The central provinces of Argentina are characterized by the presence of a high number of shallow lakes, located in endorheic basins, many of which have elevated salinities as well as eutrophic or hypereutrophic condition. The zooplankton of four saline shallow lakes of the province of La Pampa was studied on a monthly basis during a 2-year period to determine its temporal and spatial variation.The surface of these shallow lakes (< 2.5 m depth) varied between 56.8 and 215.9 ha, and some have from 8.4 to 20.8 g · l -1 . The more saline lakes have "clear" water and the less saline lakes "turbid" water. Fishes, Jenynsia multidentata, were present in only two lakes during the last two months of the studied period.The zooplankton was composed of 17 taxa of Rotifera, 5 taxa of Cladocera and 4 taxa of Copepoda. The low diversity and the faunistic composition are characteristic of saline environments. Although the studied lakes share 38% of the species, the faunistic similarity was higher between the two least saline lakes. The lowest diversity was found in the two most saline lakes.All four shallow lakes were characterized by their very high zooplankton density, especially in the least saline lakes (< 80000 ind · l -1 ). The abundance is significantly correlated with the water transparency but not with salinity.The zooplankton temporal variation was characterized by the alternation of macro-and microzooplankton, probably regulated by competition and intrazooplanktonic predation. In each lake, the spatial abundance distribution of the macro-and microzooplankton was homogeneous. It was related to the shallow depht of the lakes and their polymictic condition.The SCHEFFER model on alternative states in shallow lakes acknowledges that it cannot be applied to saline lakes because Daphnia, the main responsible for the clear water state, is not tolerant to high salinity. Our study shows that the most saline lakes, where the halophylic Daphnia menucoensis is abundant, have also the most clear waters. Another difference that we found with regards to the mentioned model is that, in turbid lakes, it could not have had a top-down control on macrozooplankton exerted by fishes because in these lakes fishes were practically absent.

es una formación medanosa ubicada en el centro de la provincia de La Pampa, en la región semiárida central de Argentina, en el que se encuentra un grupo de lagos someros, alimentados por lluvias, de distintas características físico-químicas, sobre todo la salinidad, pero que comparten un elevado grado de eutrofia. A los efectos de integrar información sobre la composición y variación de las asociaciones zooplanctónicas de ambientes de diferentes concentraciones de salinidad y grados de eutrofia, se analizaron, mediante muestreos mensuales, tres lagos someros (lagunas) del Valle Argentino, dos de ellos subsalinos y el tercero hipo-mesosalino. Las dos lagunas subsalinas son turbias, con cianofitas, y en ellas se registró la presencia de peces. Su zooplancton se caracterizó por una alta diversidad (32-37 especies) y abundancia, habiéndose verificado el predominio del microzooplancton, sobre todo de los rotíferos. En estas lagunas, el macrozooplancton estuvo representado por especies de crustáceos de talla pequeña, poco vulnerables a las interferencias alimentarias provocadas por los filamentos de cianobacterias, pero cuya baja eficiencia de filtración contribuyó a mantener el estado turbio. A pesar de su mayor concentración de sales, la tercera laguna puede caracterizarse como clara. En ésta la diversidad y abundancia fueron menores (13 especies), y la ausencia de peces depredadores permitió el predominio del macrozooplancton, integrado por especies de copépodos (Boeckella poopoensis Marsh, 1906) y cladóceros (Daphnia menucoensis Paggi, 1996) de talla grande y eficientes filtradores, los que con su acción contribuyeron a la claridad del agua. La existencia de estas dos asociaciones zooplanctónicas está influidas sobre todo por la salinidad. Las dos lagunas subsalinas compartieron 10 especies entre ellas, que no fueron registradas en la más salada. Inversamente, ésta presentó una asociación de 4 especies halófilas, ninguna de las cuales se registró en las de menor salinidad.

La ecología de los cuerpos de agua está influenciada por los aportes de nutrientes desde la cuenca. Los ambientes lénticos de escasa profundidad generalmente presentan elevados niveles de eutrofia, lo que permite alojar una fauna zooplanctónica altamente productiva. Según el modelo de los estados alternativos de los lagos someros, el zooplancton de estos ambientes se caracteriza por una composición taxonómica, un espectro de tallas y por consiguiente una biomasa que dependen de la fauna íctica presente. A pesar de que en Argentina estos ambientes son abundantes y de su importancia dada por su productividad, diversidad e interés recreativo y turístico, sólo recientemente han comenzado a estudiarse considerando estas relaciones. Esta contribución tiene por objetivo dar información colectada durante 2006, mediante muestreos mensuales de agua y zooplancton, sobre los factores de importancia limnológica, su variación e influencia sobre la transparencia del agua y la abundancia y biomasa zooplanctónica de un lago somero urbano hipereutrófico de la provincia de La Pampa y comparar la situación registrada con un estudio similar realizado entre 1995 y 1996. La laguna se caracteriza por sus bajas salinidad y transparencia y por el descenso de estos parámetros entre los dos períodos estudiados. La concentración de nutrientes es mayor que la verificada en lagunas similares de la provincia de Buenos Aires. Si bien durante los dos períodos el número de especies fue el mismo, se verificaron cambios en la composición taxonómica, registrándose en 2006 menor número de cladóceros y mayor de rotíferos y la ausencia de especies de Daphnia, lo que puede contribuir al descenso de la transparencia verificada en este período. Las especies halladas fueron de talla pequeña, típico del zooplancton sometido a predación por peces. Entre ambos períodos se verificó un descenso en la abundancia de la comunidad zooplanctónica, sobre todo en cladóceros y rotíferos.

ABSTRACT. The limnology of saline water bodies at other latitudes is fairly well known, but in Argentina such studies have only recently begun. The applicability of many conclusions regarding the functioning of these environments around the world is limited due to the scant ecological knowledge of some endemic species recorded in the assemblages of Argentine lakes. The aims of this work were to determine the effects of salinity and inorganic turbidity on the taxonomic composition, abundance, and zooplankton biomass in a shallow, hypereutrophic, mesosaline lake in the north of La Pampa province characterized by seasonality, variations in level and salinity, and the lack of macrophytes and fishes, and to compare it with other shallow lakes of the province. We found important differences with other saline lakes: the species richness was lower; the mean abundance of zooplankton was between four and six times higher; and rotifers, which were not affected by salinity or the concentration of inorganic suspended solids, were numerically predominant. Crustaceans, on the other hand, were negatively affected by these environmental factors. Biomass was twofold higher than that recorded in the same period in two shallow lakes of Pampa, with similar nutrient concentrations but lower salinities. Keywords: saline lakes, inorganic turbidity, zooplankton biomass, Boeckella poopoensis, Argentina.Variación estacional e influencia de la turbidez y la salinidad sobre el zooplancton de un lago salino de la región central de Argentina RESUMEN. La limnología de los cuerpos de agua salinos de otras latitudes es bastante conocida, pero en Argentina se ha comenzado a estudiar recientemente. Muchas conclusiones sobre el funcionamiento de estos ambientes a nivel mundial son de aplicación restringida debido a que las asociaciones registradas en los lagos argentinos tienen algunas especies endémicas, cuyo conocimiento ecológico es escaso. Los objetivos de este trabajo fueron determinar los efectos de la salinidad y la turbidez inorgánica sobre la composición taxonómica, abundancia y biomasa zooplanctónica en un lago somero mesosalino hipereutrófico del norte de La Pampa, caracterizado por su temporalidad, variaciones del nivel y salinidad, carencia de macrófitas y peces, y compararlo con otros lagos someros de la provincia. Se encontraron importantes diferencias con otros lagos salinos, la riqueza específica fue menor, la abundancia media del zooplancton fue entre 4 a 6 veces superior y el predominio numérico fue de los rotíferos, que no fueron afectados por la salinidad o la concentración de sólidos suspendidos inorgánicos, a diferencia de los crustáceos, que fueron afectados negativamente por estos factores ambientales. La biomasa fue dos veces más elevada que la registrada en el mismo período en dos lagos someros pampeanos con parecida concentración de nutrientes pero con salinidades menores.

ABSTRACT. Temporary water bodies can remain active as such for varying periods. However, they are reservoirs (as "egg banks") of species adapted to these special conditions. In central Argentina, there are numerous temporary lakes, which have only recently begun to be studied. The aim of this work was to describe the succession of changes in diversity, abundance and biomass of zooplankton as well as in the environment, over a period of eleven months, as from the filling of a temporary saline lake, and test the hypothesis that i) salinity affects negatively the richness and abundance but positively the biomass, and ii) due to changes in environmental conditions produced by advancing the hydroperiod, different species emerge from the egg bank at different times. At the beginning, when salinity was reduced and the concentration of chlorophyll-a was higher, we recorded the highest diversity, due mainly by less tolerance species. Later, as salinity increased, the macrophyta Ruppia cirrhosa developed and covered 90% of the surface of the lake, the concentration of chlorophyll-a decreased and the water transparency increased. The zooplankton richness decreased and the community was integrated only by halotolerant species. In the first two months, there was an increased replacement of species, indicated by the high value of Whittaker's beta diversity index (0.63), the density was high, with a predominance of microzooplankton, so the registered biomass was relatively reduced. Then, abundance decreased markedly, and was mostly dominated by macrozooplankton, so the biomass increased. When salinity exceeded 11 g L -1 , there was a predominance of the halophilic cladoceran Daphnia menucoensis, a species of ecological importance because it's high grazing pressure on phytoplankton. In this study, in addition to the modulating effect of salinity on the richness, abundance and zooplankton biomass, it was verified that the diversity present in the egg bank of temporary saline lakes is higher than that recorded in the water column at any time during hydrophases. Keywords: temporary lakes, saline lakes, Daphnia menucoensis, egg bank, central Argentina.Diversidad zooplanctónica y su relación con cambios ambientales luego del llenado de una laguna salina temporaria de la región semiárida de La Pampa, Argentina RESUMEN. Los cuerpos de agua temporarios pueden permanecer activos durante períodos variables, pero son reservorios ("bancos de huevos") de especies adaptadas a estas especiales condiciones. En el centro de Argentina existen numerosos lagos temporarios, que sólo recientemente han comenzado a ser estudiados. El objetivo de este trabajo fue describir la sucesión de cambios en la diversidad, abundancia y biomasa de zooplancton y en los parámetros ambientales, durante once meses, a partir del llenado de un lago salino temporario y probar las hipótesis de que i) la salinidad afecta negativamente la riqueza y abundancia pero positivamente a la biomasa y ii) debido a los cambios ambientales producidos al avanzar el hidroperíodo, difer...

Moina macrocopa is common in eutrophic water bodies. Two subspecies have been described: M. macrocopa macrocopa (Straus, 1820), whose original distribution was restricted to the Old World, and M. macrocopa americanaGoulden, 1968, recorded only in North America. In 1994, the species was found in Bolivia, and in 1997 it was found in the northeast of Argentina. As the specimens belonged to M. macrocopa macrocopa in both cases, the authors suggested that it may have been introduced from the Old World by anthropogenic means. The aim of this study was to assess the distribution of M. macrocopa in La Pampa (Argentina), and provide information on its population characteristics under natural conditions. The species was recorded in six out of more than 100 water bodies surveyed; the specimens belong to M. macrocopa macrocopa. Although this species is not considered halophilic, the water bodies where it was found present high salinity (5.7 to 21.8 g.L–1). All are shallow, temporary and hypertrophic due to the agricultural activities carried out in their basins. Densities were between 1 and 312.6 ind.L–1. Size varied between 0.6 and 1.8 mm and some specimens were larger than those of other places. The parthenogenetic fraction predominated and the number of eggs ranged between 2 and 23. No correlations were found between density, size, or number of eggs and the environmental parameters studied. The presence of M. macrocopa in Pampean lakes could be due to the introduction of fish from hatcheries in the province of Buenos Aires, where it is used as food. Considering that the invasive character ofM. macrocopa has been recognized and that this study found that it is tolerant to a wide range of environmental conditions, attention should be paid to a possible displacement of its native congeneric species, typical of saline and temporary water bodies in central Argentina.

No abstract

The zooplankton of lakes is controlled by biological and physico-chemical parameters. Among the former, predation by fish can determine the replacement of large-sized species by small-sized ones and among the latter, salinity exerts negative effects on richness and abundance. Since it has been suggested that saline lakes without fishes have higher zooplankton biomass than low salinity ones, the aim of this study was to determine the richness, abundance and biomass of zooplankton in two lakes with different salinity and test the hypothesis that in the presence of zooplanktivorous fishes and at equal concentrations of nutrients and chlorophyll-a, saline lakes have higher biomass than those with low salinity. The study was conducted in two shallow lakes of the Province of La Pampa (central Argentina): a subsaline lake and a hyposaline lake, which shared high concentrations of chlorophyll-a and total phosphorus, reduced transparency and presence of planktivorous fish. Zooplankton richness was different and higher in the subsaline lake, whereas abundance and total biomass were similar, even when the taxonomic groups were considered separately. It is suggested that the presence of a halotolerant planktivorous fish controlled the size of zooplankton due to the predation on larger species and prevented the development of higher biomass in the saline lake, which is an important difference from previously recorded situations. This study shows that, regardless of the differences in salinity, the top-down effect in the food chain may have been a factor that equalized the zooplankton biomass by allowing only the development of small species and highlights the possible importance of fish predation in determining chlorophyll-a concentrations and water transparency.