Characterization of phosphorus accumulation and release using diffusive gradients in thin films (DGT) - linking the watershed to Taihu Lake, China

Phosphorus cycle in shallow lakes affected by crucian carp (Carassius auratus): Effects of fish density and size

Crucian carp (Carassius auratus) is an omni-benthivorous fish common in many shallow lakes in China. The presence of crucian carp can contribute to the nutrient cycles in lakes and thus affect water quality. In this work, a two-by-two factorial mesocosm experiment was performed with crucian carp of different sizes and densities, to investigate their effects on the cycle of phosphorus (P). Results showed that nutrients in particulate form increased in overlying water due to crucian carp disturbance, especially for treatments with higher fish densities and larger individuals. Smaller individuals at high density have a greater ability to promote P release from sediment, due to a stronger combined effects of physical disturbance and excretion. Accumulation of feces led to sediment anaerobiosis and the reductive dissolution of iron oxide-hydroxide, which were the main factors affecting the desorption of P. Our results quantify the endogenous P diffusion fluxes across the sediment-water interface attributed to different densities and sizes of crucian carp disturbance, and suggest controlling crucian carp at low density and small size to minimize their impact on sediment P flux.

Influence of different cyanobacterial treatment methods on phosphorus cycle in shallow lake microcosms

Cyanobacterial bloom is a pressing issue affecting water supply security and ecosystem health. Phosphorus (P) released from cyanobacterial bloom during recession is one of the most important components involved in the lake P cycle. However, little is known about the consequences and mechanisms of the P cycle in overlying water and sediment due to the anthropogenic treatments of cyanobacterial blooms. In this study, treatment methods using hydrogen peroxide (H2O2), polyaluminum chloride (PAC), and the feces of silver carp were investigated for their influence on the P cycle using microcosm experiments. Results showed that H2O2 treatment significantly increased the internal cycle of sediment-related P, while PAC treatment showed minor effects. H2O2 and PAC treatment suppressed the release of P from sediment before day 10 but promoted the release of P on day 20, while silver carp treatment suppressed the release of P during the whole experiment. The reductive dissolution of iron oxide-hydroxide was the major factor affects the desorption of P. Path analyses further suggested that overlying water properties such as dissolved oxygen (DO) and oxidation-reduction potential (ORP) play critical roles in the treatment-induced sediment P release. Our results quantify the endogenous P diffusion fluxes across the sediment-water interface attributed to cyanobacterial treatments and provide useful guidance for the selection of controlling methods, with silver carp being the most recommended of the three methods studied.

Terrestrial sources regulate the endogenous phosphorus load in Taihu Lake, China after exogenous controls: Evidence from a representative lake watershed

Identifying phosphorus (P) sources and contributions from terrestrial sources is important for clean water and eutrophication management in lake watersheds. However, this remains challenging owing to the high complexity of P transport processes. The concentrations of different P fractions in the soils and sediments from Taihu Lake, a representative freshwater lake watershed, were obtained using sequential extraction procedure. The dissolved phosphate (PO₄-P) and alkaline phosphatase activity (APA) in the lake's water were also surveyed. The results showed that different P pools in the soil and sediments displayed different ranges. Higher concentrations of P fractions were measured in the solid soils and sediments from the northern and western regions of the lake watershed, indicating a larger input of P from exogenous sources, including agriculture runoff and industrial effluent from the river. Generally, higher Fe-P and Ca-P concentrations of up to 399.5 and 481.4 mg/kg were detected in soils and lake sediments, respectively. Similarly, the lake's water had higher concentrations of PO₄-P and APA in the northern region. A significant positive correlation was found between Fe-P in the soil and PO₄-P concentrations in the water. Statistical analysis indicated that appropriately 68.75% P was retained in the sediment from terrigenous sources, and 31.25% P experienced dissolution and shifted to the solution phase in the water-sediment ecosystems. The dissolution and release in Fe-P in the soils were responsible for the increase of Ca-P in the sediment after the afflux of soils into the lake. These findings suggest that soil runoff predominantly controls P occurrence in lake sediments as an exogenous source. Generally, the strategy of reducing terrestrial inputs from agricultural soil discharge is still an important step in P management at the catchment scale of lakes.

Uncertainty and Sensitivity Analysis of Input Conditions in a Large Shallow Lake Based on the Latin Hypercube Sampling and Morris Methods

We selected Tai Lake in China as the research area, and based on the Eco-lab model, we parameterized seven main external input conditions: discharge, carbon, nitrogen, phosphorus, wind speed, elevation, and temperature. We combined the LHS uncertainty analysis method and the Morris sensitivity analysis method to study the relationship between water quality and input conditions. The results showed that (1) the external input conditions had an uncertain impact on water quality. Among them, the uncertainties in total nitrogen concentration (TN) and total phosphorus concentration (TP) were mainly reflected in the lake entrance area, and the uncertainties of chlorophyll-a (Chl-a) and dissolved oxygen (DO) were mainly reflected in the lake center area. (2) The external input conditions had different sensitivities to different water layers. The bottom layer was most clearly and stably affected by input conditions. The TN and TP of the three different water layers were closely related to the flux into the lake, with average sensitivities of 83% and 78%, respectively. DO was mainly related to temperature and water elevation, with the bottom layer affected by temperatures as high as 98%. Chl-a was affected by all input factors except nitrogen and was most affected by wind speed, with an average of about 34%. Therefore, the accuracy of external input conditions can be effectively improved according to specific goals, reducing the uncertainty impact of the external input conditions of the model, and the model can provide a scientific reference for the determination of the mid- to long-term governance plan for Tai Lake in the future.

Remobilization and hypoxia-dependent migration of phosphorus at the coastal sediment-water interface

Sediment internal phosphorus (P) loading can be tightly associated with overlying water hypoxia. However, the effects of long-term seasonal hypoxia on the geochemical transition of P in P-poor coastal sediment and how this transition is linked to the early diagenesis of iron (Fe), sulfur (S) and carbon are still poorly understood. Here, we conducted a one-year monthly field investigation to study the (im)mobilization and migration of P among coastal sediment, porewater and overlying water. The coherent distribution of soluble Fe and mobile P and decoupled distribution of labile S (soluble sulfide) and mobile P in the depth profiles indicate that the redox cycling of Fe (but not S) dominates P mobility. Nevertheless, the monthly variation in the porewater soluble reactive P (SRP) presented significant positive correlations with that of the overlying water SRP. This finding highlights that hypoxia-fueled SRP migration from overlying water rather than weak diagenetic P mobilization due to deficient organic matter and solid labile P is the crucial factor responsible for internal P mobility over long time scales. Although SRP tends to migrate from overlying water to porewater, the potential risk of sediment labile P remobilization and reliberation to the overlying water is considerable.

Control of internal phosphorus release from sediments using magnetic lanthanum/iron-modified bentonite as active capping material

The non-magnetic capping materials are difficult to be recycled from the water bodies after their application, leading to the increase in the cost of the sediment remediation. To address this issue, a capping material, i.e., magnetic lanthanum/iron-modified bentonite (M-LaFeBT) was prepared by loading lanthanum onto a magnetic iron-modified bentonite (M-FeBT) and used to control the internal phosphorus (P) loading in this study. To determine the capping efficiency and mechanism of M-LaFeBT, the impact of M-LaFeBT and M-FeBT capping on the mobilization of P in sediments was investigated, and the stabilization of P bound by the M-LaFeBT and M-FeBT capping layers was evaluated. Results showed that M-LaFeBT possessed good magnetic property with a saturated magnetization of 14.9 emu/g, and exhibited good phosphate adsorption ability with a maximum monolayer sorption capacity (Q_MAX) of 14.3 mg P/g at pH 7. Moreover, M-LaFeBT capping tremendously reduced the concentration of soluble reactive P (SR-P) in the overlying water (OL-water), and the reduction efficiencies were 94.7%-97.4%. Furthermore, M-LaFeBT capping significantly decreased the concentration of SR-P in the pore water and DGT (diffusive gradient in thin films)-labile P in the profile of OL-water and sediment. Additionally, most of P bound by the M-LaFeBT capping layer (approximately 77%) was stable under natural pH and reducing conditions. The phosphate adsorption ability for M-LaFeBT was much higher than that for M-FeBT, and the Q_MAX value for the former was 4.86 times higher than that for the latter. M-LaFeBT capping gave rise to a higher reduction of DGT-labile concentration in the profile of OL-water and sediment than M-FeBT capping. The P adsorbed by the M-LaFeBT capping layer was more stable than that by the M-FeBT capping layer. Results of this study demonstrate that M-LaFeBT is promising for utilization as an active capping material to intercept sedimentary P release into OL-water.

High resolution evidence of iron-phosphorus-sulfur mobility at hypoxic sediment water interface: An insight to phosphorus remobilization using DGT-induced fluxes in sediments model

The deterioration of reservoirs in southern China due to the kinetics of Iron (Fe), Phosphorus (P) and sulphide (S) at the sediment-water interface (SWI) is a major problem that needs urgent attention. Studies on the biogeochemistry of Fe, P, and S using high-resolution profile techniques in reservoirs in this region are limited. The diffusive gradient in thin films (DGT) technique, high-resolution dialysis, DGT-computer imaging densitometry (CID), DGT-induced fluxes in sediments (DIFS) and planar optode (PO) device were used to describe the dynamics Fe-P-S in SWI during hypoxia. The results showed the release of Fe-P-S in SWI was due to sulfate reduction and iron reduction influenced greatly by hypoxia. Positive apparent fluxes were recorded indicating that the sediments release Fe-P-S to the overlying water. High positive correlations (r² > 0.7) for DGT-labile Fe and DGT-labile P in sediments revealed that iron-bound P controlled the release of P at SWI during reductive dissolution. The low correlation between DGT-labile Fe and DGT-labile S (r² < 0.4) disclosed the combative nature between sulfate reduction and iron reduction process. The low correlation occurred because of the co-precipitation between Fe and S, forming black materials such as monosulfide (FeS) and pyrite (FeS₂) in a hypoxic environment. The DIFS model showed the resupply ability (R-values) of P in sediments belonged to the partially sustained case with a steady state case of resupply at TB3 (Tc = 1088s, Kd = 1005.61 cm³/g R = 0.72, K_-1 = 0.19 day^-1) and TB4 (Tc = 712 s, Kd = 712.53 cm³/g, R = 0.78, K_-1 = 0.46 day^-1). The resupply rate belonged to the non-steady state case at TB1 (Tc = 10,990 s, Kd = 396.3 cm³/g, R = 0.35, K_-1 = 0.07 day^-1) and TB2 (Tc = 6097 s, Kd = 578.5 cm³/g, R = 0.45, K_-1 = 0.10 day^-1). The DGT-CID-PO-DIFS provided a deep insight on the mechanism of Fe-P-S and remobilization of P at SWI leading to Blackwater events and eutrophication.