Garças Reservoir trophic state dynamics: a 20-year synthesis

Abstract

(Garças Reservoir trophic state dynamics: a 20-year synthesis). The Garças Reservoir history reveals a remarkable case of human impacts resulting from the urbanization pressure over the 20th century. It is a shallow tropical system located in the Parque Estadual das Fontes do Ipiranga, a protected area within the city of São Paulo, São Paulo State, Brazil. Based on a comprehensive dataset (1997-2017), we evaluated the water quality temporal change using a trophic state index (TSI); in addition, we also verified whether the management procedures improved the water quality. Mechanical removal of macrophytes (1999) triggered an abrupt change from eutrophic-supereutrophic to the hypereutrophic state with feedback mechanisms. After five years (2005), a slowdown in the internal P loading process occurred, followed by the installation of the secondary wastewater treatment in the city zoo two years later (2007). Both processes promoted the beginning of water quality improvement. In 2011, TSI first dropped to supereutrophic state with the temporary diversion of the sewage effluent from the São Paulo State Department of Agriculture. This effluent was finally inactivated (2014) and the system sustained the supereutrophic state. TSI responded well to the historical key events related to the control of sewage effluents. After 11 years from the permanent impact of macrophytes removal, current results have demonstrated improved health of the system. It is absolutely necessary to improve the sewage treatment from the city zoo to continue the reservoir recovery considering that since 2014 it has been the only external phosphorous loading to Garças Reservoir. The history of Garças Reservoir can offer a better understanding of wrong management strategies and the vulnerability of tropical shallow systems to eutrophication.

Evaluating the periphyton as a bioreactor for removal of nutrients in a shallow hypereutrophic reservoir

“…Our findings showed a difference in the dissolved and total nutrient concentrations between sampling sites, showing an N and P availability gradient from the discharge of effluent from the secondary sewage treatment system. Comparing to long-term studies in the current reservoir (Crossetti et al, 2019;Bicudo et al, 2020), dissolved nutrient concentrations were extremely high in the inlet effluent, demonstrating the high nutrient load that discharged into the reservoir. We observed that total and dissolved nutrient concentrations were substantially reduced inside and after the macrophyte stand (55-76%) when compared to sampling sites with treated sewage.…”

“…33, e10 which is located São Paulo, Brazil (Figure 1). It is a shallow urban reservoir (Zmax = 4.7 m; Zmed = 2.1 m) and classified as hypereutrophic (Bicudo et al, 2020). The reservoir has a surface area of 88,156 m 2 , a volume of 188,785 m 3 , and a mean theoretical residence time of 71 days.…”

“…About 20 years ago, the macrophyte coverage reached 40-70% of the water surface causing mosquitoes' proliferation (Bicudo et al, 2007). To mitigate this problem, macrophytes were removed mechanically and management actions were initiated (Bicudo et al, 2020). Currently, macrophytes are trapped by a structure made of bamboo and metal mesh.…”

Periphyton responses to enrichment and nutrient dilution in two mesocosm experiments in a shallow hypereutrophic reservoir

“…The reservoir has an area of 88 156 m 2 , a volume of 188 785 m 3 , an average depth of 2.1 meters and a maximum depth of 4.7 meters. Despite the improvement of reservoirs in water quality in recent years, it is classified as hypereutrophic (Bicudo et al, 2020). Both experiments were carried out during the winter, which is characterized by mild temperatures and low rainfall.…”

Epipelon biomass responses to different restoration techniques in a eutrophic environment

“…This reservoir is shallow, super-eutrophic, polymictic, and discontinuous, with periods of thermal strati cation of days or weeks. Its average depth is 2.1 m, with a maximum of 4.7 m, an area of 88,156 m², a volume of 188,785 m³ and an average residence time of 71 days (Bicudo et al 2020). This reservoir was selected due to its eutrophic condition and frequent phytoplankton blooms, mainly cyanobacteria.…”