Effect of salinity and food concentration on competition between Brachionus plicatilis Mu ¨ ller, 1786 and Brachionus calyciflorus Pallas, 1776 (Rotifera) N. S. Ferrando A,C , S. Nandini B , M. C. Claps A and S. S. S. Sarma B A Instituto de Limnologı ´a Dr Rau ´ l A. Ringuelet (Consejo Nacional de Investigaciones Cientı ´ficas y Te ´cnicas, Universidad Nacional de La Plata), Boulevard 120 y 62, 1900 La Plata, Provincia de Buenos Aires, Argentina. B Laboratorio de Zoologı ´a Acua ´tica, Edificio Unidad de Ciencias Morfolo ´ gicas Universidad Nacional Auto ´ noma de Me ´ xico, Campus Iztacala, Avenida de los Barrios 1, Los Reyes, Estado de Me ´xico CP 54090, Me ´xico. C Corresponding author. Email: nferrando@ilpla.edu.ar Abstract. Natural populations of planktonic rotifers are affected by salinity and food density, among other stressors. Moreover, competition among congeneric species limits the abundance of certain rotifers in ecosystems without spatial heterogeneity and environmental fluctuations. We isolated Brachionus plicatilis and Brachionus calyciflorus from Salado River basin waterbodies and studied the demographic characteristics and competition between them at three salinity levels (0.75, 1.75 and 2.75 g L À1 NaCl) and at two food concentrations (0.1 Â 10 6 and 0.5 Â 10 6 cells mL À1 Chlorella vulgaris). The lowest salinity level proved to be unfavourable for the population growth of B. plicatilis, but at higher salinity levels the growth was similar to that of controls (without competition), even in the presence of the competitor and at either food concentration. By contrast, the competitor almost always decreased the density of B. calyciflorus. Salinity greatly affected the peak density of B. plicatilis, whereas the effects of salinity and competition on B. calyciflorus were similar. Biomass was inversely related to salinity for both species, and was significantly affected by competition in B. plicatilis. These results show that B. calyciflorus is more adversely affected than B. plicatilis by competition and high salinity, and explain why both species can coexist in eutrophic saline lowland rivers and pampean shallow lakes. Additional keywords: food density, growth rates, plankton, rotifers. Received 19 October 2018, accepted 21 May 2019, published online 20 August 2019 Introduction The population dynamics of rotifers, essential components of freshwater ecosystems and the pelagic food chain (Sommer et al. 2001; Gliwicz 2002), are influenced by abiotic and biotic con- ditions such as temperature, salinity, food quality, competition and predation (Dumont and Sarma 1995; Rothhaupt 1995; Kumar and Rao 2001; Kumar 2003). Rotifers are more diverse in freshwater than in saline environments (Sarma et al. 2002), although species of Synchaeta, Hexarthra, Filinia and Brachio- nus are commonly found in inland saline waterbodies (Fontaneto et al. 2006). In such ecosystems, the diversity and abundance of rotifers are controlled primarily by physical and chemical chan- ges in the environment (Walsh et al. 2008; Kaya et al. 2010). The Salado River basin located in the Buenos Aires Province in Argentina represents the second largest South American wetland in terms of rainfall accumulation and includes a large number of shallow lakes occupying 10 000 km 2 under normal conditions of river flow that are characterised by different degrees of connectivity to the river. Agriculture and cattle rearing are the principal economic activities in the catchment, contributing to a high percentage of national gross output (Gabellone et al. 2013). In order to maintain the hydrometric levels and lower salinity values, some of these lakes have sluices because, without regulation, the lakes would dry up completely in the summer (Gabellone et al. 2005). The chemical composi- tion of these pampean aquatic environments is related to the lithology of the sub-basins: the chemistry is primarily domi- nated by anions, such as chloride, sulfate, or bicarbonate, and the cation sodium (Miretzky et al. 2000; Gabellone et al. 2008). Salinity is one of the most influential controlling parameters in pampean waterbodies (Claps et al. 2009). In recent years, the basin has become more saline as a result of the mobilisation of large amounts of stored salts in the groundwater of the endorheic areas by artificial diversion to the main channel of the Salado River (Bazzuri et al. 2018). Investigations performed in lotic environments have revealed that a decrease in species richness of zooplankton along with a spatiotemporal predominance of the rotifer Brachionus plicatilis (Gabellone et al. 2014) occurs CSIRO PUBLISHING Marine and Freshwater Research https://doi.org/10.1071/MF18403 Journal compilation Ó CSIRO 2019 www.publish.csiro.au/journals/mfr