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

Abstract

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.

Feeding of two Cichlidae species (Perciformes) in an hypertrophic urban lake

“…In Lake Rodó, the abundant population of small omnivorous fish (C. decemmaculatus) maintains a high predation pressure on zooplankton, restricting the abundance of largebodied herbivorous, which in turn, allow an increase in microalgae biomass and a decrease in water transparency (SCASSO et al, 2001). In this context, C. facetum could have a clear beneficial effect on the water quality by predating over C. decemmaculatus.…”

“…depth 2.4m) and hypertrophic man-made lake, with the absence of real littoral zone due to the presence of vertical stone walls. It is located in Montevideo city and has been physically and biologically characterized by SCASSO et al (2001), who considered the system as hypertrophic, with a conductivity between 427 and 1103 µS cm…”

“…One of the first steps in the processes of top-down biomanipulation is the knowledge of the trophic function of the key species that compose the system (PERROW et al, 1997). Cichlasoma facetum (Jenyns, 1842) and Gymnogeophagus rhabdotus Hensel, 1870 are among the most abundant species in Lake Rodó (Montevideo, Uruguay), an urban system dominated by a small omnivorous fish, Cnesterodon decemmaculatus Jenyns, 1842 (SCASSO et al, 2001).…”

Shallow lakes theory revisited: various alternative regimes driven by climate, nutrients, depth and lake size

“…By contrast, top-down control of zooplankton by fish is very strong all year round in warmer lakes at low latitudes because fish are abundant and reproduce continuously in such (sub)tropical lakes (Dumont, 1994). The difference in food web structure implies that biomanipulation as used to shift temperate lakes to a clear state, seems less easy to apply to (sub)tropical lakes (Scasso et al, 2001;Jeppesen et al, 2005).…”

Lakes and society: Mirrors to our past, present and future

“…Biomanipulation may fail unless it is intense and repeated. Fish are fecund, and the multiple generations of small fish in warm lakes may preclude biomanipulation entirely as a way of restoration (Scasso et al 2001). Sources of aquatic plants in intensely modified landscapes may be scarce, and money is rarely available for proper follow up and monitoring.…”