# Effects of Multiple Stressors on the Spatial Pattern of Fish Diversity in the Middle and Lower Reaches of the Han River, China

## TL;DR

This study examines how natural and human factors affect fish diversity in the Han River, China, revealing that natural factors mainly drive α-diversity while human stressors influence β-diversity indirectly.

## Contribution

The study identifies distinct roles of natural and human stressors in shaping fish diversity patterns in human-impacted rivers.

## Key findings

- Fish communities in the Han River were grouped into three distinct regions based on cluster and NMDS analysis.
- Natural factors like water level and river width were key drivers of α-diversity, while water quality factors influenced β-diversity.
- Human stressors affected β-diversity primarily through indirect pathways, as shown by structural equation modeling.

## Abstract

Understanding the relationship between the environment and biodiversity is crucial for biodiversity conservation. However, fish diversity in rivers is affected by many factors, such as the natural environment and human activities. Therefore, we selected the middle and lower reaches of the Han River in China—an area significantly impacted by human activities—as the research region to identify the main factors affecting fish diversity and how these factors worked.

Human activities have altered rivers worldwide, but how their combined effects shape fish assemblages remains unclear. We therefore surveyed fish and habitats seasonally along the middle and lower reaches of the Han River, China, during 2022, specifically in June–August (wet season) and October–November (dry season). This study analyzed the spatial distribution pattern of fish diversity, explored the effects of natural factors (e.g., hydrology, water quality) and human stressors (e.g., dam, land use) on the spatial pattern of fish diversity, and identified the key driving factors. Cluster analysis and Non-metric Multidimensional Scaling (NMDS) showed that the fish communities could be divided into three groups: the Danjiangkou reservoir area (DRA), the middle reaches (MR), and the lower reaches (LR). For α-diversity, the LR had the highest value, followed by the DRA, with the MR being the lowest. For β-diversity, the MR had the highest value, followed by the LR, with the DRA being the lowest. Random Forest model showed that fish diversity was mainly affected by natural factors; among these factors, the key drivers of α-diversity were hydrological factors such as the water level (3.56–5.97%) and river width (4.53–4.69%), while for β-diversity, the key drivers were water quality factors, including the dissolved oxygen (10.08–12.36%), total nitrogen (6.49–9.31%), and chlorophyll a (8.26–8.40%). Structural Equation Modeling further revealed that natural factors affected β-diversity mainly through direct pathways, while human stressors affected β-diversity mainly through indirect pathways. The results revealed the differential roles of natural factors and human stressors in driving the patterns of fish α-diversity and β-diversity in human-disturbed rivers, which will provide a scientific basis for the conservation of fish diversity in the Han River.