Lake Caohai (26°47′32″–26°52′52″) is the largest semi-enclosed urban lake in Guizhou Province, China (Dong et al., 2023; Yang X. et al., 2024). The region experiences distinct wet and dry periods, with rainfall concentrated in spring and summer, and dry conditions prevailing in autumn and winter (Cao et al., 2016; Hu et al., 2021). The lake’s primary water source is precipitation (Hu et al., 2021). Its water is relatively stagnant, with a hydraulic residence time of 85.6 days (Cao et al., 2016). On average, strong winds occur on 31 days per year, mostly between January and April. The mean wind speed is 3.2 m/s, with maximum speeds reaching up to 20.7 m/s. Compared with other inland lakes, Lake Caohai is more sensitive to nitrogen levels (Li et al., 2024).

Three sediment cores were collected from Lake Caohai using a gravity corer in November 2023. Sediment samples were dried using a Techconp FD-3-85 MP vacuum freeze drier. Total nitrogen (TN) was measured using the Kjeldahl method (Ni and Wang, 2015), while total phosphorus (TP) was analyzed using the molybdenum blue method after acid digestion (Liu et al., 2025).

Water sample were collected from the surface layer using a plexiglass sampler. Water temperature and pH were measured with a YSI multi-parameter water quality analysis system. TN was analyzed using the persulfate digestion-UV spectrophotometric method (Zhang et al., 2025). Ammonium was determined by salicylate method (Zhang et al., 2025), while TP was measured using the ammonium molybdate spectrophotometry (Zhang et al., 2025).

The concentrations of TN and TP in lake sediments can partially reflect the degree of eutrophication in lake waters (Ni and Wang, 2015; Wu et al., 2018; Lin et al., 2019). In general, the more eutrophic a lake is, the higher the levels of TN and TP in its sediments (Ni and Wang, 2015; Wu et al., 2018; Lin et al., 2019). Increased nutrient inputs accelerate the accumulation of TN and TP in lake sediments (Ni and Wang, 2015; Wu et al., 2018; Lin et al., 2019). In eutrophic Lake Caohai, a highly significant positive correlation was observed between TN and TP (R² = 0.61–0.89, p < 0.05). Sediment-bound TN and TP tended to increase toward the sediment–water interface (Figures 2a,b). Sediment-bound TN, in the upper sediment layers, were at or above 4.8 g kg⁻¹ (Figure 2a), the elevated sediment-bound TN in Lake Caohai may pose risks to water quality and threaten the survival of sensitive aquatic organisms. Compared with other eutrophic lakes—such as Dianchi Lake (Wu et al., 2018), Erhai Lake (Ni and Wang, 2015), Yangcheng Lake (Su et al., 2022), Honghu Lake (Liu et al., 2025) and Hangzhou West Lake (Lin et al., 2019), and West Lake in Hangzhou (Lin et al., 2019)—sediment-bound TN in Lake Caohai is exceptionally high, whereas sediment-bound TP is relatively low. This indicates that the nitrogen load of Lake Caohai is sensitive to the anthropogenic activities.

In eutrophic Lake Caohai, government agencies and relevant departments have implemented extensive measures to reduce external nutrient loading (Yan et al., 2023). The abundance of TN and TP in overlying water were consistently lower than those in sediment pore water (Yu et al., 2022). The total exchangeable nitrogen constitutes the main components of TN in lake sediment (Wu et al., 2019), the lake sediment-bound nitrogen and phosphorus, during the dry period, shows the release characteristics of “source” (Han et al., 2017; Yu et al., 2022; Zheng et al., 2023), sediment-bound nitrogen and phosphorus release can exacerbate internal loading to some extent (Yan et al., 2023). Moreover, the release of sediment-bound nitrogen and phosphorus is considerably increased with rising pH under weakly to strongly alkaline conditions (Liu et al., 2020; Zhao et al., 2022). The slightly higher water pH in Lake Caohai is associated with the presence of carbonate and bicarbonate ions originating from abundant calcite and dolomite (Hu et al., 2021). Phytoplankton photosynthesis in surface waters reduces bicarbonate concentrations, thereby increasing water alkalinity (Hu et al., 2021; Li et al., 2024). The alkalinity of the water environment, limited aquatic plants (Dong et al., 2023), and high-intensity wind disturbance (Tong et al., 2019) in Lake Caohai facilitate the release of nutrients from sediments, thereby increasing waterborne nitrogen and phosphorus concentrations.

The concentrations of nitrogen and phosphorus compounds in lake waters can partially reflect their sources (Tong et al., 2019). Ammonium concentrations are lowest, nitrate concentrations are highest, and nitrification activity peaks at intermediate temperatures (15 °C–25 °C) (Barkow et al., 2020). The relatively lower water temperatures during the dry period (Figure 3a) inhibit the activity of nitrifying bacteria. Meanwhile, strong winds in the dry period (Figure 2c) promote the release of ammonium from sediments (Ding et al., 2018; Tong et al., 2019). Additionally, the die-off of large numbers of aquatic plants during the dry period leads to decomposition, which further increases waterborne nitrogen and phosphorus concentrations (Yang R. et al., 2024). These results partially explain why waterborne ammonium concentrations were slightly higher during the dry period (Figure 4b). Continuous heavy precipitation in the wet period (Figure 2d) facilitates the transference of accumulated nitrogen and phosphorus from soils into the water column via soil erosion, leading to marked increases in waterborne TN and TP (Tong et al., 2019; Su et al., 2022). Both waterborne TN (Figure 4a) and NH₄⁺ (Figure 4b) were considerably higher in the wet period compared with the dry period. Similar seasonal patterns of elevated waterborne nitrogen have also been observed in Yangcheng Lake (Su et al., 2022), Yilong Lake (Yang R. et al., 2024), and Dianchi Lake (Shen et al., 2024).

Elevated ammonium levels are typically associated with human and animal excreta and domestic sewage (Liu et al., 2025), whereas nitrate enrichment is associated with strong nitrification in lake water (Cao et al., 2016). Observed ammonium concentrations (Figures 3c, 4b) exceeded the recommended freshwater limit of 0.025 mg/L (Tang et al., 2015), while nitrate (Figures 3d, 4c) remained within the permissible limits of 11 mg/L (Mortada and Shokeir, 2018). Waterborne TN and TP concentrations exceeded the eutrophication thresholds (TN: 0.2 mg/L; TP: 0.02 mg/L), indicating that nutrient may exert a limiting effect on algal growth (Xu et al., 2014). When water-bound TN and TP exceed 0.80 and 0.20 mg/L, respectively, algal blooms are no longer limited by nitrogen and phosphorus concentrations (Xu et al., 2014). The rising levels of TN, NH₄⁺, and NO₃⁻ (Figures 3c,d) are associated with intensified agriculture and increased fertilizer use in the Lake Caohai watershed. A decline in water transparency (Figure 3b) also implied the poor water quality of Lake Caohai. Strict control of external nutrient loading is therefore necessary.