Effects of polyaluminium chloride (PAC) addition and benthivorous fish removal on the benthic macroinvertebrates assemblages of a tropical shallow eutrophic lake
Floculation; sedimentation; phosphorus imobilization; biomanipulation; bioturbation
The application of aluminum salts and the removal of benthivorous fish are two techniques of water quality management of eutrophic lakes that tend to reduce the internal contribution of phosphorus from the sediment to the water column and to mitigate the effects of eutrophication. However, the isolated and combined effects of these two management techniques on the structure and dynamics of aquatic communities are poorly understood. The application of aluminum polychloride (PAC) increases the flow of organic matter from the water column to the sediment, increasing the availability of detritus to the benthic invertebrates, while the removal of benthivorous fish reduces the pressure of competition and / or predation on these invertebrates. Therefore, the increase in the availability of detritus should have a stronger positive effect on the benthic macroinvertebrates in the absence than in the presence of benthivorous fish and for this reason we must observe a synergistic effect of the two techniques on the density and diversity of these invertebrates. In order to test this hypothesis, a field experiment with a 2 x 2 factorial design was carried out, combining the application or not of PAC with the presence and absence of the benthic fish Prochilodus brevis, a very common and abundant benthivorous species in the Brazilian semiarid reservoirs. The experiment was carried out in 20 mesocosmos of 6 m3 inserted in a semi-arid tropical lake and the 4 treatments were randomly allocated in the mesocosmos. Water and sediment samples were collected in each mesocosm at the beginning, middle and end of the experiment, which lasted 8 weeks. The benthic macroinvertebrates were identified and quantified in each sample and the density data of these organisms were treated with a two-way ANOVA. None of the two water quality management techniques nor the interaction between them had any significant effect on the density, richness and diversity of benthic macroinvertebrates. Therefore, the results found rejected the hypothesis of this research and suggest that the high availability of organic matter in the sediment, typical of eutrophic environments, cancels any beneficial effect for the benthic macroinvertebrates of the increase of the vertical flow of organic matter, induced by the application of the PAC, or the removal of detritus fish and other potential competitors