Eutrophication of turbid tropical reservoirs: Scenarios of evolution of the reservoir of Cointzio, Mexico
Introduction
In this century, pressure on freshwater resources has been increasing considerably, especially in the “tropical world” (Tundisi, 2003). Most lakes, rivers and wetlands are suffering from the input of sediments and nutrients such as nitrogen (N) and phosphorus (P). As a result, the eutrophication of epicontinental waters is progressively extending to tropical water bodies (Alcocer and Bernal-Brooks, 2010). Additionally, most tropical lakes and reservoirs are situated in developing countries, where there is a chronic lack of financial resources for the establishment of long term and consistent monitoring programs (Von Sperling and Sousa, 2007). As a consequence of limited data availability in tropical regions, global biogeochemical models are better constrained in temperate regions, and have greater predictive power in economically developed regions (Seitzinger et al., 2010).
Inappropriate disposal of domestic waste, untreated waste water, and increase of nutrient loads from domestic origin are typical issues of developing countries and newly industrialized countries. Primitive techniques of farm management, based on forest clearance and combustion of the remaining biomass also cause the influx of sediments and nutrients into natural freshwater tropical systems. In general, eutrophication is a type of water quality deterioration caused by excessive concentrations of nutrients. Phytoplankton can form blooms, generally accompanied by a decrease of water transparency and the appearance of oxygen depletion in the hypolimnion (Istvánovics, 2009). Therefore, eutrophication management has become an increasingly important issue in these countries, including Mexico. Numerical modeling is a good option to assess the complex interplay between physical and biogeochemical processes taking place in lakes and reservoirs (Chanudet et al., 2012). Moreover, the application of mathematical modeling is necessary in order to design management strategies, test functional hypotheses and simulate future states for a system in response to environmental alteration. An overview of several models of different levels of complexity is given by Jørgensen and Bendoricchio (2001) and Jørgensen (2010). In the hydrologic community, there have been some interesting discussions regarding the best way to use numerical models. According to Grayson and Blöschl (2000), a balance between model complexity and data availability must be found to optimize model performance. The data availability for the Cointzio reservoir is clearly not sufficient for a full three-dimensional model, and we therefore decided to use a one-dimensional model that resolves only the dynamics in the vertical dimension. We used a combination of two independent one-dimensional models: (i) the Simstrat model (Goudsmit et al., 2002) for simulating the physical processes in the lake and (ii) a slightly modified version of the BELAMO model (Kunz et al., 2011, Mieleitner and Reichert, 2006, Omlin et al., 2001a, Omlin et al., 2001b) for the simulation of the biogeochemical cycles.
In order to prevent or minimize the eutrophication of the water in the reservoir, questions that most often arise with respect to eutrophic conditions include:
- i)
Can eutrophication be controlled by limiting the inflows of key nutrients?
- ii)
By how much do we need to reduce nutrient inputs to mitigate eutrophication?
In this study, these two questions served as a guideline to assess the biogeochemical cycles taking place in the reservoir of Cointzio.
The biogeochemical functioning of the reservoir was simulated to define which factors controlled the algal development in the reservoir. After calibration on target year 2009 and validation with data of 2008, various numerical simulations related to climate changes and mitigation strategies were conducted with the objective of assessing the trophic state of the reservoir in the future. The different scenarios are presented and discussed. Some solutions of rehabilitation are proposed to restore eutrophication. In the Discussion section, results are put in a regional context, with a specific focus on the duration of mitigation plans to get significant reduction of eutrophication for similar reservoirs. The paper ends with some perspectives which could be applied to other similar reservoirs of the region or other tropical reservoirs.
Section snippets
Study site
The Cointzio reservoir (19.622°N, − 101.256°W) is located on the Trans-Mexican Volcanic Belt, at an altitude of 1920 m above sea level. The reservoir drains a volcanic watershed of 627 km2, where domestic waters are rejected without any treatment. It is an essential source for the domestic water supply (20%) of Morelia city (700,000 inhabitants) and for irrigation. It is also used to control flood for Morelia city (Fig. 1). The reservoir has a maximum depth that does not exceed 29 m. It has a
Results
In the following sections we first present results of the simulations with the Simstrat, which serve as input for BELAMO. The “Present scenario” is then discussed in order to evaluate the quality of the models before introducing scenarios for the future.
Biogeochemical analysis of the model for the Cointzio reservoir
The model calculates the DO concentration as a balance between source and sink terms due to surface aeration, depletion by sediment, consumption by respiration and nitrification, and production by photosynthesis. DO revealed to be a reliable indicator that integrates almost all the processes taken into account in the model. Based on the nutrient release and mineralization rates calculated from the model results, we can see that benthic anoxic mineralization (denitrification), benthic aerobic
Conclusions
The physical mixing model Simstrat, using a k–ε scheme, and the biogeochemical advection–diffusion–reaction model BELAMO were implemented for the study of the very turbid tropical reservoir of Cointzio in Michoacán, Mexico. This is the first application of a biogeochemical modeling approach in the Trans-Mexican Volcanic Belt — a region poorly studied despite important risks of eutrophication. Simstrat accurately reproduced water temperature profiles and pointed out the low to moderate density
Acknowledgments
This study was performed at the Laboratory of Transfers Hydrology and Environment (LTHE), Grenoble, France in the framework of the European Research project DESIRE (2007–2011) and the French ANR Research project STREAMS (2008–2010). The assistance of all the individuals participating in this program, financed by the ANR STREAMS project and the DESIRE European project, is gratefully acknowledged. Phuong Thuy Kim Doan is recipient of a fellowship from the Vietnamese government. We also thank
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