Predicting zebrafish spatial avoidance triggered by discharges of dairy wastewater: An experimental approach based on self-purification in a model river.

Wastewater discharges from dairy industries can cause a range of harmful effects in aquatic ecosystems, including a decline in biodiversity due to species evasion. Therefore, it is important to know the purification potential of rivers for the removal of pollutants released in dairy wastewater (DWW). The hypothesis adopted in this work was that the release of DWW into stretches of the Ribeirão dos Pombos River (São Paulo State, Brazil) might trigger an avoidance response, resulting in fish migrating to other regions, with the response being greater when the self-cleaning potential of the river is smaller. Therefore, the goals of the present study were to: (i) investigate how land use and seasonality of the rainfall regime influence the quality of the water in different areas of the river (P1: river source; P2: urban region; P3: rural region); (ii) assess the potential of the river to purify DWW; and (iii) evaluate the potential toxicity and repellency of DWW to the freshwater fish Danio rerio, using acute toxicity (mortality) and non-forced avoidance tests, respectively. P1 was shown to be the most preserved area. The chemical composition of the river varied seasonally, with higher concentrations of Cl- and SO42- at P3 during the rainy period. The river purification potential for DWW was higher at P2, due to greater microbiological activity (associated with higher BOD). The DWW was more acutely toxic in water from P2. The avoidance response was strongly determined by the concentration of DWW, especially for water from P2. The high capacity for self-cleaning at P2 did not seem sufficient to maintain the stability of the ecosystem. Finally, the non-forced exposure system proved to be a suitable approach that can assist in predicting how contaminants may affect the spatial distributions of organisms.

Long-term performance and acute toxicity assessment of scaled-up air-cathode microbial fuel cell fed by dairy wastewater.

Long-term performance of a scaled-up air-cathode microbial fuel cell (MFC) and toxicity removal were studied with dairy wastewater (DW) used as the substrate. The MFC in a semi-continuous flow was strategically inoculated with consortium of Shewanella oneidensis and Clostridium butyricum. The scaled-up approach delivered a maximum power density of 0.48 W/m3 (internal resistance of 73 Ω) removing 93% of total chemical oxygen demand and 95% of total biochemical oxygen demand at organic loading rate (OLR) of 0.9 kg COD/m3/d and hydraulic retention time (HRT) of 21 days. It also achieved high removal efficiency of nitrate (100%), organic nitrogen (57%), sulfate (90%) and organic phosphorus (90%). The power generation and DW degradation performance decreased with OLR of 1.8 kg COD/m3/d and HRT of 10.5 days. Furthermore, testing of acute toxicity with the microcrustacean, Daphnia similis, revealed high toxic effect of the raw DW, but no toxic effects of the MFC effluent during 95 days of operation. These outcomes demonstrated that scaled-up MFC fed with high-strength DW should be an effective system for pollutants removal and simultaneously energy recovery.

Influence of interspecific interactions on avoidance response to contamination.

An increasing number of studies have shown the ability of organisms to escape from toxic effects due to contamination, by moving spatially towards less contaminated habitats. However, this issue has been investigated in monospecific scenarios, without considering possible interactions between species during the contamination avoidance process. It is widely known that the spatial distribution of one species can be affected by another one, in different ways. Therefore, the main question addressed in the present study was as follows: Might interspecific interaction between the freshwater fish Danio rerio (zebrafish) and Poecilia reticulata (guppy) change their behavior patterns in terms of avoidance in the presence of a copper gradient? Zebrafish and guppies exposed to a copper gradient were tested for avoidance responses in a free-choice, non-forced, static, multi-compartmented exposure system, using two distinct approaches: (1) monospecific tests, in which only one species was exposed to the copper gradient, at two different population densities; and (2) multispecific tests, in which both species were tested simultaneously. In the control (with no copper) monospecific tests, both species were randomly distributed; however, in the control multispecific test, P. reticulata tended to aggregate. In the monospecific tests with a copper gradient, both species avoided copper in a similar way, with AC50 (concentration triggering avoidance in 50% of the exposed population) values between 15 and 18 µg·L-1, irrespective of the population density. However, in the multispecific tests, P. reticulata displaced D. rerio to previously avoided copper levels, consequently increasing the AC50 of D. rerio to 75 µg·L-1. This study shows the importance of understanding the interactions among species in contaminated areas, and the way that one species can prevent the avoidance behavior of another.