Nutrient pollution in two lakes of the Haihe Basin in China: causes, effects and future trends

Summary

Lakes and their drainage basins provide important ecosystem services and natural resources to support human development. However, many rivers and lakes have become polluted and eutrophied. This is causing environmental and socio-economic problems globally, as well as in China.

Haihe Basin is one of the largest river basins of China, located in North China. There are many human activities in the Haihe Basin because it is the politic-economic center of China. However, the water systems in Haihe Basin are polluted and water shortage is a serious issue. There are more than 1,000 reservoirs and natural lakes in Haihe Basin. In this thesis, I choose Guanting Reservoir (major water resource of Beijing) and Lake Baiyangdian (rapidly urbanizing) as case studies. The Guanting and Baiyangdian basins together host over 24 million people.

The increasing nutrient exports from land to the Haihe lakes and reservoirs can be attributed to rapid urbanization and agricultural production. However, there are important knowledge gaps about these nutrient flows. First, there is a lack of information on the seasonal and annual variability in these nutrient flows. Second, the knowledge about future trends in river export of nutrients is limited. Third, there is a lack of studies that estimate the critical nutrient loads of the lakes in Haihe Basin. Finally, there is a lack of studies exploring possible pathways to reduce nutrient pollution in these lakes.

Basin-scale nutrient export models are useful tools to understand the causes of nutrient exports from land to water systems, and to explore effective solutions. The MARINA family models can quantify flows of different forms of nutrients from land to coastal water areas by source and by sub-basin for current and future years. However, these models have not been applied to the sub-basins of lakes in China while accounting for seasonal variation. Another group of models are aquatic ecosystem models (such as PCLake+), that can help to estimate critical nutrient loads. Critical nutrient loads reflect the amount of nutrients that the aquatic ecosystem can absorb. Linking basin-scale nutrient export models and aquatic ecosystem models for nitrogen and phosphorus can provide new insights in how to reduce external nutrient loads so that critical nutrient loads in lakes or reservoirs are not exceeded.

The main objective of this research is, therefore, to improve our understanding of nutrient flows from land to the two lakes in the Haihe Basin, and possibilities to reduce this pollution in the future. To this end, I formulated four sub-objectives on the causes and future trends of nutrient pollution from land to the lakes (sub-objectives 1, 2, 3) and the effects of nutrient pollution on the lakes (sub-objective 4).

Sub-objectives focusing on the causes and future trends are as follows:

1. To better understand the seasonal trends in river export of dissolved inorganic nitrogen (DIN) to the two lakes of the Haihe Basin (Chapter 2). 
2. To analyze future trends in nutrient inputs into Guanting Reservoir and Lake Baiyangdian in the Haihe Basin (Chapter 3).
3. To explore options to reduce nutrient losses from livestock production to Lake Baiyangdian from 2012 to 2050, taking urbanization into account (Chapter 4).

The sub-objective focusing on the effect is as follows:

1. To compare nutrient loads for current (2012) and future years (2050) with critical nutrient loads of Lake Baiyangdian, and to discuss the possibilities for nutrient management in sub-basins of Lake Baiyangdian (Chapter 5).

Chapters 2, 3 and 4 focus on the causes and future trends of nutrient exports to Guanting Reservoir and Lake Baiyangdian. I started with developing a seasonal version of the MARINA-Lakes (Model to Assess River Inputs of Nutrients to lAkes) model which accounts for the seasonality in human activities (e.g., cropping systems, fertilizer practices), climate and hydrology (Chapter 2). I analyzed the seasonal patterns of river export of dissolved inorganic nitrogen (DIN) to two lakes. Next, I analyzed future trends in nutrient exports to Guanting Reservoir and Lake Baiyangdian by applying the MARINA-Lakes model (Chapter 3) between 2012 and 2050. In Chapter 4, I applied the MARINA-Lakes model to explore options to reduce nutrient losses from livestock production to Lake Baiyangdian from 2012 to 2050. Chapter 5 focuses on the effects of nitrogen and phosphorus loads on the aquatic ecosystem. I compared current and future nutrient loads with the critical nutrient loads of Lake Baiyangdian, and then discussed the possibilities for nutrient management in sub-basins of Lake Baiyangdian (Chapter 5).

In Chapter 6, I discussed the main findings of this thesis, and drew four main lessons. These lessons are 1) Downscaling annual inputs of nutrients to seasonal inputs supports a better understanding of temporal trends in river exports of nutrients to lakes, 2) Scenario analysis is a useful tool to explore mitigation solutions for river export of nutrients to lakes associated with future urbanization, 3) Using an indicator for reallocating manure across areas may be useful in future modeling exercises, 4) Linking two different models (MARINA-Lakes and PCLake+) for the same study area helps to better understand N and P pollution in lakes. In this chapter, I also formulated recommendations for future research and nutrient management. Future research could further improve nutrient modelling by accounting for the locations of point sources and spatial-temporal changes of critical nutrient loads. Additionally, I recommend to use the model approaches of this thesis to identify the key periods, areas and sources of nutrient pollution for comprehensive nutrient management in lake basins. My linked model approach can be used as a starting point to explore nutrient management strategies for sub-basins aiming to protect lake ecosystem health.