## 3 Results and Discussion
### Influence of Wetlands on Water Quality
#### 3.5.2 Dissolved Oxygen and Biochemical Oxygen Demand Assessment  
When comparing DO at the inlet and outlet of the Pantanal region, a downward trend is observed in most of the years studied. In the majority of the years studied, the DO values were above 5.0 mg/L, the minimum limit for Class 2 (Figure 10). It was also observed that DO values are higher during the driest times of the year.  
The BOD concentration showed an increase in relation to the inlet and outlet of the MRW wetland for 6 years and a decrease for 4 years. These values correlate with DO, where a decrease in concentration was observed in the Pantanal wetland. Due to the low population density in the Pantanal region, the influence of sewage discharge is not relevant; however, other types of anthropogenic interference cannot be excluded (Figure 11).  
Figure 10: Spatial variation of the average dissolved oxygen (mg/L) at the monitoring stations in the Pantanal (IA07, IA14, IA15, IA16, IA17).  
The reduction in DO and the increase in BOD in the wetlands of the Pantanal may be related to the natural phenomenon known as \"dequada\" or \"decoada\", as mentioned before [32]. [PERSON] [29] highlights that wetlands have large amounts of stored organic matter, about 50% of which is carbon. Carbon associated with other nutrients such as phosphorus and mainly nitrogen can reduce the amount of dissolved oxygen due to its demand in biochemical reactions involving these elements.  
In a study conducted by [PERSON] and [PERSON] [41] in the wetlands existing in the urban area of St. Bernard County, Louisiana, United States, the authors found low DO values, ranging from 3.0 mg/L to 1.8 mg/L. [PERSON] et al. [42] also observed levels considered low of DO, from 1.37 to 5.37 mg/L in two ways of the Paraguay River in Caceres (MT). During the \"dequada\" in the Paraguay River in Caceres/MT, [43] found that the average dissolved oxygen declined from 6.37 mg/L (October 2018) to about 3.0 mg/L (April 2019), returning to normal in the following months.  
[PERSON] et al. [44] conducted an analysis of \"dequada\" events along the Paraguay River and its tributaries. The authors found that in the region from the confluence of the Miranda and Aquidauna rivers to the mouth with the Paraguay River (the area studied in this work), there is a reduction in DO and an increase in the concentration of carbon dioxide released during the decomposition of organic matter causing \"dequada\" events, considered of medium to high magnitude. Thus, it is relatively common to find low values of DO in wetlands due to the low speed of the water, which reduces natural aeration in these areas. Especially in tropical areas with an abundance of herbaceous vegetation, DO levels can drop to 2.0 mg/L during the decomposition of organic matter, as floodwaters advance in the floodplain area [41; 44].  
The higher concentrations of BOD and the reduction in DO in the studied area may be associated with the decomposition of dead natural organic matter, resulting from seasonal flooding cycles. [PERSON] et al. [44] further add that the magnitude of these events (intensity  
Figure 11: Spatial variation of the measurement of biochemical oxygen demand BOD (mg/L) at the monitoring stations in the Pantanal (IA07, IA14, IA15, IA16, IA17).  
and duration) varies from year to year, occurring from December to May in the tributaries, and from February to June in the main channel of the Paraguay River.