Efeitos da técnica de coagulação-sedimentação para mitigação de florações de cianobactérias sobre as comunidades zooplanctônica e zoobentônica do Reservatório do Funil (RJ, Brasil)

Abstract

Eutrophication is the major water quality problems in aquatic ecosystems and results in harmful cyanobacteria blooms. Controlling cyanobacterial blooms is crucial to control the availability of phosphorus (P), reducing its external and internal inputs. One technique to control the internal P that has been successfully used in laboratory and field experiments is the combined use of metal coagulants and solid-phase P adsorbents through flocculation and sedimentation (Floc&Lock), which removes the dissolved (phosphate) and particulate (cyanobacteria) P-fraction from the water column and prevents P-release from sediment to water column. However, mitigation studies should be associated with assessments of side effects to non-target organisms to clarify possible interference at other trophic levels. The objective of this study is to identify and evaluate the possible unintended side effects of the Floc&Lock technique on zooplanktonic and zoobenthic communities of a reservoir. A mesocosms experiment was carried out to test different coagulant (Poly-Aluminium Chloride – PAC) and adsorbent combinations of P in solid phase (Lanthanum modified Bentonite clay). The zooplankton and zoobenthic density were evaluated at the beginning and at the end of the experiment. There was no great variation in P-dissolved values, what may indicate that the phosphorus was in its particulated form. Besides this, limnological variables were also monitored through the experiment. A change in pH was observed soon after the application of the technique, but its values returned to values close to the initial ones after seven days. Cyanobacterial biomass was reduced, but there was no variation on dissolved P values, what may indicate that the phosphorus was in a particulate form. Total phosphorus was significatively reduced in both treatments. Dissolved inorganic nitrogen was not reduced, but total nitrogen suffered a reduction seven days after the experiment begun, it could have happened due to the remotion of the nitrogen that was incorporated by the phytoplankton and zooplankton biomass in the water column. Rotifers and copepods codominance was observed in the zooplankton community, with cladocerans also present before the treatment’s application and, during the experiment, it was seen a dominance of copepods and reduction of the rotifers’ density. A reduction of the zooplanktonic community was observed, nearly 78% in PAC+LMB and 91% in PAC+LMB+RS, when comparing the initial time (Ti) with a day after the application of the technique (T1). Rotifers, copepods and cladocerans had significatively differences among control and both treatments samples. Besides this reduction at the zooplanktonic density, the three groups were present at the end of the experiment. Regarding the zoobenthic community, there were found only three Tubificidae and six Chironomidae family organisms were found in Ti an T28, respectively. The results showed that there was a density reduction of zooplankton organisms after the technique was applied, with a subsequent recovery of this community. The results were not able to indicate if there were an effect on the natural zoobenthic community from the reservoir.