Allelopathic interactions between the macrophyte Egeria densa and plankton (alga, Scenedesmus acutus and cladocerans, Simocephalus spp.): a laboratory study
Authors: Espinosa-Rodríguez Cristian A. , Rivera-De la Parra Ligia , Martínez-Téllez Aurora , Gómez-Cabral Gisela C. , Sarma S.S.S. , Nandini Sarma 
Published: March 2016

<p>Allelopathic interactions between macrophytes and zooplankton are important to understand the plankton dynamics in shallow waterbodies. <em>Egeria densa</em> is a native, perennial, submerged macrophyte in the tropical and subtropical zones of South America. It has been introduced to Central and North America and is now common in many Mexican lakes. This macrophyte produces chemical substances that negatively affect some phytoplankton species. However, it is not clear how zooplankton species adapt different life history strategies in the chemical presence of this macrophyte. Here, we tested the direct and indirect effects of allelochemicals released by <em>E. densa</em> on the population growth of <em>Scenedesmus acutus</em> and on the demographic variables of three species of <em>Simocephalus</em>, <em>S. exspinosus</em>, <em>S. serrulatus</em> and <em>S. mixtus</em> (via alga exposed to the macrophyte allelochemicals). To quantify the effect of <em>E. densa</em> on <em>S. acutus</em> we set up four treatments: control, artificial <em>Egeria</em>, natural <em>Egeria</em> and allelochemicals from <em>Egeria</em>. To test the allelochemical effects on <em>Simocephalus</em> species, we compared four treatments: Control, indirect effect (using <em>S. acutus</em> grown on <em>Egeria</em>-allelochemicals), direct effect (using <em>Egeria</em>-conditioned medium) and together with both these kinds of direct and indirect effects. <em>Scenedesmus </em>had the highest cell density in the presence of allelochemicals from <em>Egeria</em>, followed by controls. The specific algal growth rate (µ) between control and allelochemicals treatment was not significant (P&lt;0.05). However, the µ of alga in the presence of artificial or natural <em>Egeria</em> was significantly lower than controls or in treatment involving allelochemicals. The age-specific survivorship of the three cladoceran species was longer in treatments containing <em>Egeria</em>-conditioned medium. Cladocerans receiving <em>Egeria</em> conditioned-medium and algae cultured on macrophyte-allelochemicals also had a longer survivorship. Daily fecundity of <em>S. serrulatus</em> increased after reaching mid-age while <em>S. expinosus </em>and <em>S. mixtus</em> showed continuous reproduction starting from the first week. In general, <em>Egeria</em>-allelochemicals enhanced the age-specific reproductive output for all the three cladoceran species. The average lifespan of the three <em>Simocephalus </em>varied from 17 to 46 days, depending on the cladoceran species and treatment. <em>S. serrulatus </em>had lower lifespan compared to other two cladoceran species. For the three species, lifespan significantly increased in treatments containing macrophyte-conditioned medium + algae grown on the plant-allelochemicals; also under these conditions, both gross and net reproductive rates were significantly enhanced. This stimulatory effect was also evident in generation time (about 50% higher). The rate of population increase ranged from 0.23 to 0.38 per day for the three tested <em>Simocephalus</em> species but there were no significant differences (P˃0.05) among treatments. Our results suggest that the biological activity as well as physical structure of <em>E. densa</em> had negative effects on <em>S. acutus</em> population growth but had stimulatory effects on the demography of <em>Simocephalus</em>. </p>