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Language see all. The outline denotes the morphometric dimensions recorded in December The lake is surrounded by fields where cereals and oilseeds are cultivated, and, to a lesser extent, are used as pasture for cattle. This lake does not host any aquatic vegetation or fish. At the beginning of the study period December , the maximum length of the lake was 2 m, its maximum width was m, and its maximum area was These parameters gradually decreased until complete lake drying up.
Monthly samples were taken from December to July the lake dried up after the last sampling. Water samples were taken and kept refrigerated until laboratory analysis. Moreover, two liter quantitative zooplankton samples were taken at each sampling site. In addition, at each sampling spot, a qualitative sample was also taken with a similar net.
All samples were anesthetized with CO 2 and kept refrigerated until fixation. Water samples were taken twice during the study period January and July , to determine ionic composition. Macro and microzooplankton Kalff, counts were made under stereoscopic and conventional optical microscopy in Bogorov and Sedgwick-Rafter chambers, respectively. Samples were homogenized by shaking. Zooplankton density was obtained by counting the number of individuals ind per volume unit, expressed as ind L For crustaceans, available formulae relating total length to dry weight were used McCauley, ; Culver et al.
Subsequently, the volume estimate allowed for fresh weight calculation assuming a specific gravity of 1 and its subsequent conversion to dry weight assuming a dry wet ratio of 0. Software packages Past Hammer et al. At the beginning of the study, in December , the depth of the lake was around 0. However, in July , its depth decreased to 0. Furthermore, mean water pH was of 9. A maximum water temperature of Mean dissolved oxygen concentration in water was high, around 8 mg L -1 Table 1 , and fluctuated from a minimum of 6.
Water transparency was always low Table 1 , with a maximum of 0.
Phytoplankton chlorophyll-a concentration was reduced at the beginning of the study but experienced a marked autumn peak, reaching Both solid types showed a strong peak in autumn, especially in April, exceeding 1 mg L -1 altogether. A maximum species number of five was observed in February, and it declined as lake drying proceeded, so that, only one species was recorded in July. The copepod Boeckella poopoensis Marsh, was found in each month, from December to July, even when salinity rose above 90 g L Total average density and biomass values of the zooplankton community in Ojo de Agua Uriburu, were of From May on, both zooplankton density and biomass were severely reduced Fig.
No correlations were found between zooplankton total density or biomass and the assessed environmental parameters. For most of the study period, copepods accounted for the largest fraction of zooplankton density and biomass in Ojo de Agua Uriburu in five out of the six months that lasted the sampling Fig. Remarkably, rotifers accounted for the largest fraction of the zooplankton density in the lake in February Fig.
During the first four months, parthenogenetic, ovigerous M.
From May on, no single M. Among copepods, the species B. Both B. Adults and copepodites of this species, experienced a density and biomass peak in April,with parameter values of Further, B. These larvae accounted for 4. However, no nauplii larvae were recorded in the last two sampled months. Average body sizes of the two most important crustaceans in the studied lake, M. Between December and March, mean body lengths of M. In the subsequent months, larger average body sizes were observed. Specifically, M. Most shallow lakes in the province of La Pampa are temporary, and their filling and drying depends on wet and dry climatic cycles Viglizzo, In a relatively prolonged hydroperiod with greater-than-average precipitation the amount of water stored in the soil increases and the piezometric level rises.
Groundwater discharges in low-lying areas, where temporary lakes are located, constitute the main water source for these lakes, followed in importance by rain water or surface runoff Dornes et al. Ultimately, hydroperiod duration in these lakes is closely related to the amount of water stored in the ground and may extend for several years in some cases; water input from rain may not be constant, but phreatic water discharges keep the water influx into the lake Echaniz et al.
Previous to its latest drying event, the Ojo de Agua Uriburu lake was rich in chloride - carbonate sodic , and the hardness of its water was reduced given the low concentrations of bivalent cations. At the beginning of the present study, the lake was classified as hyposaline Hammer, However, only three months into the study in March , the process of salt concentration, consequence of water evaporation, drove the lake above the hypersaline threshold Hammer, The lake remained hypersaline until drying.
Because of these salinity changes, any classification of such lakes into a specific category is of relative utility, and caution should be taken, as salinity depends on the ruling hydroperiod at the time of observation. Both chemical composition and salt concentration analyses confirmed that the lake chemistry was governed by evaporation - crystallization.
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Water transparency was relatively low, likely due to the high amount of suspended solids, especially those of inorganic origin. This is a common situation for lakes in Central La Pampa. Remarkably, the lowest water transparency was recorded in April, when the highest concentration of phytoplankton chlorophyll-a coincided with a very high amount of suspended inorganic solids.
The observed high concentration of dissolved oxygen in the water, even when chlorophyll concentration was relatively low, could have been a consequence of atmospheric dissolution, favored by wind-driven turbulence, rather than a contribution of phytoplankton autotrophic organisms. This phenomenon is common in the clear lakes of La Pampa, such as El Carancho, Utracan, and La Amarga, where, despite the fact that chlorophyll-a concentrations are below 1.
This situation could be due to the large amount of inorganic, suspended solids rendering waters of an intense dark brown color and, in turn, favor heating by solar radiation. Zooplankton richness in Ojo de Agua Uriburu was low. However, this is common among fishless, saline lakes in Central Argentina.
These lakes are inhabited by the crustaceans B. Whereas these crustacean species are endemic to Neotropical halophilic lakes, the rotifers are cosmopolitan and are broadly distributed given their wide tolerance to salinity Fontaneto et al. Therefore, the present study provides the first record for this species in the hypersaline range. Despite having been recorded as the most halotolerant Moinid in Argentina Paggi, , M. Consequently, in the last two months of the sampling period, only the halotolerant B.
However, caution should be taken before reaching any conclusion, since the absence of significant statistical correlations among the studied environmental variables water temperature and salinity , and the zooplankton parameters density and biomass is by no means a confirmation of a negative effect of increasing salinity on these biological parameters. The largest mean body sizes for both B. Whereas in ectotherm vertebrates, individuals of small size are likely to occur at lower environmental temperatures Gillooly et al. In the present study, the observed increase in body size for B. Such an effect could be obtained by negatively influencing reproduction, as evidenced by the lack of ovigerous, parthenogenetic females of M.
Moreover, low temperature could not affect the reproduction of these two species, since ovigerous M. In addition, in three mesosaline lakes of LaPampa, with relatively stable salinity, the highest population densities of B. A feature of many shallow lakes of the region is that the combination of their temporary nature and their high salinity hinders their colonization by fish Menni, ; Rosso, This situation, verified in Ojo de Agua Uriburu, may also lead to relatively large-sized zooplankton crustaceans, such as B.
Grazers are considered to play a key role in decreasing the amount of phytoplankton species of the genus Daphnia Muylaert et al. It is likely that, as a relatively large size grazer, M. The environmental and biological changes that occur during lake drying have already been previously studied in two lakes of La Pampa. In comparison with the information obtained in the present study, variation in lake dynamics, due to differences in environmental conditions, can be highlighted.
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