An Integrated Analysis of the Eutrophication Process in the Enxoé Reservoir within the DPSIR Framework

Tiago Ramos,Ramiro Neves,Manuela Morais,David Brito,Maria Gonçalves,Hanaa Darouich,Manuel Fernandes,Fernando Pires,José Martins,Maria Castelo Branco

doi:10.3390/w10111576

Abstract

The Enxoé reservoir in southern Portugal has been exhibiting the highest trophic state in the country since its early years of operation. The problem has attracted water managers’ and researchers’ attention as the reservoir is the water supply for two municipalities. Extensive research was thus conducted over the last few years, including field monitoring and modelling at the plot, catchment, and reservoir scales. This study now frames all partial findings within the Driver-Pressure-State-Impact-Response (DPSIR) framework to better understand the eutrophication process in the Enxoé reservoir. Agriculture and grazing were found to have a reduced role in the eutrophication of the reservoir, with annual sediment and nutrient loads being comparably smaller or similar to those reported for other Mediterranean catchments. Flash floods were the main mechanism for transporting particle elements to the reservoir, being in some cases able to carry up three times the average annual load. However, the main eutrophication mechanisms in the reservoir were P release from deposited sediment under anoxic conditions and the process of internal recycling of organic matter and nutrients. Reducing the P load from the catchment and deposited sediment could lead to a mesotrophic state level in the reservoir. However, this level would only be sustainable by limiting the P internal load ability to reach the photic zone.

Highlights

Agriculture is commonly pointed out as the major contributor for surface water eutrophication, with inefficient practices resulting in high nutrient surpluses ( phosphorus (P) and nitrogen (N)) that are transferred to water bodies through diffuse processes, promoting algae blooms, oxygen depletion and biodiversity loss
This study presented an integrated analysis of the eutrophication process in the Enxoé River and reservoir, with the DPSIR framework providing a common context for research carried out in the region over the last few years
The drivers provided the identification of the major causes affecting water quality in the Enxoé reservoir

Summary

Introduction

Agriculture is commonly pointed out as the major contributor for surface water eutrophication, with inefficient practices resulting in high nutrient surpluses ( phosphorus (P) and nitrogen (N)) that are transferred to water bodies through diffuse processes (runoff and leaching), promoting algae blooms, oxygen depletion and biodiversity loss. In this matter, P is usually considered the single most limiting nutrient for phytoplankton growth, namely cyanobacteria. Lake reservoirs are vulnerable to eutrophication, with increasing nutrient loading from upstream catchments promoting the development of primary producers (phytoplankton, cyanobacteria, aquatic plants), conditions of hypoxia and anoxia and biodiversity loss. The low water velocities registered in reservoirs, their high residence time and the existence of thermal stratification help promoting the two main processes that drive N and P biogeochemical cycles: the primary production and the settling of particulate matter [14,15,16]

Objectives

Methods

Findings

Conclusion