Behavioural responses of freshwater zooplankton vary according to the different alarm signals of their invertebrate predators

Single behavioural interactions between two freshwater planktonic crustaceans and invertebrate predators from different zones of the common environment were studied. The planktonic prey organisms were the cladoceran Ceriodaphnia dubia and the copepod Notodiaptomus conifer. The three invertebrate predators were the shrimp Macrobrachium borellii, the crab Trichodactylus borellianus and the larva of the dipteran midge Chaoborus. Feeding experiments were first performed to document the effects of exposure time and prey density on predation. All the selected predators fed on C. dubia and N. conifer, and predation rates were dependent on prey density and exposure time. The ability of microcrustaceans to modify their swimming and avoidance behaviour when faced with cues from each predator was then analysed. The cladoceran was more vulnerable to decapod predation, whereas the prey microcrustaceans were taken equally by the dipteran larvae. The analysis showed that the microcrustaceans detected the presence of at least one invertebrate predator through the predator alarm signals. The cladoceran responded to chemical signals from the three predators but copepods modified their behaviour only in the presence of infochemicals of M. borellii. The different outcomes suggest that macrocrustaceans have different vulnerabilities when faced with the same predator and so have evolved specific and different strategies to reduce invertebrate predation.Fil: Gutierrez, Marìa Florencia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; ArgentinaFil: Rojas Molina, Florencia Mercedes. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; ArgentinaFil: de Azevedo Carvalho, Debora. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; Argentin

Spatial variability of chlorophyll-a and abiotic variables in a river–floodplain system during different hydrological phases

Chlorophyll-a (Chl-a) and abiotic variables were measured in the main channel and floodplain waterbodies of the Middle Paraná River to analyse the system dynamics and to assess their spatial variability during different hydrological phases, including an extreme flood. We wanted to test that the flood does not always have a homogenising effect in a river-floodplain system. An explanatory model for Chl-a was performed according to Akaike?s Information Criterion (AIC), and the relation of water level with the coefficient of variation (CV) among sites for each variable was explored. The model explained 64% of Chl-a variability. Water level, depth:euphotic zone ratio (Zd:Zeu) (inverse correlation) and conductivity (direct correlation) were the significant explicative variables. The CV of Chl-a decreased with flood from the main channel to the floodplain, but for turbidity, Zd:Zeu, pH, dissolved oxygen, soluble reactive phosphorus and Chl-a:pheophytin-a ratio, it increased. However, within the floodplain, CV of turbidity, Zd:Zeu and pH decreased during flood. These suggest that the homogenising effect frequently observed during inundation cannot be generalised and that the floodplain may maintain its identity even during flood. The extreme flood and its overlap with the warm season and sedimentological pulse probably contributed to the heterogenity in the spatial gradient.Fil: Mayora, Gisela Paola. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; ArgentinaFil: Devercelli, Melina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; ArgentinaFil: Giri, Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; Argentin

Feeding selectivity of the invasive mussel Limnoperna fortunei (Dunker, 1857) on a natural phytoplankton assemblage: what really matters?

The aims of this work were to analyse the feeding selectivity of L. fortunei in a natural assemblage of phytoplankton in a short-term microcosm experiment and to assess whether this selectivity is affected by the presence of Rotifera as a secondary, palatable feeding resource. This bivalve preferred Desmidiales, Chlorococcales, Euglenophyceae and Chrysophyceae algae with a maximum linear dimension from 20 to 100 µm. Organisms between 500 and 40 × 103 µm3 belonging to Desmidiales, Chrysophyceae and Euglenophyceae were also positively selected. Volvocales, Cryptophyceae and one group of medium-size Euglenophyceae (Trachelomonas sp.) had a high, negative selectivity index independent of their cell shape or size (Ivlev’s index of feeding selectivity  0.8). We concluded that the impact on phytoplankton community structure could be severe, considering that the presence of zooplankton does not have an effect on the majority of phytoplankton groups and that the mussel tends to feed on both items to improve its diet. The negative selection of some phytoplankton taxa is possibly related to the morpho-physiological characteristics of their cell shells.Fil: Frau, Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; ArgentinaFil: Rojas Molina, Florencia Mercedes. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; Argentina. Universidad Nacional del Litoral. Facultad de Bioquímica y Ciencias Biológicas; ArgentinaFil: Mayora, Gisela Paola. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; Argentin