in a shallow eutrophic lake (San Miguel del Monte, Argentina). Duplicate samples of zooplankton were obtained from three stations with different limnological characteristics. The density of zooplankton community was dominated by rotifers and small cladocerans related to the trophic status of this shallow lake. The zooplankton biomass showed similar values to other eutrophic shallow lakes. The annual biomass distribution was bimodal (winter and summer peaks). The contribution of crustaceans, mainly copepods, determined the total biomass. The total zooplankton biomass was significantly correlated with chlorophyll «a» in the sampling station located at the deep part of this shallow lake. There were factors and processes promoting changes in the zooplankton biomass, such as food availability, grazing ability, fish predation, eutrophication process with alternation of turbid and clear water periods, and also the addition of organisms from the Salado River during floods.
Los ríos de llanura tienen como característica particular presentar una interrelación con su valle de inundación y con los ambientes lénticos asociados a él (Amoros & Bornette, 2002; Vázquez et al., 2008). durante las inundaciones grandes, cantidades de sedimento son desplazados por los ríos y en esos sedimentos existe una gran variedad de estadios de resistencia. Pequeñas inun
Zooplankton play a key role in aquatic ecosystems constituting an essential intermediate component in aquatic food webs. Cladocerans in particular are widely studied as model of generalist filter feeders. At the regional level, the cladoceran Daphnia spinulata is found in numerous habitats with wide conductivity range (160 μS cm −1 to 14.000 μS cm −1 ), but information about the species's tolerance range or life history is scarce. Our objective was to generate essential information about D. spinulata's life history and the main parameters related to two contrasting salinity levels representative of water bodies of the pampean plain.Results obtained in laboratory bioassays showed that individuals at low conductivity (700 μS cm −1 ) had more moults, a lower mortality and a higher life expectancy at birth, a higher generation time, and an earlier peak density than those at high-conductivity treatments (5000 μS cm −1 ). The maximum density reached was similar for both conductivities indicating a similar growth capability attained by different strategies. Moreover, the results indicated that D. spinulata at different conductivities was able to reproduce, with the offspring thus contributing to population growth. This study provided a knowledge of the life history strategies of this endemic cladoceran at two different conductivities, thus generating crucial information on the population dynamics and for evaluating the species's role as a possible salinity indicator in pampean water bodies. Nevertheless, future investigations will be necessary in order to identify the absolute limits of tolerance and the optimum conductivity for this species and determinate which stressors will act as a mictic stimuli. ResumenEl zooplancton cumple un papel esencial en los ecosistemas acuáticos, constituyendo un eslabón intermedio en las tramas tróficas acuáticas. Los cladóceros, en particular, son ampliamente estudiados como modelos de filtradores generalistas. A nivel regional, el cladócero Daphnia spinulata es encontrado en numerosos ambientes con un amplio rango de conductividad (desde 160 μS cm −1 hasta 14.000 μS cm −1 ), pero la información en relación a sus rangos de tolerancia o historia de vida, es muy escasa. Nuestro objetivo fue
In the present work, we provide the first approach about the life-history of Brachionus plicatilis in South America. We tested with laboratory experiments the response of the pampean strain of B. plicatilis for two of its main stressors (conductivity and temperature). We evaluated the effects of eight conductivity values from 1 to 17 mS.cm-1 and two temperatures (15 and 25 °C) to compare its abundance with those obtained in the pampean lotic and lentic environments, where this rotifer is frequent or dominant. The results demonstrated that the increase in population-growth rate and the peak of abundance occurred at the highest temperature and at medium conductivity. Minimum values were obtained at the lowest temperature and conductivities analyzed, but the final density attained was nevertheless similar to those recorded in the pampean environments at the optimum conductivity and during the spring and summer seasons. Males, mictic females, and resting eggs were observed at the minimum and maximum conductivities, revealing the strategy of this species for maintaining dominance in environments with fluctuating salinity. The experiments also indicated the possible behavior of this relevant member of the zooplankton community within a scenario of increasing temperature and salinity related to the climate changes occurring in the pampean region.
Polyphenols -products of organic-matter decomposition entering water bodies from autochthonous and especially allochthonous sources -affect primary producers, bacterioplankton, and zooplankton to consequently modify food webs. Cladocerans are widely used in research experiments because they constitute the most frequent prey of high-trophic-level organisms in the majority of lakes, and certain species symbolise the ecologic prototype of the generalist filter feeder. In our study area's shallow lakes, cladocerans, though generally of low abundance, do attain significantly high biomasses. We accordingly evaluated the mortality of the cladoceran Daphnia menucoensis Paggi, 1996 exposed to different tannin concentrations along with the bacterial abundance. In two experiments D. menucoensis females exposed to high tannin concentrations reproduced inefficiently, exhibited high mortality, and manifested altered behaviour, such as reduced reflexes and diminished mobility. Though tannins apparently affected algal abundances, the cladoceran herbivory strongly reduced those numbers. Positive effects on bacteria, however, were not recorded. High-allochthonous-organic-mattercontaining water bodies support trophic webs because bacterioplankton provide an alternative energetic base for the zooplankton, as possibly occurs in shallow lakes of the SaladoRiver basin, characterised by cyclic hydrologic periods alternating between draught and flooding and intensive land use. In conclusion, tannins diminish D. menucoensis's survival, locomotion, and the capacity to respond to stimuli. ARTICLE HISTORY
Los bañados de desborde fluvial (BDF) desempeñan un papel integral en la ecología de las cuencas fluviales y en su capacidad de autodepuración. La demanda antrópica de espacio y agua pone en riesgo su biota y los beneficios ecosistémicos que brindan. Los objetivos del trabajo fueron analizar los ensambles de microorganismos que habitan en BDF, relacionar sus variaciones al impacto antrópico y evaluar cómo la microbiota puede ser indicadora de estos cambios ambientales. Se estudiaron los ensambles de diatomeas bentónicas, fitoplancton, zooplancton, ostrácodos y hongos de cuatro BDF: dos de ubicación periurbana con uso del suelo agrícola y hortícola (Del Gato y Carnaval), y dos de ubicación rural y uso ganadero (Cajaravillas y Chubichaminí). En la mayoría de los grupos las diferencias vinculadas a la ubicación geográfica se expresaron claramente, resultando en una agrupación de ensambles en: BDF rurales (con un número de especies sensibles a la contaminación y a la eutrofización comparativamente mayor) y BDF periurbanos (donde fueron más importantes las especies muy tolerantes a la contaminación y a la eutrofización). En estos últimos, tanto la participación de los grandes grupos taxonómicos como los niveles tróficos estuvieron más restringidos y relacionados a la vía detritívora por sobre la fotosintética, y allí también se acentuaron las estrategias para afrontar el estrés hídrico. Los resultados evidencian los efectos de la presión humana, la importancia que adquieren los microorganismos y la necesidad de una planificación adecuada del uso del territorio para evitar la pérdida de funciones y beneficios en estos ambientes.
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