## 3 Results and Discussion
### Influence of Wetlands on Water Quality
#### 3.5.4 Total Phosphorus Assessment  
The spatial behavior of TP concentrations varied from year to year in the studied area. Four years of decrease in upstream/downstream concentrations are observed (2006, 2010, 2015, 2017), and seven years of increase in upstream/downstream concentrations (2005, 2007, 2009, 2013, 2014, 2018). It is also noted that TP values were higher during rainy seasons compared to the drier period (Figure 13).  
Figure 13: Spatial variation of the mean Total Phosphorus (mg/L) at the monitoring stations of the Pantanal (IA07, IA14, IA15, IA16, IA17).  
The spatial behavior of Total Phosphorus concentrations varied year to year in the study area. There are six years of decline in upstream/downstream concentrations (2006, 2008, 2010, 2011, 2015, 2017) and seven years of increase in upstream/downstream concentrations (2005, 2007, 2009, 2013, 2014, 2018). It is also noted that Total Phosphorus values were higher in rainy seasons compared to the drier period (Figure 13).  
In the study conducted by [PERSON] [12] in the wetland area of Riacho Fundo, Brasilia, Brazil, an average retention of 34% for TP between the inlet and outlet points analyzed by the authors was observed. After an extensive literature review on nutrient removal in wetlands, [PERSON] and [PERSON] [13] identified that 84% of the studied wetlands showed phosphorus retention, 10% indicated an increase in phosphorus, and in 6% of the studied areas, there was no difference between the input and output of wetlands. Studies in the literature associate wetlands with phosphorus retention mechanisms [48; 49]. These mechanisms combine biological, physical, and chemical factors such as absorption and release by vegetation, periphyton, and microorganisms; sorption and exchange reactions with soils and sediments; chemical precipitation in the water column; and sedimentation and transport [50; 51].  
In wetlands, phosphorus is removed from surface waters and transferred to sediments and plants. Phosphorus removal can be limited by the concentration of iron, aluminum, or calcium in sediments [47]. The variability in phosphorus concentrations found in the studied area may be associated with the processes of assimilation and release of P in aquatic plants in the Pantanal. According to [PERSON] and [PERSON] [52], the Pantanal is one of the Brazilian regions with the highest diversity of aquatic macrophytes due to the diversity of habitats in the Pantanal plain, with annual variations in water levels, temperature, and air humidity. Most of these species, for example, die in the dry season, releasing biomass rich in nutrients. Aquatic vegetation in wetlands plays a significant role in phosphorus assimilation, as it absorbs P directly from the water column. Due to rapid turnover, P storage is short-term in short-cycle emergent plants, such as macrophytes, and much of the P is released back into the water column after vegetative decomposition, providing short-term storage. Unlike macrophytes, woody tree formations in wetlands provide long-term storage, with absorption rates ranging from 1 to 15 kg.ha\({}^{-1}\).year\({}^{-1}\)[50]. Periphyton may play an important role in regulating P concentrations in the water column, as it assimilates organic and inorganic forms of P and can induce marked changes in pH and oxygen concentration in the water column and in the soil-water interface of flooding. These alterations can potentially influence P solubility by precipitating phosphorus along with CaCO\({}_{3}\)[50; 53]. Thus, quantifying P cycling in wetlands proves to be a major challenge in terms of measuring the many physicochemical forms of phosphorus, as well as the vast array of biological and chemical processes that control the absorption, storage, and release of this element in wetlands [54].