Allelopathic interactions between the macrophyte Egeria densa and plankton (alga, Scenedesmus acutus and cladocerans, Simocephalus spp.): a laboratory study

Allelopathic interactions between macrophytes and zooplankton are important to understand the plankton dynamics in shallow waterbodies. Egeria densa 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 E. densa on the population growth of Scenedesmus acutus and on the demographic variables of three species of Simocephalus, S. exspinosus, S. serrulatus and S. mixtus (via alga exposed to the macrophyte allelochemicals). To quantify the effect of E. densa on S. acutus we set up four treatments: control, artificial Egeria, natural Egeria and allelochemicals from Egeria. To test the allelochemical effects on Simocephalus species, we compared four treatments: Control, indirect effect (using S. acutus grown on Egeria-allelochemicals), direct effect (using Egeria-conditioned medium) and together with both these kinds of direct and indirect effects. Scenedesmus had the highest cell density in the presence of allelochemicals from Egeria, followed by controls. The specific algal growth rate (µ) between control and allelochemicals treatment was not significant (P<0.05). However, the µ of alga in the presence of artificial or natural Egeria was significantly lower than controls or in treatment involving allelochemicals. The age-specific survivorship of the three cladoceran species was longer in treatments containing Egeria-conditioned medium. Cladocerans receiving Egeria conditioned-medium and algae cultured on macrophyte-allelochemicals also had a longer survivorship. Daily fecundity of S. serrulatus increased after reaching mid-age while S. expinosus and S. mixtus showed continuous reproduction starting from the first week. In general, Egeria-allelochemicals enhanced the age-specific reproductive output for all the three cladoceran species. The average lifespan of the three Simocephalus varied from 17 to 46 days, depending on the cladoceran species and treatment. S. serrulatus 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 Simocephalus 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 E. densa had negative effects on S. acutus population growth but had stimulatory effects on the demography of Simocephalus.