Santiago Andrés Echaniz

El zooplancton de un lago somero hipereutrófico de la región central de Argentina: cambios después de una década

The water bodies ecology is influenced by the contributions of nutrients from the basin. The shallow lentic ecosystems generally presents high levels of eutrophy, which allows to sustain a highly productive zooplanktonic fauna. According to the model of the alternative states of shallow lakes, zooplankton of these environments is characterized by a taxonomic composition, a spectrum of sizes and therefore a biomass that depends on the fishes present. Although in Argentina these ecosystems are abundant and of their importance given by their productivity, diversity and by their recreational and tourist interest, only recently they have begun to be studied. This contribution must by objective give information collected during 2006, by means of monthly samplings of water and zooplankton, on the factors of limnologic importance, their variation and influences on the water transparency and the abundance and zooplanktonic biomass of a shallow urban hipereutrophic lake of La Pampa province and to compare the situation registered with a similar study made between 1995 and 1996. The lagoon is characterized by low salinity and transparency and the reduction of these parameters between both periods studied. The nutrients concentration is greater than the verified in similar environments of Buenos Aires province. Although during both periods the species number was the same, it was verified changes in the taxonomic composition, registering in 2006 smaller number of cladocerans and greater of rotifers and the absence of Daphnia species, which can contribute to the reduction of the water transparency in this period. The found species were of small size, typical characteristic of zooplankton under fish predation. Between both periods a reduction in the abundance of the zooplankton community was verified, mainly between the cladocerans and rotifers.

Seasonal variation and influence of turbidity and salinity on the zooplankton of a saline lake in central Argentina

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 two- fold higher than that recorded in the same period in two shallow lakes of Pampa, with similar nutrient concentrations but lower salinities.

EL ZOOPLANCTON DE TRES LAGOS SOMEROS DE DIFERENTE SALINIDAD Y ESTADO TROFICO EN LA REGION SEMIARIDA PAMPEANA (ARGENTINA)

El Valle Argentino es una formacion medanosa ubicada en el centro de la provincia de La Pampa, en la region semiarida central de Argentina, en el que se encuentra un grupo de lagos someros, alimentados por lluvias, de distintas caracteristicas fisico-quimicas, sobre todo la salinidad, pero que comparten un elevado grado de eutrofia. A los efectos de integrar informacion sobre la composicion y variacion de las asociaciones zooplanctonicas 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 registro la presencia de peces. Su zooplancton se caracterizo por una alta diversidad (32-37 especies) y abundancia, habiendose verificado el predominio del microzooplancton, sobre todo de los rotiferos. En estas lagunas, el macrozooplancton estuvo representado por especies de crustaceos de talla pequena, poco vulnerables a las interferencias alimentarias provocadas por los filamentos de cianobacterias, pero cuya baja eficiencia de filtracion contribuyo a mantener el estado turbio. A pesar de su mayor concentracion de sales, la tercera laguna puede caracterizarse como clara. En esta la diversidad y abundancia fueron menores (13 especies), y la ausencia de peces depredadores permitio el predominio del macrozooplancton, integrado por especies de copepodos (Boeckella poopoensis Marsh, 1906) y cladoceros (Daphnia menucoensis Paggi, 1996) de talla grande y eficientes filtradores, los que con su accion contribuyeron a la claridad del agua. La existencia de estas dos asociaciones zooplanctonicas esta influidas sobre todo por la salinidad. Las dos lagunas subsalinas compartieron 10 especies entre ellas, que no fueron registradas en la mas salada. Inversamente, esta presento una asociacion de 4 especies halofilas, ninguna de las cuales se registro en las de menor salinidad.

Zooplankton richness, abundance and biomass of two hypertrophic shallow lakes with different salinity in central Argentina

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.

Distribution and biological aspects of the introduced species Moina macrocopa (Straus, 1820) (Crustacea, Cladocera) in the semi-arid central region of Argentina

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.

Biology of Boeckella poopoensis Marsh, 1906 (Copepoda, Calanoida) in natural conditions in temporary saline lakes of the central Argentina

Boeckella poopoensis Marsh, 1906 is the dominant copepod in saline lakes from northern Patagonia to southern Peru. It is a tolerant species, which has been registered at salinities between 20 and 90 g.L-1, and is important because it integrates the diet of flamingos and fishes of commercial and sport interest. The aims of this study were to analyze the characteristics of populations of B. poopoensis in the central region of Argentina and to establish their relationships with environmental parameters. Monthly samples during 2007 were taken in four temporary lakes of La Pampa province. Environmental parameters and density, size, biomass, number and size of eggs were determined. The salinity ranged between 13.38 and 32.90 g.L-1. In three lakes that had previously continuously contained water, B. poopoensis was registered throughout the whole study. In the fourth lake, which was filled in January, it was present only when salinity exceeded 15 g.L-1. The population of the lake that was filled differed from that of the other lakes in terms of the density and biomass of adults and copepodites. The number of ovigerous females represented a higher percentage of the population during the colonization of the lake that had been dried and these produced the highest number of eggs. In the three lakes in which B. poopoensis was always recorded, its characteristics were more influenced by the availability of food than by temperature or salinity. It was found that the strategies of the species vary throughout the hydroperiod; at the beginning, thrives when the salinity rises and impedes the presence of less tolerant species. At this point, the production of relatively small eggs is high, allowing rapid colonization. When the lakes become relatively stable, B. poopoensis allocates more energy to reach larger sizes and although egg production is not so high, they are larger, allowing it to maintain stable populations.