## 1 Introduction  
The urban water environment is an important part of the terrestrial hydrosphere, which is closely related to human activities, and is a fundamental material basis for developing industrial and agricultural production and maintaining the life of urban residents [1, 2, 3]. Because of its unique location, the urban water environment integrates natural and human landscapes, serves as a major leisure area, and contributes to the economic development of cities. However, human impact on urban water environment is becoming increasingly severe. In the past two decades, eutrophication has occurred in urban water bodies in several countries to varying degrees, severely affecting the quality and function of urban water environments [4, 5, 6]. Eutrophication in urban water bodies in China is also relatively severe. Rapid urbanization, coupled with intensive industrial and agricultural activities in urban areas, causes excessive nitrogen (N) loading into urban rivers, lakes, and other small inner circulating water bodies in cities [7, 8, 9, 10]. The eutrophication of urban water bodies has become an urgent problem that urban water environment protection should solve. Sedimentary organic matter contains rich biological, physical, and chemical information. It is not only the central link of nutrient circulation in water bodies but also the main accumulation bank of nutrients [11, 12, 13]. Moreover, it can directly reflect the pollution history of water bodies and release various pollutants upward to the overlying water bodies under certain conditions, which is a major secondary pollution source affecting water quality [14, 15]. Consequently, sedimentary organic matter preserves key information such as the original productivity and change process of water nutrition and can be used as an effective carrier to reconstruct the changes in water environmental productivity and evolution of material sources [16, 17]. In general, the sedimentary organic matter sources can be categorized into internal and external sources, among which internal sources are mainly biological residues of water bodies and external sources are mainly granular and dissolved organic matter carried in the recharge process of external water sources [18, 19, 20]. Under the influence of human activities, nutrient salts are rapidly imported into urban water bodies, resulting in ecological environment changes such as water eutrophication, food chain structure changes, and seasonal hypoxia of bottom materials, which leave records in sediments. Variation of biogenic elements, especially carbon (C) and nitrogen (N), and their isotopes in sediments is an effective indicator of water productivity, nutrient level, and material source [21]. The C/N ratio and stable isotopes of C and N in sediments are often used as indicators of changes in the source or cycle of sedimentary organic matter in aquatic systems over time because different sources of sedimentary organic matter have different C/N ratios and stable isotopic compositions [22, 23, 24]. For example, the C/N ratios of marine and terrestrial organic matter are usually 5-8 and \(>\)15, respectively. Under the influence of human activities, river sediments often show higher C/N ratios than marine sediments. Organic carbon isotope (\(\delta^{13}\)C) characteristics and C/N ratios are commonly used to assess the source of organic matter in ecosystems, while nitrogen isotope (\(\delta^{15}\)N) characteristics are mainly used to assess the anthropogenic emissions and nutrient structures of aquatic organisms [25, 26, 27]. However, previous studies have mostly focused on natural waters, such as large coastal bays, rivers, and lakes, and only few studies have focused on small artificial water bodies in medium-sized cities. Based on the aforementioned factors, this study selected four typical urban water body in Shaoxing City, a medium-sized city of east China, as the research object. As a Chinese city with a long history of thousands of years, Shaoxing is well known for its water environment [9, 10].