Limnological comparison of two shallow lakes (Buenos Aires, Argentina) during turbid and clear water phases

CLAPS, M. C.; ARDOHAIN, D. M.; BENÍTEZ, H. H.; CANO, M.G.; CASCO, M. A.; GABELLONE, N.; MAC DONAGH, M.; SOLARI, L.; RUIZ, G.

Buenos Aires

Congreso; International Conference Environmental change and rational water use.; 2005

International Geographical Union

body { background: #FFFFFF; margin: 0px; padding: 0px; } The differences of the shallow lakes of the Buenos Aires province can be assigned to their geological features, evolution and land use. Two alternative state of equilibria related to the presence or absence of submerged macrophytes could be observed in these lakes involving changes in their  physical, chemical and biological characteristics. The main objective of this study was to compare two lakes with different hydrological and geological features during both phases. The San Miguel del Monte lake (35° 27´ S - 58° 48´W) has an area of 655 ha, and its mean depth is 1.3 m. A town (10,000 inhabitants) is located on the shore. During floods of the Salado River, the lake can be connected to the river. The Lacombe lake is arreic (35º 55` S - 58º 65` W), with an area of approximately 130 ha. Its maximum depth was 2,5 m. Extensive cattle breeding and agriculture practices are developed in the surroundings. Emergent (Scirpus californicus) and submerged macrophytes(Potamogeton pectinatus and Myriophyllum quitense).were recorded in both lakes. Sport fishing is performed all year in both lakes on planktivorous and predator fishes. The study was carried out in both water bodies during rainy periods. Plankton samples were collected for qualitative and quantitative analysis. Total phosphorus (TP), conductivity, particulate organic matter and suspended solids were measured. Transparency (Secchi disk) increased during the growth of P. pectinatus, reaching 0.7 m in Monte and 1.0 m in Lacombe, whereas the minimum occurred during the turbid phase (0.20 m in Monte and 0.30 m in Lacombe). Values of conductivity decreased throughout the sampling period (Monte: 2500 to 1190; Lacombe: 4240 to 1294 µS/cm). The decrease of TP concentrations in both lakes is related to the growth of P. pectinatus. The composition of suspended solids differed in the lakes (Monte: clay and particulate organic matter; Lacombe: particulate organic matter). In Monte, the highest density of phytoplankton was reached in spring with dominance of Oscillatoria limnetica and Rhapidiopsis mediterranea and late summer (Aphanocapsa delicatissima, Coelosphaerium kuetzingianum and Chroococcus dispersus). In Lacombe, a much smaller phytoplankton peak was recorded in winter, due to the striking contribution of Lyngbya limnetica and A. delicatissima. The zooplankton is dominated by rotifers (Monte: Brachionus caudatus, B. havanensis, and Filinia longiseta, Lacombe: B. havanensis and Keratella lenzi) and nauplii stages of copepods in both shallow lakes. The importance of cladocerans increases when the macrophytes beds are developed. In spite off marked hydrological and geological differences, these lakes showed similar change patterns in biological and physical characteristics. Decrease of TP, conductivity and suspended solids and increase of transparency were recorded. In a first view, the plankton of these lakes showed clear quantitative and qualitative differences. Nevertheless, the annual cycle analysis reveals the same responses to the alternative phases. The unpredictably external forces (rainfall, flood conditions) coupled with internal characteristics (clear and turbid phases) facilitate to understand the main differences between Monte and Lacombe lakes.