Saline Lakes, Temporary Lakes, Halophilic Zooplankton - Scientific ...

International Journal of Ecosystem 2013, 3(4): 72-81 DOI: 10.5923/j.ije.20130304.03

Variations in Zooplankton and Limnological Parameters in a Saline Lake of La Pampa, Central Argentina, During an Annual Cycle Santiago A. Echaniz* , Gabriela C Cabrera, Paola L. Aliaga, Alicia M. Vignatti Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Pampa. 6300. Santa Rosa, provincia de La Pampa. República Argentina

Abstract Most of the lakes of La Pampa province are shallow, temporary, saline and hypertrophic. It should be studied in the framework o f the alternative states model, which indicates a turbid state, with high phytoplankton biomass when planktivorous fishes are present and a clear state in their absence, allowing the development of large zooplankton species, with high filt ration efficiency. Since in La Pampa the ecology of this type of ecosystem has recently begun to investigate, t he objective of this study is to analyze environmental and biological information, collected during an annual cycle in a p ampean shallow saline lake and test the hypotheses: i) increased environ mental stress due to decreased depth and salinity increased, which made richness, density and biomass of zooplankton dimin ish at the end of the study and ii) because of the temporality and high salinity, the lake lack fish, allowing the presence of large -sized cladocerans which determine a reduced phytoplankton chlorophyll-a concentration. The lake is chlorinated-sodium do minated and hypertrophic. The water level decreased 25% and salinity doubled. Lo w concentrations of chlorophyll-a resemb le San Joséto clear lakes, but inorganic suspended solids reduced the transparency. The diversity was reduced and the presence of large cladocerans may be due to the lack of fish. Zooplankton biomass was similar to other pampean lakes without fish but doubled that of lakes that have fish and small zooplankton. The lo w influence of environ mental factors on the species recorded may be due to their salinity wide tolerance ranges.

Keywords Saline Lakes, Temporary Lakes, Halophilic Zooplankton, Boeckella poopoensis, Daphnia menucoensis, Moina Eugeniae

1. Introduction In the province of La Pampa, in the semiarid central region of Argentina, there are a large nu mber of lakes, whose most remarkab le characteristics are their scarce depth (for wh ich they can be considered shallow lakes), their temporality, their relatively high salinity, and their high trophic status[1, 2, 3, 4, 5]. Due to their shallow depth, it is useful to study them within the framework o f the model of alternati ve states[6, 7, 8], wh ich ind icates that the presence of planktivorous fish in the ecosystem causes a trophic cascade or top down effect, since these fish prey especially on zoop lan kton species of g reater size and greater filtration efficiency. This leads to an increase in ph ytop lan kt on ch lo rop hy ll-a con cent rat ions and to a decrease in water transparency, which lead to a turbid state[6, 8, 9, 10, 11]. Conversely, the absence or low * Corresponding author: [email protected] (Santiago A. Echaniz) Published online at http://journal.sapub.org/ije Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved

density of planktivorous fish favors the development of large cladocerans, particularly of the genus Daphnia, whose high grazing rates lead to low phytoplankton concentrations of chlorophyll-a. As a result, the water t ransparency is high, thus leading to a clear state[6, 8, 9, 10, 11]. On the other hand, most Pampean lakes are temporary, since they contain water during periods that may range fro m a few months to several years (hydrophases)[3, 12, 13, 14]. This is because they are located in arheic basins and are fed mainly on precipitation and, to a lesser extent, on phreatic water. Thus, the water level of these lakes increases during rainy periods and decreases gradually during drier periods, sometimes becoming comp letely dry[3, 13, 14]. Pampean lakes can be considered saline because their concentration of dissolved solids is greater than 3 g.L-1 [15]. In addition, salinity is one of the ab iotic factors with g reater influence on the ecology of these environments because it can be drastically mod ified by changes in water levels caused by temporality, which in turn increase environmental stress[3, 16, 17]. In this sense, the species richness and abundance of zooplankton decrease as the concentration of dissolved solids increases [15, 18, 19, 20,

International Journal of Ecosystem 2013, 3(4): 72-81

21, 22, 23], a situation that has also been verified in lakes of La Pampa province[3, 14, 17]. Although the limno logy of relatively similar water bodies fro m other latitudes is well known, there are few studies on them in Argentina. These include studies on some relat ively stable saline lakes of the province of Buenos Aires [24, 25, 26] and Santa Fe [27], saline lakes of the Northwest[28, 29], and the extensive saline lake of Mar Chiquita, in the province of Córdoba[30]. In addition, although many conclusions on the functioning of other saline, shallow and temporary ecosystems of the world are applicable to those of Argentina, the zooplankton species assemblages reported are different, especially among crustaceans, which show some endemic elements of the neotropical region[1, 5, 17, 31, 32, 33, 34, 35], on which eco logical knowledge is relatively scarce. Taking into account that some shallow, temporary, saline lakes of La Pampa province have recently begun to be studied, the aims of this work were: to describe changes in physical, chemical, and bio logical (taxonomic co mposition, species richness, density and biomass of zooplankton) parameters, establish lin ks among them, and verify the hypotheses that: i) due to the increase in environ mental stress produced by the increase in salin ity resulting fro m the decrease in depth, richness, density and biomass of zooplankton in the lake studied decrease towards the end of the study period, and ii) due to the temporality and high salinity, the lake has no predatory fish, and thus zooplankton includes large cladocerans, wh ich determines that the lake has reduced phytoplankton chlorophyll-a concentration and high water transparency (clear state).

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2. Materials and Methods 2.1. Study Area The lake of Estancia San José(63º55'W, 36º21'S) (Figure 1) is located in the northeastern region of La Pampa, in the phytogeographic province of the Pampa Plains [36]. When the study began had an area of 288.5 ha and a maximu m depth of 3 m. The maximu m length was 2626 m, the maximu m width 1923 m and the length of the coast line was 8590 m. It is an environment characterized by large fluctuations in water level and salinity, since it is mainly fed on rainfall and phreatic inputs are unimportant. It lacks rooted vegetation and fish fauna. Since it is located in a very flat landscape, water inputs cause large movements of the coast line, so that its outline is a soft beach with patches of halophytic vegetation. Its entire perimeter is surrounded by fields used for growing cereals, oilseeds and extensive livestock. 2.2. Fiel d and Laboratory Work Monthly samples of water and zooplankton were collected between December 2005 and December 2006 (with exception of August), at three sites (F igure 1). The water temperature, the concentration of dissolved oxygen (Lutron® OD5510 o ximeter), the water transparency (Secchi disk), and the pH (Co rnning® PS15 pH meter) were recorded at each station, and water samples, which were kept refrigerated until analysis, were collected for physico chemical determinations.

Figure 1. Geographical location of the Estancia San JoséLake, in La Pampa province. A, B and C: sampling sites

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Santiago A. Echaniz et al.: Variations in Zooplankton and Limnological Parameters in a Saline Lake of La Pampa, Central Argentina, During an Annual Cycle

In addition, a qualitative sample of zooplankton was taken by vertical and horizontal dragging, with a net of 22 cm in diameter of mouth and 0.04 mm mesh opening, and quantitative samples were taken with a Schindler -Patalas plankton trap of 10 L with a net of 0.04 mm mesh opening, by means of t wo vertically aligned extract ions, so as to integrate the water colu mn. Samp les were anesthetized with CO2 and kept refrigerated until their analysis. The maximal depth of the lake was measured by probing and the len gth and width with a GPS Garmin® E-Trex Legend. The dissolved solids concentration (salinity) was determined by the gravimetric method with drying at 104°C of 50 mL of prev iously filtered water. The concentration of chlorophyll-a was determined by extraction with aqueous acetone to 90% and subsequent reading in a spectrophotometer[37, 38], total n itrogen (TN) by the Kjeldahl method and total phosphorus (TP) using the method of ascorbic acid, p revious digestion with potassium persulfate. The content of suspended solids was determined with M icroclar® FFG047WPH fiberglass filters, dried at 103-105°C until constant weight and later calcined at 550ºC[39]. The determination of the species present in each sample was carried out under conventional optical microscop e in open chambers that allow the manipulat ion of the material. Counts of macro- and microzooplankton[7] o f each samp le were carried out under stereoscopic and conventional optical microscopes in Bogorov and Sedgwick-Rafter cameras respectively. Once analy zed, the samples were fixed with formaline 5% and then deposited at the plankton collection of the Facultad de Ciencias Exactas y Naturales de la Universidad Nacional de La Pampa. To determine the biomass of zooplankton, a min imu m of 30 specimens of all species were measured with a Leit z ocular micro meter and formulas that relate the total length with the dry weight of the specimens were used[40, 41, 42, 43, 44]. We used the classification of continental waters based on salinity proposed by Hammer[15]. To represent the water chemistry we developed a Maucha graph[45, 46]. In order to examine relationships between environmental factors and attributes of zooplankton, nonparametric correlation coefficients of Spearman (rs ) were calculated[47, 48, 49]. To determine possible nutrient limitations to algal growth we calculated the ratio total nitrogen to total phosphorus (TN:TP)[46]. With the aim to characterize the lake as a function of its transparency, we calculated the relationship between the mean depth and that of the photic zone: Zm/Zphot [50]. For the calculation of the depth of the photic zone, we mu ltip lied the reading of the Secchi disk by a factor of 3[7, 51]. We used Past[52] and Infostat[53] softwares.

3. Results 3.1. Environmental Parameters

The mean depth of the lake along the study period was close to 2.7 meters (Table 1). It was highest in January but decreased markedly, especially during spring, reaching a minimu m in December 2006 (Figure 2). The mean dissolved solid concentration was just over 22 g.L-1 (Tab le 1), but varied widely, since it showed a min imu m just over 17 g.L-1 in January, coincid ing with greater depth, and a maximu m of almost 31 g.L-1 in December 2006 (Figure 2). The correlation between both parameters was significant (r s = -1; p = 0.0000). Table 1. Mean, minimum and maximum values of the limnological parameters determined in Laguna San José

Figure 2. Monthly variation of the maximum depth of the lake and the total dissolved solids concentration

The minimu m temperature of the water was close to 6°C (July) and the maximu m was higher than 27°C (December 2005) (Table 1 and Figure 3). The dissolved oxygen concentration was relatively h igh throughout the study period; with minimu m and maximu m in May and February respectively (Table 1 and Figure 3). Na+, which represented almost 91% o f total cat ions, and Cl , which represented 60% of total anions, predominated in the ionic composition of the water. A mong anions, SO 4 = also showed a relat ively high concentration, representing nearly 24% of total anions (Figure 4). The pH remained relatively stable during the study period, with a mean value of 9.13 (±0. 19).


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May (Table 1 and Figure 6). The mean concentration of phytoplankton chlorophyll-a was close to 20 mg.m-3 , but had a wide variation, since it had a min imu m near 4 mg.m-3 in Feb ruary and a maximu m of almost 80 mg .m-3 in March (Table 1 and Figure 6). This parameter was significantly correlated with the concentration of organic suspended solids (rs = 0.64; p = 0.0247).

Figure 3. Monthly variation of the water temperature and dissolved oxygen concentration

Figure 6. Monthly variation of the chlorophyll a and organic suspended solids concentrations

Figure 4. Ionic composition of the water of Estancia San JoséLake

Mean water transparency was greater than 0.4 m, with a minimu m in July and a maximu m in February (Tab le 1 and Figure 5). We found significant correlat ions between transparency and the concentration of chlorophyll-a (rs = -0.65; p = 0.0217) and the concentration of inorganic suspended solids (rs = -0.64; p = 0.220). The calcu lation of the Zm/Zphot relationship gave a value of 1.00. The mean concentration of inorganic suspended solids was close to 16 mg.L-1 , with a minimu m in April, when it was lower than 1 mg.L-1 , and a maximu m in June, when it was higher than 62 mg.L-1 (Table 1 and Figure 5).

The concentrations of nutrients in the water were very high. TP showed a mean close to 9 mg .L-1 , with a minimu m in February and March and a maximu m in May (Table 1 and Figure 7), and was correlated with the concentration of inorganic suspended solids (rs = 0.70; p = 0.0104). TN showed a mean slightly higher than 17 mg.L-1 , with a minimu m in January and a maximu m in May (Table 1 and Figure 7). We found a significant correlat ion between the concentration of this nutrient and that of organic suspended solids (rs = 0.88; p = 0.000). The mean TN:TP rat io was very low (around 2), with a minimu m in January and a maximu m in March (Table 1 and Figure 7).

Figure 7. Monthly variation of the nutrients concentration and the TN:TP ratio Figure 5. Monthly variation of the water transparency and inorganic suspended solids concentration

3.2. Zooplankton

The mean concentration of organic suspended solids reached almost 18 mg.L-1 , with a minimu m close to 4 mg.L-1 in February and a maximu m exceeding 40 mg .L-1 in

We recorded eight taxa, two corresponding to cladocerans, two to copepods and four to rotifers (Figure 8). Species richness was slightly higher during spring and summer

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(Figure 8), but we found no significant correlations between the number of species and water temperature or salin ity. Copepods were recorded in all the samp ling occasions, whereas cladocerans were found in ten of the sampling occasions (Figure 8). Rot ifer richness was significantly correlated with water temperature (rs = 0.74; p = 0.0052), since they were recorded only in spring - summer (Figure 8). Among crustaceans, the calanoid copepod Boeckella poopoensis was present throughout the study period, followed by the harpacticoid copepod Cletocamptus deitersi, which was present during eleven months (Figure 8). Among cladocerans, Moina eugeniae was also relatively frequent, since it was recorded in ten of the sampling occasions, followed by Daphnia menucoensis, which was found only on six occasions, with the particularity that it was recorded in December 2005 but not in December 2006 (Figure 8). All rotifers had relatively low frequencies. Among them, Brachionus plicatilis and Hexarthra fennica were found in 33.3% of the samples (four occasions), followed by Brachionus dimidiatus, present only in two occasions (Figure 8).

The total mean bio mass of the zooplankton commun ity was 4659.08 (±3417.30) µg.L-1 , with a minimu m of 179.87 µg.L-1 in February and a maximu m of 12923.14 µg.L-1 in May (Figure 9). Bio mass was significantly correlated with water transparency (rs = -0.67; p = 0.0153) and with the concentration of organic suspended solids (rs = 0.74; p = 0.051). The largest contribution to biomass was that of copepods (59.3%), fo llo wed by that of cladocerans (35.4%). Rotifers contributed only 2.63% of total bio mass . We found no significant correlation between total density and total biomass of the zooplankton community (rs = -0.007; p = 0.9828). Cladocerans showed a relatively low density, despite which they contributed a high proportion of the biomass of the community (Table 2). Both parameters were correlated with the concentration of total dissolved solids (rs = 0.68; p = 0.0143 and rs = 0.71; p = 0.0095). No rep resentatives of this group were recorded during February and March and reached the highest density in May (343.33 ind.L-1 ). However, this peak d id not coincide with the maximu m biomass, wh ich was recorded in September (5124.57 µg.L-1 ). Among cladocerans, M. eugeniae was the species that contributed the greatest density and biomass (Table 3), followed by D. menucoensis, a species that, despite showing a lower density, contributed a relatively high bio mass (Table 3). Table 2. Density and biomass of the different zooplankton groups recorded in Laguna San José

Figure 8. Taxa and month in which they were registered

Total mean density during the study period was 1532.11 (±1747.59) ind.L-1 , with a minimu m o f 220.33 ind.L-1 in December 2005 and a maximu m of 5135.01 ind.L-1 in March (Figure 9). Total mean density was not significantly correlated with any of the environmental parameters studied. The largest contribution to total density was that of rotifers (57.8%), fo llo wed by that of copepods (adults and copepodites) (21.2%).

Figure 9. Monthly variation of the total zooplankton density and biomass

Copepods (adults and copepodites) reached an intermediate density, close to 320 ind.L-1 , but contributed the largest biomass (Table 2). The minima and maxima of both parameters were recorded in December 2005 (1.50 ind.L-1 and 18.74 µg.L-1 ) and May (1862.67 ind.L-1 and 10551.86 µg.L-1 ) respectively. The two variab les were correlated with the concentration of organic suspended solids (rs = 0.83; p = 0.0008 and rs = 0.81; p = 0.0014). B. poopoensis contributed with the highest density and biomass (Table 3), whereas C. deitersi showed significantly low mean density and biomass (Table 3). Nauplius larvae showed a relat ively low density and contributed very little to mean bio mass (Table 2). They were not recorded in February and reached a maximu m density of 1030 ind.L-1 and a maximu m b io mass of 710.70


µg.L-1 in April. Density and bio mass of nauplii were not significantly correlated with any of the environmental parameters studied. Table 3. Mean, minimum and maximum values of density and biomass of the zooplankton species recorded in Laguna San José

Rotifers reached the highest densities, but, given their small size, contributed very little to total bio mass (Table 2). They were not recorded between April and October and had maximu m density and biomass in March (4963.34 ind.L-1 and 759.38 µg.L-1 respectively). Both parameters were positively correlated with water temperature (rs = 0.73; p = 0.0067 and rs = 0.78; p = 0.0027). The h ighest density and biomass were provided by B. plicatilis (Table 3), present only on four occasions.

4. Discussion The lake studied is located near the western boundary of the Pampean Plain[36] and, although it shares some features with the typical pampasic lakes of the province of Buenos Aires[25, 26, 54], such as the shallowness and polymixis, it differs by its temporality and wide variations in water level and salinity. These features allow classifying it as a shallow lake typical of the province of La Pampa, whose most important features are that it is fed on precip itation and, to a lesser extent, on groundwater and that, due to the absence of effluents for being in an arheic basin, water losses are especially produced by evaporation[14, 17, 34, 35, 55]. This phenomenon is especially important, given that the lake is situated in a region with annual average rainfalls of around 700 mm[56] that are exceeded by a potential evapotranspiration greater than 850 mm[57]. The temporary character of the lake was reflected by a decrease in the water level of almost 25% and an increase in

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the concentration of dissolved solids, which, at the end of the study, was almost two-fold higher than the initial value. This led Laguna San Joséto be hyposaline[15] during the first five months of the study period and then to become mesosaline [15] during the rest of the study. The relationship between the average depth of a lake and the depth of the photic zone has been proposed as a parameter to separate clear lakes fro m turbid ones in the framework of the model of the alternative states of shallow lakes[50]. Thus, if the calculated value is lower than 1, it is a clear lake, whereas if it is greater than 1, it is a turbid lake. Since the lake studied showed a value equal to 1, this relationship did not allow defining whether it is a clear or turbid lake. Taking into account its chlorophyll-a concentration, Laguna San Joséis similar to some shallow lakes of the region, which can be unequivocally classified as clear[1]. Ho wever, it is also similar to other relatively nearby lakes with regards to the negative influence of inorganic suspended solids on the transparency of the water[14, 55]. In addition, both Laguna San Joséand these environments have a relat ively flat bottom and are located in open and flat landscapes[1, 14, 55], in a region characterized by strong and frequent winds[58]. Besides, Laguna San José and these environments lacked aquatic vegetation during the study period[1, 14, 55] that could moderate the waves [59]. A ll these features favor the resuspension of bottom sediments by effect of the wind[60, 61], which could help to explain the relatively low water transparency of this lake produced especially by inorganic suspended solids. The chemical co mposition of the water, with the predominance of the monovalent cation Na + and low concentrations of the bivalent cations Ca ++ and Mg ++ (which indicate relatively low hardness) and of the anion Cl-, makes Laguna San Josédifferent fro m generalizat ions that indicate the predominance of Ca ++ and HCO3 - in most of the internal waters of the wo rld[7]. However, this feature makes it similar to most of the shallow lakes in the province, with the exception of those located in dune areas and those receiving water drainage directly fro m cit ies [1]. The ionic co mposition indicates that the mechanisms involved in the control of the chemical co mposition of the water body s tudied include evaporation and crystallizat ion, wh ich is typical in arid or semiarid reg ions like the northeast of La Pampa, where evapotranspiration rates are higher than rainfall[7, 46, 62], and of many of the environments of the Chaco-Pampean plain[63]. The high concentrations of nutrients found in Laguna San José, which allowed categorizing this lake as hypertrophic[6 4], were similar to those found in other environments of La Pampa[1, 5, 14, 65], especially in the case of total phosphorus, whose concentrations were higher than those reported by Quirós et al.[50] and Sosnovsky & Quirós[66] as some of the highest recorded in the literature. Such high concentrations could be due to the high impact o f the dragging of faeces excreted by the animals that graze in its basin, especially during storms [67, 68, 69], since cattle can excrete between 9 to 16 kg.ind.year-1 of phosphorus[70]. On

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the other hand, the resuspension of sediments by effects of the wind is particularly important in shallow lakes [61, 71, 72], in which the removal of sediments favours the resolubilizat ion of nutrients fro m the internal load [73], which collaborates to its internal eutrophication[74]. Similar to that observed in a nearby shallow lake[55], the highest concentrations of nutrients in the water were measured in the second half of the year, a period with winds of great intensity[58] and concentrations of inorganic suspended solids were the highest. In addition, since Laguna San Joséis an environment located in an arheic basin, water outlets only occur by evaporation, leading to processes of nutrient accumulat ion. The TN:TP rat io calculated throughout the study was very low, which would indicate a relat ive limitation by nitrogen to the growth of phytoplankton algae[7, 46]. Th is limitation could exp lain the relat ively lo w concentrations of chlorophyll-a found. This is supported by the fact that March and April, when the values of the TN:TP ratio were h igher (thus indicating a decrease in the limitation by n itrogen), were the months in which we recorded the highest concentrations of phytoplankton chlorophyll-a. The diversity of zooplankton was low, which is a common situation in high salinity environments [15, 16, 23]. This situation has also been shown in La Pampa, since the zooplankton of subsaline environ ments that do not exceed 2 g.L-1 may include more than 20 taxa [1, 4, 13, 65], whereas richness of hypo- and mesosaline environments is considerably lo wer[5]. The zooplankton of San José showed the association of species typical of this type of ecosystem: an assemblage characterized in particular by the presence of native halophilic crustaceans such as D. menucoensis, M. eugeniae and B. poopoensis[1, 5, 14, 55]. D. menucoensis is a very common species in saline ecosystems of Northern Patagonia and central region of Argentina[1], whereas M. eugeniae is restricted to saline waters of the central region[1, 5, 17, 32]. B. poopoensis is highly tolerant to salinity and has a very wide geographic distribution, fro m the north of the Patagonian plateau to the south of Peru[28, 75, 76]. The predominance among rotifers of the genus Brachionus is a characteristic in co mmon with other similar water bodies of the province[1, 5, 17, 35]. A mong them, strongly euryhaline species such as B. plicatilis and B. dimidiatus predominate[77], although they are not typical in the region because of their cosmopolitan distribution[78]. B. plicatilis together with Hexarthra fennica comprise the association of rotifers found most frequently in lakes of salinities higher than 20 g.L-1 in the central region of Argentina[5]. The presence of large cladocerans may be due to the lack of fish, given the saline and temporary nature of Laguna San José. In Pampean environments with greater permanence of water and fish fauna, large cladocerans are usually absent or occur in very low densities [1, 35, 79] due to predation by planktivorous fish[6, 8, 50]. A direct consequence of the

presence of large cladocerans, which are efficient filter feeders, is the reduction of phytoplankton biomass and the consequent clearance of water. This situation was also observed in Laguna San Jos é, where phytoplankton chlorophyll-a concentrations were almost 10 times lower than those recorded in Don Tomás or Bajo de Giu lian i, lakes of La Pampa province characterized by the presence of silverside (Odontesthes bonariensis)[4]. The relat ive nu merical do minance of rotifers found in the summer is a feature in common with other environments of lower salin ity of the province, since in other mesosaline lakes these organisms tend to be less numerous than crustaceans[1, 65, 80]. This do minance led zooplankton density to be highest during the months of higher temperature, unlike that observed in Pampean lakes with predominance of crustaceans, where this type of seasonal pattern is less frequent[1, 5]. Although there were differences in total abundance, the zooplankton bio mass found in Laguna San José was relatively similar to that found in the same period in other two saline lakes of the reg ion, Estancia Pey-Ma and Aime, where relatively large species were also recorded[3, 14]. However, while the abundance in Laguna San Joséwas quite lower than that recorded in the same period in Don To más and Bajo de Giu liani, the relat ively h igh proportion of large crustaceans led the bio mass of Laguna San José to be two-fold higher than that of the other two lakes, which showed a higher proportion of rotifers and smaller crustaceans because of predation[4]. In addition, the macro zooplankton biomass of Laguna San José was more than six-fo ld higher than the maximu m determined by Quirós et al.[50] in a group of 23 o rganic turbid shallow lakes of Buenos Aires province, with salinities between 0.3 and 27 g.L-1 and concentrations of chlorophyll-a of up to 405 mg .m-3 . Th is supports the hypothesis that although saline lakes have low concentrations of chlorophyll-a, lower algal bio mass and therefore lo w primary productivity, they can support high zooplankton bio masses[81]. A remarkable feature of Laguna San José was the low influence of environmental factors on the density and biomass of zooplankton. This may probably be due to the fact that all the species recorded were found within their tolerance ranges and therefore may have not suffered fro m the environmental stress exerted by the variation in abiot ic parameters recorded during the study period, in part icular the increase in salin ity.

ACKNOWLEDGEMENTS We thank the Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Pampa for the financial support of the project and Deanna family, owners of the establishment in which the lake is located.


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