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Body size is a master trait controlling biological communities and ecosystem functioning. Mean population size not only depends on the size of individuals, but also on the size distribution of individuals within the population. Mean community size is additionally influenced by the composition of species (larger- or smaller-sized species). Shallow lakes within semi-arid landscapes are prone to experience large changes in temperature and salinity, which affect the zooplankton size structure. Higher temperatures are expected to result in smaller average body size, while the effect of salinity appears to depend on the range under study. Here we analyze zooplankton body size patterns across shallow lakes from the semi-arid central region of Argentina. All community size descriptors point to decreasing size and a narrow size range at higher temperatures. On the other hand, the maximum average community body size occurred at intermediate (~30 gL−1) salinity levels. The combined effect of both variables resulted in a bell-shaped pattern, with maximum community body size toward lower temperatures and intermediate salinities. Based on future temperature scenarios, one may anticipate an overall decrease in community body size. But such prediction is strongly conditioned by regional and local trends in salinity. [ABSTRACT FROM AUTHOR]

Dormancy is an essential component of the life cycle for populations that disappear from the plankton during winter. The hatching of resting eggs is the main mechanism for restoring active population components. Here, we analyzed the hatching dynamics of zooplankton from sediments of a shallow lake as affected by temperature. Sediment samples collected every other month were incubated at in situ temperatures and at 20°C. The highest number of individual's hatchings was observed for copepod nauplii, the cladoceran Bosmina sp., and the rotifer Brachionus havanaensis. Temperature showed a positive significant effect on the hatchlings of B. havanaensis. Other zooplankton species were less affected by temperature, and there was no relationship between their emergence from the incubated sediments and occurrence in the water column. We also tested the effect of short-term (6 and 12 weeks) and long-term (a year) storage duration on hatching success. The hatching success of B. havanaensis was not affected by the storage time. In contrast, the hatchlings of Bosmina sp. and nauplii increased after short storage time. The evidence collected here suggests that many zooplankters were not affected by temperature, while for B. havanaensis temperature turned out to be a master variable that links planktonic and passive phases. [ABSTRACT FROM AUTHOR]

The Central Plains of Argentina is a heterogeneous environment, but the lakes there share some fundamental features: they are all shallow and polymictic as being well exposed to wind. First, we provide a synthesis of the climate, geology, and hydrological network. We also discussed shallow lakes origin and their limnological and biological salient features. Second, we focus on Pampean shallow lakes from a global perspective, comparing the limnological variables: total phosphorus concentration (TP), total nitrogen concentration (TN), chlorophyll a (Chl a) concentrations, and Secchi disk reading (SD) from a compiled database. No significant differences in the Chl a vs. TP relationship were found between Pampean and other shallow lakes. Otherwise, the chlorophyll yield per unit of phosphorus of Pampean lakes is similar to the world shallow lakes average. Moreover, the relationship SD vs. Chl a differed significantly between Pampean and the remaining world lakes, about 50-60%. When confronted against other lakes worldwide, Pampean shallow lakes depart from most of them as having higher TP, TN, and Chl a concentrations and much lower SD transparency, and therefore they stand as extremes of the trophic-state continuum. Despite their highly turbid state, these lakes provide valuable ecosystem services that are highly appreciated and mobilize important economic resources. [ABSTRACT FROM AUTHOR]

The effects of external physical forcing variables (solar radiation and winds) on short-term dynamics of total suspended solids (TSS), Chlorophyll- a (Chl- a), turbidity levels, and water transparency were studied during 15 days in a highly turbid, shallow lake (Laguna Chascomús, Argentina). Water samples were taken three times per day (8, 14 and 20 h.). Solar radiation and wind velocity showed a repeatedly bell-shaped diurnal pattern, with significant higher values during morning and afternoon, respectively. TSS and turbidity displayed a general decreasing trend during the sampling period, while water transparency showed the opposite trend. Also Chl- a displayed a decreasing trend and was closely correlated to TSS levels. We assayed a first-order kinetics model to detrend the series, obtaining the rate of change during the night, morning, and afternoon. We observed higher values on afternoon compared to morning for TSS, Chl- a, and turbidity levels and the opposite pattern for water transparency. We conclude that this pattern may result from a combination of biological activity, as it took place after a period of intense photosynthetic activity, together with resuspension by winds during the afternoon, (windiest time of the day). [ABSTRACT FROM AUTHOR]

The relationship between the timing of recurrent biological events and seasonal climatic patterns (i.e., phenology) is a crucial ecological process. Changes in phenology are increasingly linked to global climate change. However, current evidence of phenological responses to recent climate change is subjected to substantial regional and seasonal biases. Most available evidence on climate-driven phenological changes comes from Northern Hemisphere (NH) ecosystems and typically involves increases in spring and summer temperatures, which translate into earlier onsets of spring population developments. In the Argentine Pampa region, warming has occurred at a much slower pace than in the NH, and trends are mostly restricted to increases in the minimum temperatures. We used zooplankton abundance data from Lake Chascomús (recorded every two weeks from 2005 to 2015) to evaluate potential changes in phenology. We adopted a sequential screening approach to identify taxa displaying phenological trends and evaluated whether such trends could be associated to observe long-term changes in water temperature. Two zooplankton species displayed significant later shifts in phenology metrics (end date of Brachionus havanaensis seasonal distribution: 31 day/decade, onset and end dates of Keratella americana seasonal distribution: 59 day/decade and 82 day/decade, respectively). The timing of the observed shift in B. havanaensis phenology was coincident with a warming trend in the May lake water temperature (4.7°C per decade). Analysis of abundance versus temperature patterns from six additional shallow Pampean lakes, and evaluation of previous experimental results, provided further evidence that the lake water warming trend in May was responsible for the delayed decline of B. havanaensis populations in autumn. This study is the first report of freshwater zooplankton phenology changes in the Southern Hemisphere (SH)., (© 2018 John Wiley & Sons Ltd.)

In turbid Pampean lakes, incident solar radiation is a major driver of plankton seasonal dynamics. Higher light availability in summer translates into higher primary production, and therefore more food for zooplankton grazers. However, experimental evidence suggests that food produced under the high irradiance conditions prevailing in summer are less suitable to sustain rotifer population growth than that produced under the lower irradiance conditions typical of winter. Here, we analysed time series datasets corresponding to three shallow lakes from the Salado river watershed. This analysis provided evidence for similar seasonal patterns of rotifer relative abundance over a large geographic area. In addition, we performed life table experiments to test the hypothesis that natural seston produced in winter could sustain higher population growth rates than seston produced in summer. We suggest that the natural seasonal changes in temperature and food generate successive time windows, which may be capitalized by the different grazer species, resulting in predictable phenology of grazer populations.