Algal blooms pose serious threats to water environments and the balance of aquatic ecosystems. Moreover, environmental factors may be the primary driver of bloom outbreaks. Studying the characteristics and driving factors of the evolution of cyanobacterial blooms can provide a scientific basis for the restoration of lake water environments and aquatic ecosystems. Based on the Landsat extended time series and practical ecological investigation, this study determined the bloom area and dominant species of cyanobacterial blooms in Hulun Lake, and analyzed their evolution characteristics and primary influencing factors. The results show that the area of bloom outbreak tended to decrease from 2018 to 2020, with the peak area remaining below 150 km²; in contrast, the bloom area showed an increasing trend in 2021–2022, with a maximum of 1970.55 km² in June 2022. Pearson correlation analysis of bloom area and driving factors showed that the wind speed was the main influencing factor of bloom outbreaks. In 2022, there were five dominant species of cyanobacteria in summer and three in autumn. Redundancy analysis of the dominant species of cyanobacteria and water quality indicators showed that COD, DO, TP, WT, pH, and TN were the main influencing factors of the dominant species of cyanobacteria in summer, and COD, DO, TP, and WT were the main influencing factors in autumn. In general, cyanobacteria in Hulun Lake are sensitive to changes in wind speed and nutrient concentration. An important approach towards effectively reducing the intensity of cyanobacterial blooms in Hulun Lake and coping with the ecological risks associated with the changing environment would be to reduce external loads and regulate the ecosystem structure of the lake. In the future, while studying the law of cyanobacterial bloom outbreaks, attention should be paid to the impact of the cyanobacterial physiological status on bloom outbreaks.

This study uses the Kherlen River as a case study to investigate the relationship between soil nutrients in riparian zones and water quality in inlet sections of lakes. Field sampling and experimental analyses were conducted during the high-water period (July) of 2023. An investigation was conducted on both the water quality of the river segments entering the lake and the soil nutrients. Methods such as the comprehensive water quality index (WQI), spatial heterogeneity analysis, and gray relational analysis were employed to assess water quality, soil nutrient characteristics, and their interrelationships, respectively. The results indicated that during the high-water period, the average concentrations of the permanganate index (COD_Mn), total nitrogen (TN), total phosphorus (TP), and dichromate oxidizability (COD_Cr) in the Kherlen River exceeded the Class V surface water quality standard thresholds. The overall WQI of the Kherlen River was 22.54, reflecting generally poor water quality, with a Global Moran’s I of 0.21, indicating a spatially clustered distribution. In the watershed, the Global Moran’s I values for pH values, TOC, TN, and TP at soil depths of 0–10 cm and 10–20 cm were 0.52, 0.90, 0.86, and 0.94 and 0.51, 0.57, 0.77, and 0.78, respectively. A significant positive correlation was found among soil nutrients, exhibiting a strong spatial aggregation characteristic, with nutrient concentrations decreasing with increasing soil depth. Moreover, the WQI of the Kherlen River demonstrated a significant correlation (R² > 0.6) with soil nutrient indicators, underscoring the substantial impact of riparian soil nutrients on river water quality. Based on these findings, targeted water management and ecological restoration measures are proposed to improve the water quality of the Kherlen River and Hulun Lake, providing new insights and scientific evidence for the restoration and sustainable development of lake ecosystems.

Groundwater is one of the key sources of water recharge in Hulun Lake. In order to trace the location of the confined aquifer of the deep groundwater that recharges the lake, hydrogeochemical characteristic analysis and hydrogen and oxygen stable isotope sampling and analysis were performed on the lake water, phreatic water and multi-layer cretaceous confined water in the same region of the Hulun Lake basin. The hydraulic relationships between the lake and various aquifers were then revealed through the use of hydrogen radioisotopes. The results show that the lake water, phreatic water and confined water are of the HCO₃−Na type, and the content of stable isotopes (δD, δ¹⁸O) and radioisotopes (δ³H) is in the order of “confined water < phreatic water < lake water”. The main influencing factor of hydrochemical evolution in the phreatic water is the dissolution of feldspar; its age is about 26.66 years, and its renewal rate is nearly 3.75%. The main influencing factor of hydrochemical evolution in the K₁y₁, K₁y₂ and K₁d₁ Cretaceous confined water is evaporite dissolution (i.e., halite, gypsum); their renewal rate is less than 1%, and the discharge condition deteriorates with the increase in the aquifer roof burial depth. Phreatic water in the Jalainur Depression Zone supplies Hulun Lake under the condition of the existence of permafrost cover. The K₁d₂ confined water of the Lower Cretaceous–Damoguaihe Formation Coal Group II, with the deepest roof burial depth (441 m), shows significant differences in hydrochemistry, δD, δ¹⁸O and δ³H from the other K₁y₁, K₁y₂ and K₁d₁ Cretaceous confined waters in the same basin. The renewal rate (nearly 4.32%) of the K₁d₂ confined water is better than that of the phreatic water, and its hydrochemical characteristics are similar to those of the lake water and phreatic water, indicating that the Cuogang Fault and Xishan Fault, caused by crustal faults, resulted in the hydraulic relationship between the K₁d₂ confined water, lake water and phreatic water, resulting in drastic interannual changes in the lake water level. This study of lake–groundwater interactions in cold and arid regions can provide a theoretical basis for lakes’ sustainable development.

The water level of Lake Hulun has changed dramatically in recent years. The interannual interaction between groundwater and lake water is an important factor affecting Lake Hulun’s water level. Vertical recharge between groundwater and the lake is particularly important. Based on an analysis of differences between the hydrogeochemical and water quality characteristics of the spring water, the lake water, and the surrounding groundwater, the source and recharge mechanism of the spring water in the vertical recharge lake are determined. The results show that spring water is exposed at the bottom of Lake Hulun, and there are obvious differences between spring water and lake water in lake ice thickness, ion characteristics, and water quality characteristics. For example, the ice thickness at the spring site is only 6.8% of the average ice thickness of the lake, and there is a triangular area directly above the spring water area that is not covered by ice; the ion contents of the spring water at the lake bottom were less than 50% of those in the lake water; and the NH₄⁺-N content of the spring water at the lake bottom was only 3.0% of the mean content of the lake water. In addition, the total nitrogen (TN), dissolved oxygen (DO), and NH₄⁺-N contents of the spring water at the lake bottom all fall outside the range of contents of the surrounding groundwater. In general, the source of the spring water at the lake bottom is not recharged by the infiltration recharge of the phreatic aquifer but by the vertical recharge of the confined aquifer. Additionally, the Lake Hulun basin may be supplied with confined water through basalt channels while it is frozen. The vertical groundwater recharge mechanism may be that spring water at the lake bottom is first supplied by the deep, confined aquifer flowing through the fault zone to the loose-sediment phreatic aquifer under the lake, and finally interacts with the lake water through the phreatic aquifer.

In cold-region lakes, high organic matter concentrations with low bioavailability are common, yet the underlying causes and stabilisation mechanisms remain unclear. This study conducted a 60-day microbial treatment experiment in Hulun Lake using algae (DOMa), grass (DOMg), and manure (DOMm) as DOM sources. Fourier transform ion cyclotron resonance mass spectrometry and 16S rRNA analysis were employed to characterise DOM composition and bacterial communities. The bioavailability of DOMa, DOMg, and DOMm was 86.1%, 84.08%, and 70.9%, respectively. Differences in degradation cycles were mainly associated with the slowly biodegradable fraction; the half-lives of DOMa, DOMg, and DOMm were 49.51 days, 77.02 days, and 198.04 days, respectively. At the molecular level, proteins and lipids were rapidly utilised by microorganisms, leading to the generation of lignin, condensed aromatic hydrocarbons, and tannins, with many new molecules falling within the carboxylic acid-rich alicyclic molecule (CRAM) region. The overall community succession patterns of different DOM sources were highly similar, with initial DOM composition differences leading to variations in microbial communities during intermediate degradation stages (5~10 days). Moreover, microbiological processes facilitated the convergence of DOM source compositions and the accumulation of refractory organic matter. It is hypothesised that the regional climatic characteristics of the freeze–thaw cycle exacerbate organic matter accumulation by compressing the “effective degradation time”. These findings elucidate the causes of high organic matter and low bioavailability in cold-region lakes.

The increasing input of endogenous and exogenous dissolved organic matter (DOM) into lakes, driven by intensified human activities and climate warming, is profoundly affecting the functioning of lacustrine ecosystems. However, the fate of DOM under the combined influence of light and microorganisms—especially in northern steppe lakes—remains poorly understood. Here, we integrated ultrahigh-resolution mass spectrometry (FT-ICR MS) and high-throughput sequencing in a microcosm experiment using water from Hulun Lake, the largest arid steppe lake in northern China, to compare the molecular transformation of endogenous algal-derived DOM (ADOM) and exogenous livestock manure-derived DOM (MDOM). Treatments included dark controls, light-only, microbial-only, and light–microbial coupling. Using high-resolution mass spectrometry and multi-group structural equation modeling, our results demonstrate that light regulates microbial processing of DOM through source-dependent mechanisms. Photodegradation suppressed the bioavailability of ADOM by 4.9%, largely due to combined substrate competition and reactive oxygen species (ROS) stress on key bacterial degraders. In contrast, light synergistically enhanced the bioavailability of MDOM by 5.4% through substrate facilitation. Molecular-level analysis revealed that lignin-like molecules from different sources exhibit distinct responsiveness to photochemical and microbial processing, which emerged as the key mechanistic driver for the contrasting degradation patterns. Microbial co-occurrence networks further linked specific bacterial taxa to DOM component turnover and ROS sensitivity. These findings highlight the central role of light in modulating DOM balance and source–sink dynamics in arid boreal lakes, providing novel, mechanism-based insights into the biogeochemical cycling of DOM in lake ecosystems.