Sedimentary Differentiation Characteristics of Organic Matter and Phosphorus in Eutrophic Lake Special Zones

Abstract

Lake eutrophication, often driving harmful algal blooms (HABs) and ecosystem degradation, involves complex biogeochemical shifts within sediments. Changes in the sedimentary dissolved organic matter (DOM) composition during transitions from macrophyte to algal dominance are thought to critically regulate internal phosphorus (P) loading, yet the underlying mechanisms, especially in vulnerable plateau lakes like Qilu Lake, require further elucidation. This study investigated the coupled cycling of carbon (C) and P in response to historical ecosystem succession and anthropogenic activities using a 0–24 cm sediment core from Qilu Lake. We analyzed the total organic carbon (TOC), total phosphorus (TP), sequential P fractions, and DOM fluorescence characteristics (EEM-PARAFAC), integrated with chronological series data. The results revealed an asynchronous vertical distribution of TOC and TP, reflecting the shift from a submerged macrophyte-dominated, oligotrophic state (pre-1980s; high TOC, low TP, stable Ca-P dominance) to an algae-dominated, eutrophic state. The eutrophication period (~1980s–2010s) showed high TP accumulation (Ca-P and NaOH85 °C-P enrichment), despite a relatively low TOC (due to rapid mineralization), while recent surface sediments (post-2010s) exhibited a high TOC, but a lower TP following input controls. Concurrently, the DOM composition shifted from microbial humic-like dominance (C1) in deeper sediments to protein-like dominance (C3) near the surface. This study demonstrates that the ecosystem shift significantly regulates P speciation and mobility by altering sedimentary DOM abundance and chemical characteristics (e.g., protein-like DOM correlating negatively with Ca-P), reinforcing a positive feedback mechanism that sustains internal P loading and potentially exacerbates HABs. DOM molecular characteristics emerged as a key factor controlling the internal P cycle in Qilu Lake, providing critical insights for managing eutrophication in plateau lakes.

1. Introduction

Eutrophication represents a critical environmental challenge confronting global lake ecosystems. Statistical analyses indicate that 54%, 53%, 46%, and 28% of lakes in Asia, Europe, North America, and Africa, respectively, are threatened by eutrophic conditions [1]. Eutrophication represents a critical environmental challenge confronting global lake ecosystems, impairing the water quality, and it can produce toxins harmful to humans and wildlife [2]. The global occurrence of HABs severely jeopardizes drinking water security, exemplified by the 2007 cyanobacterial bloom in China’s Lake Taihu that triggered a drinking water crisis in Wuxi City [3,4]. Lake Qilu, a vital water resource for local agriculture and for maintaining regional biodiversity in Yuxi City, Yunnan Province, has not been immune to these pressures [5]. Historical records and recent studies indicate its progressive eutrophication since the 1980s, transitioning from a macrophyte-dominated to an algae-dominated state, with water quality often classified as inferior to Grade V [6]. This degradation poses considerable challenges to its ecological functions and water resource sustainability.

Phosphorus (P) is widely recognized as the key limiting factor controlling eutrophication in many freshwater systems [7]. Its sources include both exogenous inputs, such as agricultural runoff from extensive vegetable cultivation around Qilu Lake, which contributes significantly to nutrient loads, and endogenous release from bottom sediments. A detailed understanding of sedimentary phosphorus speciation, which dictates phosphorus’s bioavailability and potential for internal loading, is therefore fundamental to addressing eutrophication and forms a primary focus of this research. Understanding and managing internal P loading from sediments is crucial for effective lake restoration, as this internal source can sustain eutrophic conditions even after external P inputs are reduced. The mobilization of P from sediments involves complex processes regulated by environmental factors, including the redox potential, pH, temperature, bioturbation, and, critically, the amount and chemical nature of sedimentary organic matter (OM) [8].

Sedimentary OM–phosphorus interactions constitute a core, yet intricate, regulatory mechanism in lacustrine nutrient cycling [9]. To comprehensively assess these OM-P interactions, our study quantifies bulk sedimentary organic matter via the total organic carbon (TOC), and characterizes the composition and potential reactivity of dissolved organic matter (DOM) using fluorescence spectroscopy and a parallel factor analysis (PARAFAC). DOM derived from terrestrial vegetation, autochthonous algae, macrophytes, and microbial activities modulates phosphorus bioavailability through adsorption–complexation–mineralization pathways [10]. Numerous studies globally have explored sedimentary P and OM dynamics in eutrophic lakes, often focusing on factors like redox conditions, OM lability, and the dissolution of Fe oxides in influencing internal P cycling and benthic DIP fluxes. For instance, the exchange of DOM at the sediment–water interface, driven by the microbial processing of sediments and concentration gradients, plays a significant role in estuarine and coastal carbon cycling.

A crucial aspect, particularly relevant to long-term lake dynamics, is the impact of ecosystem regime shifts. When lake ecosystems transition from macrophyte-dominated to algal-dominated states—a common trajectory under increasing nutrient pressure—fundamental alterations occur in the primary source and chemical composition of the OM deposited in sediments [11]. Consequently, analyzing shifts in the TOC, DOC, and specific DOM components is vital for understanding how these ecosystem transitions impact P speciation and the internal loading potential. This shift, often towards more labile, protein-rich algal material replacing potentially more refractory macrophyte detritus, can significantly modify P speciation, mobility, and the potential for internal loading [12]. Organic matter further mediates phosphorus cycling by regulating redox conditions, particularly through iron/manganese oxide reduction under anaerobic environments, which promotes phosphorus release [12]. Algal-derived organic components (e.g., polysaccharides and proteins) enhance phosphate mobility via competitive adsorption, thereby establishing positive feedback mechanisms in sediment–water column phosphorus cycling [13]. Even microbially derived humic substances and their degradation products regulate phosphorus dynamics through metal ion complexation and mineral dissolution [14,15]. Recent research also indicates that the stability of OM-Fe-P associations, which influences P sequestration, is dependent on OM source and formation pathways (adsorption vs. coprecipitation), with algal OM potentially leading to less stable associations compared to terrestrial humic acids under reducing conditions [16,17].

Despite considerable research, a complete mechanistic understanding of how P speciation and transformation kinetics are regulated by OM from different sources (algae vs. macrophytes vs. terrestrial inputs) and with varying chemical properties (e.g., lability, functional group composition) remains elusive. This knowledge gap hinders the development of effective eutrophication control strategies, particularly concerning internal phosphorus loading driven by specific DOM fractions.

Lake eutrophication and its drivers exhibit marked regional heterogeneity. Plateau lakes, such as Qilu Lake in Southwest China, represent unique environments often characterized by specific hydro-climatic conditions (e.g., closed or semi-closed basins, strong solar radiation, distinct thermal regimes) and potentially different sensitivities to nutrient enrichment [18]. These factors can influence OM production, decomposition rates, and P cycling pathways distinctly from lowland lakes. Previous paleolimnological studies on Qilu Lake, utilizing proxies like the sediment grain size, pigments, and diatoms, have documented its ecological transition from macrophyte to algal dominance and significant environmental changes over the past 70 to 200 years, driven by both climate change and intensified human activities, including land reclamation by dyking the lake, hydrological regulation, and increased nutrient inputs from agriculture. For example, Li et al. divided the sedimentary environment of Qilu Lake over the past 70 years into three periods, reflecting varying degrees of anthropogenic impact and climatic conditions [19]. Qian et al. identified that nutrient enrichment has been a major driver for diatom assemblage shifts, with hydrological changes and habitat availability also playing synergistic roles [20]. These documented historical changes, coupled with its characteristics as a sensitive and heavily polluted plateau lake, establish Qilu Lake as an ideal system for deciphering the detailed mechanisms of OM-P coevolution during eutrophication.

While the general links between OM and P are known, and the eutrophication history of Qilu Lake has been investigated using various proxies, detailed investigations into the specific coevolution of the DOM molecular characteristics (identified by PARAFAC) and phosphorus speciation across these distinct historical ecosystem states in sensitive plateau lakes remain less common. The novelty of this study lies in its integrated approach, combining a well-established chronological framework with detailed P fractionation and advanced DOM characterization to precisely track how historical shifts in OM sources and composition have mechanistically regulated internal P cycling and bioavailability in Qilu Lake. This allows for a more nuanced understanding of the positive feedback loops, potentially driven by changes in DOM quality, that sustain eutrophication in such systems. This study systematically investigated the spatiotemporal evolution of key phosphorus fractions (P speciation) and organic matter quantity (TOC) and quality (DOC, DOM fluorescence characteristics identified by PARAFAC) in Qilu Lake sediments. By elucidating the coevolution mechanisms between these specific P forms and OM components during ecosystem transition, this research not only advances the theoretical understanding of eutrophication processes, but also provides scientific foundations for lake ecological restoration and water quality management.

2. Materials and Methods

2.1. Study Area Overview

Lake Qilu (24°05′–24°14′ N, 102°34′–102°53′ E), a plateau freshwater lake located in Yuxi City, Yunnan Province, China, occupies an elevation of 1797 m above sea level. As one of the nine major lakes on the Yunnan–Guizhou Plateau, it belongs to the Xijiang River system in the Pearl River Basin and is classified as a tectonic fault lake. The lake covers a surface area of 36.86 km² with maximum and average depths of 6.8 m and 4.03 m, respectively [18]. This closed-basin system receives hydrological inputs primarily through precipitation, surface runoff, spring discharge, and groundwater, with its main inflow tributaries being the Hongqi River, Yaochong River, and Daxin River.

2.2. Sample Collection and Analysis

In June 2023, a sediment core (length: ~50 cm, capturing approximately 0–24 cm of consolidated sediment depth relevant for recent history) was retrieved from the central lake area, designated as a national monitoring station (24°10′0.48″ N, 102°46′26.76″ E), using a gravity corer designed to minimize the disturbance of the sediment–water interface and stratification. The retrieved core was immediately sealed. In the laboratory, the core was carefully sectioned at 1 cm intervals for the top 2 cm, 2 cm intervals between 2 cm and 12 cm, and 4 cm intervals below 12 cm. Subsamples for the geochemical analysis were placed in sterile polyethylene bags, transported to the laboratory on ice, and subsequently lyophilized (freeze-dried) at −80 °C for 7 days using a freeze-dryer (FD-1A-50, Biocool, Beijing, China). The dried sediment samples were then gently homogenized by grinding with an agate mortar and pestle and sieved through a 100-mesh nylon sieve (150 μm pore size) prior to the analysis.

By integrating ²¹⁰Pb and ²²⁶Ra activity measurements with the constant rate of supply model, Klamt et al. determined that the 0–25 cm sediment column corresponds to approximately 54 years of deposition (1959–2013), with an average sedimentation rate of 0.46 cm yr⁻¹ (Table S1) [18]. Based on the study by Klamt et al. and the lake ecosystem data of Qilu Lake (Table S2), the chronological information of the sediment core in this region can be determined (Table S1) [6,18,21,22]. The pre-1980s period (1971–1980 AD), corresponding to the 20–24 cm depth interval, is characterized by macrophyte dominance. The 1980s (1980–1988 AD), represented by the 16–20 cm interval, marks a transitional phase. This is followed by a period of algal dominance, further subdivided as follows: the 1990s (1988–2001 AD) correspond to 10–16 cm (12–16 cm: 1988–1997; 10–12 cm: 1997–2001); the 2000s (2001–2010 AD) to 6–10 cm (8–10 cm: 2001–2006; 6–8 cm: 2006–2010); and the 2010s (2010–2019 AD) to 2–6 cm (4–6 cm: 2010–2014; 2–4 cm: 2014–2019). The most recent period, the 2020s (2019–2023 AD), is represented by the surface 0–2 cm layer.

2.3. Phosphorus Fractionation and Analysis

Sediment phosphorus (P) speciation was determined using the sequential extraction method originally developed by Psenner, which has been widely applied to lake sediments [23]. Briefly, 1 g of oven-dried sediment was placed into a 50 mL centrifuge tube, and extractions were carried out at a solid-to-liquid ratio of 1:50. The sequential extraction steps (illustrated in Figure 1) began with the extraction of NH₄Cl-P (labile P), which was performed using 1 mol/L NH₄Cl at 25 °C for 0.5 h under continuous shaking (THZ-420, GUOWANG, Changzhou, China). BD-P (redox-sensitive phosphorus) was extracted with 0.11 mol/L Na₂S₂O₄/NaHCO₃ (BD solution) at 40 °C for 1 h with continuous shaking. NaOH25 °C-rP (Al-bound P) was extracted using 1 mol/L NaOH at 25 °C for 16 h with continuous shaking. The organic phosphorus in the NaOH25 °C extract (denoted as NaOH25 °C-nrP) was quantified via the differential method. Specifically, an aliquot of the supernatant was digested with potassium persulfate (K₂S₂O₈, 50 g/L solution, 4 mL:25 mL v/v extract ratio) at 121 °C for 30 min in an autoclave (BXM-30R, Boxun, Shanghai, China). The total phosphorus (NaOH25 °C-TP) in the NaOH extract was determined after digestion (via persulfate digestion). Then, NaOH25 °C-nrP was calculated as follows: NaOH25 °C-nrP = NaOH25 °C-TP—NaOH25 °C-rP. HCl-P (Ca-bound P) was extracted using 0.5 mol/L HCl at 25 °C for 16 h with continuous shaking. NaOH85 °C-P (residual P) was oscillated with 1 mol/L NaOH at 85 °C for 2 h with continuous shaking. The reagents used in the experiment were all of analytical purity (Jingrui AR, Yuxi, China).

Following each extraction step, the suspensions were centrifuged at 4000× g for 15 min (H20250R, cence, Changsha, China), and the supernatants were filtered through 0.45 μm GF membranes. The P concentrations (as orthophosphate, PO₄³⁻-P) in the NH₄Cl, BD, NaOH25 °C, HCl, and NaOH85 °C extracts, as well as the total P in the digested NaOH25 °C extract (for the NaOH25 °C-nrP calculation), were determined spectrophotometrically using the molybdenum blue method at 880 nm on a ultraviolet spectrophotometer (312 UV-Vis, KEWLAB, Melbourne, Australia) [24]. The overall recovery of phosphorus by the sequential extraction, determined by comparing the sum of fractions to independently measure the total phosphorus, ranged from 88 to 119% [14]. Such recovery rates are within the generally accepted range for the Psenner method applied to sediments, indicating a good protocol reliability. The total phosphorus (TP) in the bulk sediment samples was quantified independently by following the Standards, Measurements and Testing (SM&T) protocol of the European Community Bureau of Reference. This involved sequential dry-ashing in a muffle furnace (450 °C, 3 h; LV 9/11, Nabertherm, Lilienthal, Germany), followed by acid digestion with 3.5 mol/L HCl (25 °C, 16 h) and a subsequent spectrophotometric analysis [25].

Rigorous quality control measures were implemented throughout the analytical procedures. High-purity reagents were used for all the extractions and analyses. The analytical instruments, including the spectrophotometer and TOC analyzer, were calibrated daily according to manufacturer guidelines. Procedural blanks were analyzed periodically to monitor for potential contamination, and instrumental readings were checked for stability before the final values were recorded.

2.4. Organic Matter Extraction and Analysis

The sequential extraction protocol used for phosphorus fractionation (Figure 1) simultaneously served to fractionate sedimentary organic matter (OM) based on its association with the sediment phases and the potential lability. The supernatants collected from the NH₄Cl, BD, NaOH25 °C, HCl, and NaOH85 °C extraction steps were also analyzed for DOM characteristics.

Prior to the fluorescence analysis, the extract was filtered through a pre-combusted 0.45 µm GF/F membrane. Subsequently, the organic matter in the extract was analyzed using a fluorescence spectrophotometer (F-7100, Hitachi High-Tech, Tokyo, Japan). Three-dimensional excitation–emission matrix (EEM) fluorescence spectroscopy combined with a parallel factor analysis (PARAFAC) was employed to characterize the organic components. The spectral acquisition parameters were set as follows: an excitation/emission slit width of 5 nm, an excitation wavelength range of 200–450 nm, and an emission wavelength range of 250–600 nm with 5 nm intervals. A blank extract was used as the baseline during the experiment to minimize the Rayleigh and Raman scattering interference.

The total organic carbon (TOC) in bulk sediment samples—after acidification to remove carbonates—and the dissolved organic carbon (DOC) in the filtered extracts were quantified using a high-temperature combustion method with a total organic carbon analyzer (TOC-L CPH, Shimadzu, Tokyo, Japan).

2.5. Data Processing

PARAFAC modeling was performed in MATLAB R2023a (9.14.0.2206163) as previously described. Descriptive statistics were calculated for the measured parameters. To investigate the relationships among variables, a Pearson correlation analysis was conducted using OriginPro 2024 SR1 (10.1.0.178), with the statistical significance set at p < 0.05 unless otherwise specified. Vertical profile plots were also generated using OriginPro 2024 SR1.

3. Results

3.1. Vertical Distribution of TP and TOC in Sediment Cores

The sediment core (0–24 cm) exhibited pronounced vertical heterogeneity in the TP, TOC, and DOC concentrations (Figure 2 and Figure 3). The TOC and DOC profiles displayed a bimodal distribution pattern with depth. Surface sediments (0–2 cm) showed elevated TOC and DOC levels (mean values: 44.76 g/kg and 520.73 mg/kg, respectively), followed by a progressive downward decline. Notably, a secondary enrichment zone emerged in the subsurface layers (16–20 cm), where the TOC concentrations peaked at 78.41 g/kg, while the DOC increased moderately to 504.31 mg/kg. The fractionation analysis of sedimentary DOC revealed distinct thermal lability characteristics. NaOH-extractable DOC (25 °C and 85 °C fractions) constituted 29–54% of the total DOC across all depths. Spatial partitioning was evident: the surface layers (0–2 cm) were dominated by the 25 °C-extractable fraction, whereas the 85–extractable fraction predominated in deeper horizons (4–20 cm).

In contrast, the TP exhibited an inverse vertical trend relative to the organic carbon fractions (Figure 2). The minimum TP concentrations were recorded in surface sediments (0–2 cm: 1101.43 mg/kg), increasing with depth before declining markedly in the deepest stratum (20–24 cm: 455.94 mg/kg).

3.2. Vertical Distribution of DOM in Sediment Cores

Three fluorescence components were identified through PARAFAC in five organic matter extraction solutions (Figure 4), namely C1 (microbially derived humic substances, associated with microbially derived humic acids), C2 (terrestrially derived humic substances), and C3 (tryptophan-like protein substances) [26,27,28].

Figure 5A–F depict the vertical distribution patterns of the relative abundance and fluorescence intensity (Fmax) of three fluorescent components under varying extraction conditions. C1 (microbially derived humic substances) exhibited a pronounced depth-dependent increase in Fmax, rising markedly from 3457.44 RU in the surface layer (0–1 cm) to 10,796.59 RU in the deeper layer (20–24 cm) (Figure 5A). C2 (terrestrially derived humic substances) maintained relatively stable Fmax values (1446.28–1676.64 RU) throughout the sediment profile, with a notable increase observed within the 2–16 cm depth interval (Figure 5C). C3 (protein-like substances) predominated in surface sediments (0–1 cm: 4898.89 RU; 1–2 cm: 4885.79 RU), followed by a sharp decline at 2–4 cm and a moderate recovery at 20–24 cm (Figure 5E). These depth-specific variations reflect the differential preservation and transformation processes of organic matter components within the sediment matrix.

An analysis of the sequential extracts revealed that the identified fluorescent DOM components and their respective fluorescence intensities (Fmax) differed notably depending on the extraction solution employed. C1 predominated in the NaOH 85 °C and NaOH 25 °C extraction solutions, with a U-shaped depth profile observed in the NaOH 25 °C extract, whereas the NaOH 85 °C extract displayed an inverse trend (Figure 5B). C2 showed a higher fluorescence intensity in the NH₄Cl and NaOH 85 °C extraction solutions compared to the other conditions, with NaOH 85 °C dominating in the deeper sediment layers (2–24 cm) (Figure 5D). C3 was primarily distributed in the NH₄Cl and NaOH 25 °C extraction solutions (Figure 5F), with its relative contribution varying markedly with depth. Specifically, the C3 contribution in the NaOH25 °C extract increased from 21% to 50%, while it declined from 79% to 48% in the NH₄Cl extract.

The vertical distribution profile indicates that C3 dominated in surface sediments (0–2 cm), with a relative contribution of 51–54%. However, its proportion decreased sharply to 7–15% at depths beyond 2 cm, where C1 emerged as the predominant component (67–77%). In contrast, the relative contribution of C2 remained relatively stable across all sediment depths, consistently ranging between 12% and 18% (Figure 6).

3.3. Vertical Distribution of P Fractions in Sediment Cores

The vertical distribution profiles of the six P fractions, determined by the Psenner sequential extraction method, are presented in Figure 7 [23]. The NH₄Cl-P fraction, operationally defined as labile or loosely adsorbed phosphorus, represents readily exchangeable phosphate and is present in sediment porewater [29]. Although NH₄Cl-P accounts for only about 1% of the total phosphorus content, making it the lowest among all phosphorus fractions, it is highly susceptible to environmental disturbances and can readily remobilize into the overlying water [30]. Therefore, it is considered a key indicator of the phosphorus release flux at the sediment–water interface. In Lake Qilu sediments, the NH₄Cl-P content remains extremely low (<1%), with relatively higher values observed in the surface (0–2 cm) and deeper (16–20 cm) layers, showing average concentrations of 2.70 mg/kg and 3.03 mg/kg (Figure 7), respectively. As a redox-sensitive P fraction, BD-P primarily binds to iron and manganese (hydr)oxides [31]. Its absolute concentration shows relatively gentle vertical variation, ranging from 97.15 to 119.46 mg/kg. However, its relative contribution is relatively higher in the surface (0–2 cm) and deeper (16–24 cm) sediment layers, with average proportions of 14.26% and 27.89% (Figure 7B), respectively.

NaOH25 °C-rP is primarily associated with aluminum (hydr)oxides and is released through ligand exchange with OH⁻ or dissolved phosphate [32]. This phosphorus fraction exhibited a vertical distribution pattern characterized by an initial increase followed by a decline, with lower concentrations and relative proportions observed in the surface (0–2 cm) and deeper (16–24 cm) layers compared to intermediate depths. NaOH25 °C-nrP represents humic acid-bound organic phosphorus with pronounced biogenic sedimentary characteristics [33]. Its average relative proportion was 7.61%, ranking only slightly higher than NH₄Cl-P (Figure 7B).

HCl-P is predominantly composed of calcium-bound phosphorus, originating from terrestrial detrital rocks and authigenic apatite [34]. Nevertheless, its proportion in the sediments increased sharply with depth, rising from 30% in the surface layer (0–2 cm) to 49% in the deeper layers, making it the dominant phosphorus fraction. In terms of absolute content, it exhibited an initial increase followed by a decrease, with enrichment occurring at a depth of 2–16 cm.

NaOH85 °C-P, classified as an inert phosphorus fraction, consists of insoluble phosphorus and stable organic phosphorus [35]. Its highest relative proportion (>20%) was observed in the intermediate layer (2–16 cm), reflecting its depositional stability.

4. Discussion

4.1. Mechanisms Driving the Inverse Distribution of TP and Organic Carbon

The vertical differentiation of TOC in lake sediments is governed by a complex coupling mechanism involving early diagenetic processes and dynamic external inputs [36]. In typical sedimentary sequences, surface sediments exhibit TOC enrichment due to the continuous deposition of fresh organic matter from the overlying water column. However, with increasing depth, organic carbon mineralization—driven by microbial activity and chemical transformation—gradually decreases the TOC content. This pattern has been well-documented in studies of representative lakes such as Erhai and Dianchi [14,37]. Under certain conditions, the vertical distribution of the TOC may deviate from the typical trend, exhibiting fluctuations or peaks due to episodic inputs of allochthonous organic carbon [36]. The vertical profiles of the TOC and TP in Lake Qilu sediments exhibited a notable inverse relationship, particularly the high TOC/low TP in deeper layers (16–24 cm) and the low TOC/high TP in intermediate layers (2–16 cm), contrasting with the surface enrichment of both (0–2 cm, albeit with TP beginning to decrease). This pattern deviates from simple depositional trends and reflects complex biogeochemical interactions and historical ecosystem shifts [38].

The high TOC content in the lower part of the sediment core (16–24 cm), corresponding to a period (pre-1980s) when the lake was likely less impacted and potentially macrophyte-dominated, probably stems from significant organic matter input from submerged macrophytes and associated epiphytic algae, coupled with contributions from benthic organisms like gastropods [39]. Submerged macrophytes contribute substantially to sediment carbon via litterfall and root exudates [40,41]. The concurrently low TP during this oligotrophic phase suggests limited external phosphorus inputs.

Conversely, the period reflected in the 2–16 cm sediment depth (approx. 1980s–2010s) marks a phase of increasing eutrophication and ecosystem transition towards algal dominance [21]. While algal production significantly increases the organic matter input, this algal-derived material is often highly biodegradable [42]. Eutrophication-induced hypoxia likely accelerated the anaerobic decomposition and mineralization of this labile organic matter within the sediments [43], preventing significant TOC accumulation despite high primary production. Simultaneously, increased external P inputs during this period likely led to the accumulation of TP, primarily bound in inorganic forms within the sediment (Figure 7B). Additionally, a substantial accumulation of gastropod remains was observed at this depth, suggesting that biogenic carbonates contributed to an increased Ca content in the sediment, thereby facilitating phosphorus retention. Studies have shown that competition between organic matter and phosphorus for binding sites favors Ca–organic matter interactions over Ca–phosphorus binding [37]. Consequently, biogenic Ca may play a crucial role in phosphorus stabilization. However, the enrichment of organic matter in both the surface and deep layers inhibits the binding of phosphorus with Ca, resulting in a lower proportion of Ca-bound phosphorus at both ends. This trend is reflected in the significant negative correlation between the TOC and HCl-P (p < 0.01, Figure S1) and the enrichment of Ca-bound phosphorus at mid-depth layers.

During this period, algal-derived organic matter became the dominant source of organic matter input to the lake’s sediments, yet its residues were highly biodegradable and subject to rapid mineralization. Decomposition products from algal biomass and emergent macrophytes were metabolized by heterotrophic bacteria, leading to the formation of humic-like substances. The concurrent decline in protein-like fluorescence components indicates a reduction in fresh organic matter input [44]. Eutrophication-induced hypoxia further accelerated anaerobic decomposition within the sediments, promoting rapid organic carbon mineralization and thereby preventing its long-term accumulation in the sedimentary record. Furthermore, the high TP content was primarily retained in sediments in inorganic forms, such as mineral adsorption, while its association with organic matter decreased.

The 0–2 cm sediment layer is particularly noteworthy, as it corresponds to the period when the Yuxi government implemented multiple ecological restoration projects in the surrounding area [45]. These projects effectively reduced the TP input into the sediments, alleviating regional water pollution loads. Additionally, they altered lake hydrodynamics by extending the water residence time, which in turn enhanced the algal activity. The accumulation of algal exudates and metabolic byproducts resulted in protein-like fluorophores becoming the dominant component in the water column. Furthermore, the combined effects of intensified algal metabolic activity and the deposition of refractory residues from emergent macrophytes contributed to the observed increase in the TOC content during this period.

4.2. Evolutionary Trends in P Fractions Within Sedimentary Cores of Lake Qilu

The vertical variations in P fractions in sediments primarily reflect the historical impacts of anthropogenic activities on internal pollution processes [46]. NH₄Cl-P remained negligible throughout, indicating that readily exchangeable P represents a very small pool in these sediments [46]. BD-P, representing redox-sensitive forms primarily bound to Fe/Mn oxides [47], showed relatively stable absolute concentrations, but higher relative contributions in surface and deeper layers. The stability in absolute terms might suggest a dynamic balance between release under reducing conditions and re-adsorption, possibly influenced by OM complexation or interactions with Al phases [48,49]. The slight decrease in deeper layers could relate to the transformation of amorphous Fe-oxides to more crystalline forms with a lower P sorption capacity over time [50]. The relative enrichment near the surface and deeper down might reflect recent inputs/transformations and conditions during past periods, respectively.

The more stable P forms (NaOH25 °C-rP, NaOH25 °C-nrP, HCl-P, and NaOH85 °C-P) exhibited distinct peaks in the intermediate sediment depths (2–16 cm), coinciding with the period of highest TP accumulation (1980s–2010s). The peak in NaOH25 °C-rP (often associated with Al-P and reactive Fe-P) suggests significant P binding onto Al/Fe hydroxides during this eutrophic phase, potentially sourced from increased external inputs or internal recycling. NaOH25 °C-nrP (organic P) followed a similar trend, reflecting P incorporation into microbial biomass and refractory organic compounds during decomposition. The peak in the absolute concentration of the HCl-P fraction (operationally targeting Ca-bound P, largely apatite forms) in the 2–16 cm zone (corresponding to peak eutrophication) suggests an enhanced accumulation, potentially through significant formation or preservation, of apatite-like minerals. This interpretation is supported by the concurrently high total phosphorus availability and the observed presence of biogenic carbonates (from gastropod remains) in this layer. Such conditions, with the elevated dissolved P and available Ca often associated with carbonate buffering, are known to favor the authigenic precipitation or enhanced preservation of Ca-phosphate minerals like apatite in some eutrophic lacustrine environments [51]. NaOH85 °C-P also peaked here, representing the accumulation of refractory P minerals and stable organic P over this period. The subsequent decline of these stable fractions in the deepest layers (pre-1980s) reflects the lower P availability during the earlier oligotrophic period. The decline in the surface layer reflects reduced recent P inputs, consistent with pollution control efforts implemented in the catchment in recent years [45].

4.3. DOM–Phosphorus Coupling Mechanisms in Sediments and Their Ecosystem Feedback

The observation that the types and fluorescence intensities of DOM components varied across the different extracts (as shown in Figure 5B,D,F) is consistent with the principles of sequential chemical extraction. Each extractant, by design, targets different sediment phases and binding strengths, leading to the release of distinct organic matter pools that co-extract with the phosphorus fractions [16]. For instance, milder extractants are expected to release more labile or loosely bound DOM, while stronger alkaline conditions access more recalcitrant or matrix-bound humic-like substances [52]. The differing fluorescence signatures (C1, C2, C3) observed in these extracts thus likely reflect the varying lability, source, and association of these DOM components within the sediment matrix.

The biogeochemical interactions between DOM components and P species, regulated by adsorption, complexation, and microbial processes, appear strongly linked to the ecosystem state and corresponding DOM characteristics in Qilu Lake sediments.

Protein-like organic matter often exhibits a high affinity for adsorption onto common sedimentary mineral phases, such as iron and aluminum (hydr)oxides, through mechanisms such as ligand exchange and electrostatic interactions [16]. This adsorption can displace previously bound phosphate, thereby enhancing its mobility within the sediment matrix. In contrast, high-molecular-weight terrigenous humic substances, rich in carboxyl and phenolic hydroxyl functional groups, form complexes with metal ions such as Al and Ca, occupying adsorption sites on mineral surfaces and thereby inhibiting phosphate sorption [17,53]. As a result, these humic substances show significant negative correlations with Al-P, Ca-P, and NaOH85 °C-P (Figure S1).

The stable microbially derived organic matter represented by NaOH85 °C-C1 may reduce the NH₄Cl-P content by forming insoluble complexes with phosphate or enhancing its adsorption onto mineral surfaces [40]. For instance, humified organic matter can bind with Fe/Al oxides, transforming phosphorus into more stable forms [54]. As organic matter ages and humification progresses, its interaction with phosphorus may shift from release to fixation. NaOH85 °C-C1, as a long-term accumulated stable organic component, can physically or chemically sequester phosphorus within sediments, thereby reducing the proportion of exchangeable phosphorus.

Meanwhile, terrestrially derived humic substances, during sedimentation, co-precipitate with mineral particles (e.g., clays or carbonates), forming stable humic-rich sediment layers enriched with Al and Ca (NaOH85 °C-C2) [55]. This results in significant positive correlations between NaOH85 °C-C1, C2, Al-P, Ca-P, and NaOH85 °C-P (p < 0.01, Figure S1). In contrast, algal-derived DOM can accelerate phosphorus mobilization through microbial mineralization while simultaneously facilitating its re-immobilization via interactions with mineral surfaces, thereby maintaining a dynamic equilibrium of phosphorus in sediments [56].

The transition from macrophyte-dominated to algal-dominated ecosystems triggers a positive feedback mechanism that exacerbates phosphorus release. Algal blooms intensify oxygen depletion, promoting Fe³⁺ reduction and phosphate release [57]. Meanwhile, algal polysaccharides (e.g., hyaluronic acid, alginate) are rich in carboxyl (-COOH) and hydroxyl (-OH) functional groups, which can form stable water-soluble complexes with Ca²⁺, inhibiting Ca-P precipitation and resulting in a significant negative correlation between NaOH25 °C-C1, C3, and Ca-P [58]. Additionally, legacy phosphorus pools (NaOH25 °C-rP, HCl-P) can be mobilized through ligand exchange, with stable Ca-P converting into bioavailable forms during eutrophication [59]. This transformation establishes a vicious cycle, where algal-derived organic matter sustains high internal phosphorus loading, further stimulating productivity and organic matter accumulation. These findings highlight the need for a dual management strategy that simultaneously regulates nutrient fluxes and DOM composition. Traditional sediment phosphorus inactivation methods may fail due to the ability of algal-derived DOM to remobilize legacy phosphorus. Therefore, interventions such as macrophyte restoration are necessary to disrupt this feedback loop and restore ecosystem stability.

5. Conclusions

The historical sedimentary record of Qilu Lake clearly reveals the profound impact of coupled anthropogenic activities and natural ecosystem processes on its carbon and phosphorus cycles. The lake transitioned from a macrophyte-dominated state with a low TP and a high, stable TOC (pre-1980s) to an algae-dominated eutrophic state characterized by high TP accumulation and distinct DOM signatures. Surface sediments (0–2 cm), representing the most recent period (2020s, 2019–2023 AD), show renewed TOC enrichment dominated by protein-like DOM (52% of fluorescent components) linked to ongoing algal activity, while deeper sediments (16–24 cm, pre-1980s) preserve a record of earlier conditions with a high TOC from microbial humic substances (77%) and low TP concentrations corresponding to the limited historical phosphorus inputs.

The ecological transition period (2–16 cm, approx. 1980s–2010s) was pivotal. During this time, the accumulation of algal biomass and its derived DOM appeared to facilitate phosphate release through mechanisms such as the complexation of Ca²⁺ by algal polysaccharides (inhibiting hydroxyapatite precipitation, consistent with the observed significant negative correlation between Ca-P and protein-like DOM, p < 0.01) and ligand competition for metal-bound phosphorus. This reinforces a positive feedback loop: “algal bloom–labile DOM input–enhanced phosphorus release–intensified algal proliferation”.

This study systematically elucidated the multi-scale regulatory mechanisms controlling the coupled carbon–phosphorus cycle in Qilu Lake sediments, highlighting that the molecular characteristics of DOM are key variables governing the distribution of endogenous phosphorus. These findings have significant implications. For future research, advanced molecular techniques (e.g., XANES and FT-ICR MS) are indeed crucial to further quantify specific DOM-P interactions and identify the precise nature of OM–mineral–P complexes. Moreover, this work underscores that effective lake management and restoration strategies for Qilu Lake and similar multiply stressed plateau lakes must consider not only external nutrient load reduction, but also the internal biogeochemical cycles driven by the quality of sedimentary organic matter. Interventions aimed at altering DOM inputs and composition, such as promoting macrophyte restoration to shift the DOM towards more refractory forms, could be key to disrupting detrimental feedback loops and mitigating internal phosphorus loading. Understanding these DOM-mediated processes is also vital for predicting lake ecosystem responses to ongoing climate change and anthropogenic pressures.