Effluents from carcinoculture and its effects on the Lagoa dos Patos estuarine system
Aquaculture is the fastest growing sector of food of animal origin in the world. Shrimp farming, as part of this sector, has generated several benefits in socio-economic terms, but it has also generated special attention to the environmental aspects involved with the activity. The captive production of Pacific white shrimp Litopenaeus vannamei has been increasingly consolidated around the estuary of Lagoa dos Patos, in southern Brazil. This estuary has ecological, industrial, port, agricultural and fisheries importance, which makes it necessary to understand the potential polluting impact of the activity in this region. In this study, parameters of water quality, composition and abundance of phytoplankton, protozooplankton,mesozooplankton and macrozoobenthos were evaluated space-time in the body of water that receives effluents from a shrimp farm of the species Litopenaeus vannamei located on the banks of the Lagoa dos Patos estuary. The levels of total organic carbon (TOC), total nitrogen (NT), copper (Cu) and zinc (Zn) in the sediment of the cultivation nursery, in the sedimentation basin and in the effluent receiving estuary were also evaluated. Cu and Zn concentrations of shrimp tissue cultivated at the end of the production cycle were also evaluated. The samples were taken before the effluent discharge (AD) and; 1 day (1 PD), 5 days (5 PD), 10 days (10 PD), 20 days (20 PD), 30 days (30 PD), 60 days (60 PD) and 90 days (90 PD) after discharge of effluents.It is worth mentioning that each study had its own schedule and thus not all of the analyzes mentioned above had collections on all the dates mentioned. Specifically for the evaluation of macrozoobenthos, a second production cycle was followed for sampling. The collection points in the estuary were distributed from the mouth of the effluent discharge channel to a distance of 250 m from it. Data on temperature, dissolved oxygen, pH, salinity, chlorophyll a, total ammoniacal nitrogen (NAT), nitrite, nitrate, total phosphorus, total suspended solids and water turbidity were measured. In addition to the temperature and salinity that undergo seasonal variations in estuarine environments, NAT,chlorophyll a and turbidity were the water quality parameters that underwent the most marked changes as a result of the discharge of effluents. However, in general, these changes were restricted to a distance of up to 20 m from the effluent discharge channel for a maximum period of 5 days. Diatoms, cyanobacteria, chlorophytes and ciliate were identified and quantified. The predominant community was that of chlorophytes, followed by diatoms, cyanobacteria and ciliate. There was an increase in the concentration of the different groups on the first post-discharge day (1 PD). However, this increase may be related to the enrichment of nutrients in estuarine waters caused by abundant rains, and not exactly the discharge of effluents.Meso-zooplankton abundance was low and represented exclusively by copepods, the vast majority of the species Acartia tonsa. The only exception was the 30 PD sampling, which suffered a significant increase, without however being related to the abiotic parameters evaluated. Although significant differences (p <0.05) were observed in some samples, they do not appear to have any relationship with the discharge of effluents. Seven macrozoobenthic groups were observed (Polychaeta, Tanaidacea, Isopoda, Gastropoda, Bivalvia, Malacostraca and Ostracoda) over two production cycles (2012 and 2013). The density and richness of species suffered little variability along the sampling points of the estuary for both stages. Over time,in 2012 it is possible to observe an increase of these indices in winter compared to summer. Throughout the 2013 collections, the density of the macrobenthes had an opposite pattern and the species richness suffered little variability. The TOC concentration ranged from 0.12 to 0.67% and the NT concentration was below the equipment's detection limit (<0.1 ppm) in all sampling sites and campaigns. The levels of Cu and Zn in the sediment were quantified through weak extraction (fraction of labile or potentially bioavailable metals) and semi-strong extraction (fraction of metals most strongly adsorbed). The Cu in the labile fraction varied from 0.12 to 1.27 μg / g and Zn varied from 0.52 to 3 μg / g, while the most strongly adsorbed fraction of Cu varied from 0.3 to 2.65 μg / ge and Zn of 30.44 to 121.4 μg / g. Significant differences (ANOVA, p <0,05) were observed in the TOC results between some sample points in the 1PD and 10PD campaigns, without however having any relation with the discharge of effluents. Pearson's correlation analysis showed no relationship between the discharge of effluents and an increase in the TOC, NT, Cu or Zn values in the sediment, except for the relationship found between the TOC and Cu and Zn values in the sediment on the first day after discharge (1 PD). At the end of the cycle, specimens of shrimp collected from two fattening ponds revealed average concentrations of 6.63 ± 0.2 μg / g of Cu and 19.76 ± 0.2 μg / g of Zn in nursery 1, and 7, 6 ± 0.51 μg / g of Cu and 19.13 ± 0.32 μg / g of Zn in the nursery 2. Although some water quality parameters have changed as a result of effluent release, these effects were acute and punctual,restricted to a distance of up to 20m from the margin and observed up to 5 days after effluent disposal. In addition, the values were within the range required by the current Environmental Resolution. Phytoplankton, protozooplankton, mesozooplankton and macrozoobenthos suffered only variations due to the natural oscillations of the abiotic parameters that occur in estuarine systems. Finally, the levels of COT, NT, Cu and Zn in the sediment and Cu and Zn in the shrimp tissue were within the maximum values stipulated in the legislation, showing that the discharge of effluents from shrimp production around the Lagoa dos Patos estuary did not bring adverse effects to the environment.