predatio n pressure on zooplankton, restricting the abundance of large-bodied herbiv ores, which, in turn, allowed an increase in phytoplanton biomass and a dec rease in water transparency. Based on this observation, together with the p hosphorus 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 Rodo. 5. A nutrient reduction programme, by the mechanical harvest of floating pl ants, and a removal of small omnivorous fishes and stocking strictly with p iscivores, could be key factors in the achievement of a stable clear-water phase. However, if blooms of Microcystis or other similar genera occur in s ummer, additional measures (e.g. reduction of the hydraulic residence time) will be needed to improve water transparency. Copyright (C) 2001 John Wile y & Sons, Ltd.