Abstract
Abstract Aim The Central Pampa of Argentina has three recognized phytogeographic regions that arise due to the decrease in rainfall towards the west. The area has numerous lakes that are mainly temporary, with hydroperiods that relate to climatic cycles, although some of them have changed due to anthropogenic influence. Some of these lakes have been studied with special reference to zooplankton, but information on their physical and chemical aspects is scarce. Consequently, managing and evaluating the anthropogenic effects on these ecosystems is challenging. The objective of this study was to explore the limnological characteristics of lakes in different regions in the Central Pampa that experience different anthropic influences. Methods Ten lakes were sampled seasonally (January, April, July, and October) during 2007. In situ measurements included transparency, pH, temperature, and dissolved oxygen concentration, and 2-L water samples were collected to determine salinity, ion composition, suspended solids, nutrient concentration, and phytoplankton chlorophyll- a concentration. Results Salinity ranged from 0.32-136.72 g L-1, with Na+ being the dominant ion in nine lakes and Cl- and HCO3- predominating in the higher- and lower-salinity lakes respectively. Nutrient concentrations were high (total Kjeldahl nitrogen: 7.97-34.69 mg L-1; total phosphorous: 4.07-14.82 mg L-1), and all lakes were hypertrophic. We determined three lake classes: i) lakes transformed from low-salinity lakes into hypersaline ones through human inactivation of the fluvial system that fed it; ii) mesosaline temporary lakes lacking fish, with low concentrations of chlorophyll-a and influenced by agricultural activities, and iii) subsaline and hyposaline lakes, highly modified by urban sewage, converted in permanent lakes (which allowed fish fauna development) and with reduced water transparency (due to high concentrations of phytoplankton chlorophyll-a). Conclusions The chemical diversity of the studied lakes is low, and their predominance of Na+ and Cl- indicated that evaporation and crystallization control the water chemistry. Additionally, this study showed the consequences of the anthropic impact, which alter water chemical composition, trophic structure and, thus, the ecological characteristics of lakes.
Highlights
The central region of Argentina, where the province of La Pampa is located, is characterized by the predominance of semi-arid conditions that increase towards the west
The absolute values ranged from a minimum of 0.32 to a maximum of 136.72 g L-1, registered during autumn in La Arocena (LAR) and La Amarga (LAM), respectively (Table 2)
No seasonal pattern was detected, but relatively wide variations involving declines could be observed in some lakes, such as those observed at Estancia Pey-Ma (EPM), where salinity decreased from 48.31 g L-1 (January) to 31.22 g L-1 (October), and at LAM, where salinity ranged from 90.52 g L-1 (October) to 136.69 g L-1 (April)
Summary
The central region of Argentina, where the province of La Pampa is located, is characterized by the predominance of semi-arid conditions that increase towards the west This phenomenon arises due to a decreasing precipitation gradient that starts at 750 mm yr-1 in the northeast and declines to less than 300 mm yr-1 in the west (Roberto et al, 1994; Casagrande et al, 2006). The third region, the Monte in the western part of the province (Cabrera, 1976) (Figure 1), is characterized by low and dispersed vegetation, predominantly “jarillas” (Larrea spp.) This region has high aridity and a low population density, with agriculture consisting of extensive cattle, sheep, and goat production
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