Controlling internal nitrogen and phosphorus loading using Ca-poor soil capping in shallow eutrophic lakes: Long-term effects and mechanisms.

Clean soil is a potential capping material for controlling internal nutrient loading and helping the recovery of macrophytes in eutrophic lakes, but the long-term effects and underlying mechanisms of clean soil capping under in-situ conditions remain poorly understood. In this study, a three-year field capping enclosure experiment combining intact sediment core incubation, in-situ porewater sampling, isotherm adsorption experiments and analysis of sediment nitrogen (N) and phosphorus (P) fractions was conducted to assess the long-term performance of clean soil capping on internal loading in Lake Taihu. Our results indicate that clean soil has excellent P adsorption and retention capacity as an ecologically safe capping material and can effectively mitigate NH4+-N and SRP (soluble reactive P) fluxes at the sediment-water interface (SWI) and porewater SRP concentration for one year after capping. The mean NH4+-N and SRP fluxes of capping sediment were 34.86 mg m-2 h-1 and -1.58 mg m-2 h-1, compared 82.99 mg m-2 h-1 and 6.29 mg m-2 h-1 for control sediment. Clean soil controls internal NH4+-N release through cation (mainly Al3+) exchange mechanisms, while for SRP, clean soil can not only react with SRP due to its high Al and Fe content, but also stimulate the migration of active Ca2+ to the capping layer, thus precipitating as Ca-bound P (Ca-P). Clean soil capping also contributed to the restoration of macrophytes during the growing season. However, the effect of controlling internal nutrient loading only lasted for one year under in-situ conditions, after which the sediment properties returned to pre-capping conditions. Our results highlight that clean Ca-poor soil is a promising capping material and further research is needed to extend the longevity of this geoengineering technology.

From macrophyte to algae: Differentiated dominant processes for internal phosphorus release induced by suspended particulate matter deposition.

In shallow lakes, eutrophication leads to a shift of the macrophyte-dominated clear state towards an algae-dominated turbid state. Phosphorus (P) is a crucial element during this shift and is usually concentrated in the suspended particulate matter (SPM) in water. However, the dominant processes controlling internal P release in the algae- (ADA) and macrophyte-dominated (MDA) areas under the influence of P-concentrated SPM remains unclear. In this study, we conducted monthly field observations of P exchange across the sediment-water interface (SWI) with the deposition of SPM in the ADA and MDA of Lake Taihu. Results revealed that both algae- and macrophyte-originated SPM led to the depletion of oxygen across the SWI during summer and autumn. Redox-sensitive P (Fe-P) and organic P (Org-P) were the dominant mobile P fractions in both areas. High fluxes of P across the SWI were observed in both areas during the summer and autumn. However, the processes controlling P release were quite different. In MDA, P release was mostly controlled by a traditional Fe-P dissolution process influenced by the coupled cycling of iron, sulfur, and P. In the ADA, Org-P control was intensified with the deterioration of algal bloom status, accompanied with the dissolution of Fe-P. Evidence from the current study revealed that the dominant process controlling the internal P release might gradually shift from Fe-P to a coupled process of Fe-P and Org-P with the shift of the macrophyte- to an algae-dominated state in shallow eutrophic lakes. The differentiated processes in the MDA and ADA should be given more attention during future research and management of internal P loadings in eutrophic lakes.

Patterns of internal nitrogen and phosphorus loadings in a cascade reservoir with a large water level gradient: Effects of reservoir operation and water depth.

Internal nutrient loadings pose a high risk of being an additional N and P source, exacerbating eutrophication and deteriorating water quality. In this study, we selected the Daheiting Reservoir (DHTR) in North China, with a pronounced water level gradient, to investigate internal N and P loadings, estimate N and P fluxes across the sediment‒water interface based on the pore water profiles, and reveal the potential effects of water discharge from an upstream reservoir and high-intensity cage aquaculture on the risks of internal N and P release. The results indicated that DHTR presented with severe internal nutrient loadings, and the N and P fluxes showed significant spatiotemporal variations. NH4+-N and soluble reactive phosphorus (SRP) fluxes were higher in deep areas (averages of 26.14 and 9.9 mgm-2d-1, respectively) than in shallow areas near inflows (averages of 5.0 and 1.24 mgm-2d-1, respectively). Unexpectedly, the estimated NH4+-N and SRP fluxes were the lowest in summer (averages of 3.94 and 0.33 mgm-2d-1, respectively), which may have been influenced by seasonal thermal stratification and copious discharge from the hypolimnion of the upstream reservoir (Panjiakou Reservoir). Comparison of annual internal and external N and P loadings revealed that water discharge from the upstream Panjiakou Reservoir was the dominant source of N and P to the reservoir, contributing up to 83.6% of N input and 55.4% of P input. The internal P loading also contributed to water eutrophication to a great extent, accounting for 34.7% of the total P input. Our results highlight the impact of upstream reservoir discharge operation on downstream reservoir water quality and the importance of controlling the internal nutrient loading in cascade reservoirs, and further provide theoretical and practical foundations for the development of policies and strategies to conserve reservoir ecosystems.

Internal nitrogen and phosphorus loading in a seasonally stratified reservoir: Implications for eutrophication management of deep-water ecosystems.

Water eutrophication is a serious global issue because of excess external and internal nutrient inputs. Understanding the intensity and contribution of internal nitrogen (N) and phosphorus (P) loading in deep-water ecosystems is of great significance for water body eutrophication management. In this study, we combined intact sediment core incubation, high-resolution peeper (HR-Peeper) sampling, and analysis of N and P forms and other environmental factors in the water column and sediments to evaluate the contributions of internal N and P loading to water eutrophication by N and P fluxes across the sediment-water interface (SWI) of the Panjiakou Reservoir (PJKR), a deep-water ecosystem where eutrophication threatens the security of the local drinking water supply in North China. The results indicated that the PJKR showed obvious thermal and dissolved oxygen (DO) stratification in the warm seasons and full mixing in the cold seasons. The mean DO concentration was 9.9 and 3.55 mg/L in the epilimnion and hypolimnion, respectively, in warm seasons and 10.7 mg/L in cold seasons. The sediment acted as a source of soluble reactive phosphorus (SRP), NH4+-N, and NO2--N and a sink of NO3--N. The SRP fluxes were 5.28 ± 4.34 and 2.30 ± 1.93 mg m-2·d-1 in warm and cold seasons, respectively, and the dissolved inorganic nitrogen (DIN) fluxes were -0.66 ± 47.84 and 44.04 ± 84.05 mg m-2·d-1. Seasonal hypoxia accelerated the release of P rather than N from the sediments in warm seasons, which came mainly from Fe-P and Org-P under anoxic conditions. The strong negative NO3--N flux (diffusion from the water column to the sediment) implied an intensive denitrification process at the SWI, which can counteract the release flux of NH4+-N and NO2--N, resulting in the sediment acting as a weak dissolved inorganic nitrogen (DIN) source for the overlying water. We also found that internal N loading accounted for only ∼9% of the total N loading, while internal P loading accounted for 43% of the total P loading of the reservoir. Our results highlight that efforts to manage the internal loading of deep-water ecosystems should focus on P and seasonal hypoxia.

Adsorption of humic acids to lake sediments: Compositional fractionation, inhibitory effect of phosphate, and implications for lake eutrophication.

Humic acid (HA) and phosphate interactions play a vital role in the biogeochemical cycle of carbon and nutrients and thus the trophic state of a lake. The adsorption behavior of HAs to sediments in the absence and presence of phosphate was investigated in this study. Three types of HAs were used, AHA from algae-dominated lake sediments, MHA from macrophyte-dominated lake sediments, and a reference HA (RHA) with terrestrial sources. The adsorption capacity of lake sediments was highest for AHA, which can be explained by that AHA contained more carboxyl-containing molecules, proteinaceous compounds and polysaccharides that were preferentially adsorbed by minerals. Phosphate showed a stronger inhibitory effect on MHA adsorption than on AHA adsorption, suggesting that AHA can more effectively replace phosphate adsorbed to sediments. Our findings show that the functional groups of organic compounds control not only their fractionation and burial but also their ability to replace phosphate in sediments. We propose a novel mechanism to explain the legacy effect of lake eutrophication. That is, as lakes shift from a macrophyte-dominated state to more eutrophic, algae-dominated state, increasing algae-derived organic compounds can promote the release of phosphate from sediments, forming a positive feedback loop that sustains internal phosphorus loading and hence lake eutrophication.

Dramatic temporal variations in methane levels in black bloom prone areas of a shallow eutrophic lake.

Global lakes serve as a key natural source of methane (CH4) and suffer from increasing hypoxia due to unprecedented anthropogenic activities and climate change. A black bloom is a temporary hypoxia triggered by a longstanding algal bloom, which facilitates CH4 production by creating reducing conditions and abundant algae-sourced organic carbon. One-year investigations were conducted to examine temporal CH4 dynamics in the water and sediment pore water in black bloom prone areas (BBPAs) in Lake Taihu, China, where there had been at least two recorded black bloom events. The CH4 in the water changed significantly with time (p < 0.001), with the highest concentrations appearing in warm months when an abnormal lower dissolved oxygen content was observed at different sites, which were one to two orders of magnitude higher than other months. Compared with the control site, there were significantly higher CH4 concentrations in BBPA waters (p < 0.001), which was consistent with the higher CH4 in the sediment pore water. Methane dynamics in the water showed significant positive correlations with temperature, total phosphorus, total nitrogen, ammonia-N, and soluble reactive phosphorus (p < 0.05), but showed a significant inverse correlation with dissolved oxygen (p < 0.01). Redundancy analysis indicated dissolved oxygen made the largest contribution to CH4 dynamics in the BBPAs. A significant increase in the CH4 in water will turn BBPAs into temporary hot spots with substantial CH4 emissions with the appearance of black blooms. The results provide new insights into understanding future CH4 dynamics under globally prevailing algal blooms and climate change.

Vertical profiles of phosphorus fractions in the sediment in a chain of reservoirs in North China: Implications for pollution source, bioavailability, and eutrophication.

The level of eutrophication in reservoirs is dependent on their internal and external P loads. Identifying the P pollution characteristics and its fractional composition in sediments is therefore necessary to determine the potential bioavailability and dominant sources of P for effective water pollution control. In this study, we investigated the P pollution characteristics in the overlying water and sediment in a chain of reservoirs (the Panjiakou (PJK), Daheiting (DHT) and Yuqiao (YQ) Reservoirs) in North China. Our results showed that the P concentrations in the overlying water of the YQ Reservoir was higher than that of the PJK and DHT Reservoirs, but the sediment P loading and P bio-availability were lower than the PJK and DHT Reservoirs. However, the sediment P release risk in the YQ Reservoir was higher than the DHT and PJK Reservoirs. The YQ Reservoir was mainly polluted by internal sediment P release and external sources predominantly derived from the inflowing polluted Sha River Basin. Various forms of P in the DHT Reservoir decreased with depth, and the P in the overlying water column was mainly sourced from internal P release due to sediment accumulation of excess P from human activities. In recent years, the proportion of bio-available P (BAP) in the PJK and YQ Reservoirs had increased, and the proportion of the more inert Al-P and Ca-P in the PJK Reservoir decreased. Ca-P in the YQ Reservoir had also decreased, indicating that inert P has been gradually transformed into active P in the PJK and YQ Reservoirs in recent years. The observed differences in P loading and sedimentary P fractions indicate different pollution characteristics and sources between the three reservoirs. We therefore recommend site-specific remediation strategies for effective control on P pollution in the three eutrophic reservoirs.

Dredging method effects on sediment resuspension and nutrient release across the sediment-water interface in Lake Taihu, China.

Environmental sediment dredging is one of the most common methods for the remediation of contaminated sediments in lakes; however, debate continues as to whether the effectiveness of dredging methods contributes to this phenomenon. To determine sediment resuspension and nutrient release following dredging with a variety of dredging methods, four dredging treatments at wind speeds of 0-5.2 m/s were simulated in this study, namely suction dredging (SD), grab dredging (GD), ideal dredging with no residual sediments (ID), and non-dredging (ND). Field sediments from suction and grab dredging areas (including post-dredged and non-dredged sediments) of Lake Taihu were used to assess the release abilities of soluble reactive phosphorus (SRP) and ammonia nitrogen (NH4+-N) from the sediment-water interface. The effects of residual sediments on nutrient concentrations in water were also evaluated. The results reveal that inhibition of resuspension of particulate matter and nutrients released through sediment dredging decreases with increasing levels of residual sediment. Total suspended particulate matter content in the mean water columns of ID, SD, and GD under wind-induced disturbance (1.7-5.2 m/s) decreased by 67.5%, 56.8%, and 44.3%, respectively; total nitrogen and total phosphorus in ID (SD) treatments were 19.8% (12.9%) and 24.5% (11.2%) lower than that in ND treatment. However, there were ~ 1.6 and 1.5 times higher SRP and NH4+-N in the GD treatment compared with the ND treatment at the end of the resuspension experiment (0 m/s). A significant increase in the SRP and NH4+-N release rates at the sediment-water interface was also observed in field sediments from a grab dredging area, indicating that GD may pose a short-term risk of nutrient release to the water body. Hence, dredging methods with less residual sediments both during and after dredging improves the dredging quality.

Water-level alterations modified nitrogen cycling across sediment-water interface in the Three Gorges Reservoir.

Water-level regime alteration-associated redox fluctuation plays a primary role in governing exchange and transformation of nitrogen (N) in water-level fluctuation zones (WLFZs), while few understanding of how hydrological regimes under reservoir operation affected N cycling across the sediment-water interface (SWI), giving rise to uncertainties in reservoir N nutrient management. Batch microcosm simulation experiments with intact sediment cores from WLFZs of the Three Gorges Reservoir (TGR) were conducted for 24 days to identify holistic flooding-drying process mechanism on N-cycling patterns. Our results showed a distinct transition of N-cycling mode across the SWI, shifting from biological denitrogen loss dominated in initial period of flooding to enhance endogenous N retention. A dramatic source-sink switch of nitrous oxide (N2O) occurred in the first 1.5 days during the flooding period. However, combined accelerating migration of NH4+-N from sediment to overlying water, and subsequently enhanced transformation of NH4+-N to NO3--N formed from flooding to drying rotation, thereby increasing N loading to overlying water. The reason for this investigation could be attributed to intensive N loss through coupled nitrification and denitrification in oxic-anoxic microenvironments after flooding. With oxygen replenishment from atmosphere during drying phase, persistent ammonification of organic N in sediments provided sufficient source of NH4+-N for the formation of NO3--N fraction in a more oxic overlying water. Therefore, water-level regime alteration by reservoir operation was capable of weakening N removal from water body and lengthening internal N turnover time across redox-variable SWI. These findings elucidate new understanding of holistic hydrological regime mechanisms on N cycling across SWI and provide insight to biogenic N nutrient management for improving the green credentials of hydroelectric reservoir.

Exchanges of nitrogen and phosphorus across the sediment-water interface influenced by the external suspended particulate matter and the residual matter after dredging.

Dredging is frequently implemented for the reduction of internal nitrogen (N) and phosphorus (P) loadings and the control of eutrophication. Residuals during dredging activities and external pollution loadings after dredging both commonly contribute to influence the effectiveness of dredging and have been widely discussed. In the current study, the exchanges of N and P across the sediment-water interface (SWI) to these two factors were compared in a six-month field incubation experiment. The results showed that the continuous deposition of external suspended particulate matter (SPM) led ammonium nitrogen (NH4+N) and soluble reactive phosphorus (SRP) fluxes across the newly formed SWI to increase by factors of 4.16 and 12.71, respectively, while residual material caused the same fluxes to increase by factors of 2.06 and 5.06. Both the deposition of external SPM and the residual matter led to higher increase of the fluxes of P across the SWI than those of the fluxes of N across the SWI after dredging. The SPM easily adsorbed P in the water due to extensive adsorption of water soluble organic matter (consisting primarily of easily-decomposed humic-like substances), iron, and aluminum. However, the decomposition of organic matter in the SPM after the deposition on the dredged sediment accelerated the dissolution of redox-sensitive P and organic P across the SWI after dredging. Both the increase in the fluxes of N and P across the SWI would further increase the concentrations of N and P in the overlying water and thereby aggravate the eutrophication status in lakes. More frequent dredging operations might be necessary to reduce the fluxes of N and P from the sediment due to the continuous influence of the external SPM and the residual matter.

An investigation of the effects of capping on internal phosphorus release from sediments under rooted macrophytes (Phragmites australis) revegetation.

In eutrophic lake restorations, in situ capping is an often considered method to control sediment internal phosphorus (P) pollution for mitigating eutrophication status. Subsequent aquatic macrophyte revegetation can directly derive P from the sediment for growth. However, the effects of capping with clean soils on internal P release from sediments under rooted aquatic macrophyte revegetation are still unclear. In the present study, the influences of sediment P remobilization by P. australis revegetation on P inactivation by capping were investigated based on an entire growth simulation study. Our findings showed during the growth of P. australis, tests conducted on total phosphorous (TP), calcium-bound P (Ca-P), loosely bound P (loose-P), organic P (Org-P), and iron-adsorbed P (Fe-P) found significant changes (p < 0.001). Specifically, the mean contents of TP and Ca-P decreased by 291.1 and 224.2 mg kg-1, respectively, while those of Fe-P increased from 26.4 to 124.8 mg kg-1. In addition, sediment mobile-P contents increased coincidentally with the growth of P. australis during the whole course of experiment. Further analysis indicated calculated diffusion fluxes of soluble reactive phosphorus (SRP) generally increased with incubation time, although capping effectively induced the reduction of SRP concentration in pore water and its release to waters. Therefore, sediment P remobilization by P. australis revegetation was able to enhance P lability in lake sediments, with intermediate activation ability compared to other correlated water bodies. This phenomenon was most likely attributed to solubilization of sediment P by organic acids secreted from P. australis rhizosphere. Overall, sediment P remobilization by rooted macrophytes is unfavorable for capping to control internal P release to water column during eutrophic lake restorations.

Fifteen-year study of environmental dredging effect on variation of nitrogen and phosphorus exchange across the sediment-water interface of an urban lake.

Environmental dredging has been applied widely in Chinese lakes to reduce their internal nutrient loads. However, the efficacy of dredging to reduce internal loading of nitrogen (N) and phosphorus (P) and to improve water quality has been questioned by some researchers. In this study, the long-term (∼15 years) effects of dredging to reduce internal N and P loading in a closed, polluted urban lake were investigated. The results showed that the release of soluble reactive phosphorus (SRP) could be suppressed quickly after dredging, and that the dredging effect was sustained for about 18 months. A significant release of NH4+-N was discovered during the first 2-8 months after dredging, followed by maintenance of low-level release rates for about 21-32 months. The continuous inflowing of external pollution loading led to the increase in the release rates of SRP and NH4+-N. The external pollution loading was therefore reduced three years after dredging to strengthen the remediation effect. After that, high diffusive flux from the sediment was observed for both NH4+-N and SRP during summer seasons for about six years, followed by a decreasing trend. The NH4+-N concentration in the overlying water was reduced after the reduction of external loading, while a high concentration of SRP in the overlying water was still observed during summer seasons. In conclusion, the mid-term (<3 years) reduction of internal N and P loading could be achieved by dredging if the external pollution loading were not reduced. Achieving long-term control would require modification of external loading.

[Simulation research on the release of internal nutrients affected by different dredging methods in lake].

A simulated experiment was carried out to study release features of internal source under different sediment dredging methods and the difference between two lake areas in Lake Taihu was also studied. The contaminated sediments were sampled from two sites in Meiliang Bay which were the Inner Bay (A) and the Outer Bay(B). Release rates of phosphorus after ideal dredging and suction dredging are about 20% and 72% of the control and the phosphorus release rate in Inner Bay(A) is about 80% of Outer Bay(B). Release rates of ammonia after ideal dredging and suction dredging are about 40% and 83% of the scallop dredging, but dredging process may even promote the release of ammonia in a short time, the ammonia release rate in Inner Bay(A) is about 150% higher than that in Outer Bay(B). Under the microcosm experiment condition, the ideal dredging method and the suction dredging method may have a better control of internal source in contrast with the scallop dredging. Altogether, sediment dredging may be a useful approach to decrease the release of internal source in the selected sites when the external nutrients are effectively controlled. Consider all kinds of dredging projects, the suction dredging should be the ideal option for sediment dredging in Lake Taihu.

[Influence of dredging on sediment resuspension and phosphorus transfer in lake: a simulation study].

A simulated experiment was conducted to investigate the impacts of sediment dredging on sediment resuspension and phosphorus transfer in the summer and winter seasons under the common wind-wave disturbance, and the contaminated sediment used in this study was from Meiliang Bay, Taihu lake. The result showed that 20 cm dredging could effectively inhibit the sediment resuspension in study area, dredging in winter has a better effect than that in summer, and the higher values of the total suspended solid (TSS) in undredged and dredged water column during the process of wind wave disturbance were 7.0 and 2.2, 24.3 and 6.4 times higher than the initial value in summer and winter simulation respectively. The paired-samples t-test result demonstrated that total phosphorus (TP) and phosphate (PO4(3-)-P) loading positively correlated to TSS content in dredged (P<0.01) and undredged water column (P<0.05), which proved that internal phosphorus fulminating release induced by wind-wave disturbance would significantly increase the TP and PO4(3-)-P loading in the water column. The effect of dredging conducted in summer on the TP and PO4(3)-P loading in the water column was negative, but not for winter dredging (P<0.01). The pore water dissolved reactive phosphorus (DRP) profile at water-sediment interface in summer simulation was also investigated by diffusive gradients in thin films (DGT) technique. Diffusion layer of the DRP profile in undredged sediment was wider than that in dredged sediment. However, the DRP diffusion potential in dredged sediment was greater than that in undredged sediment, showing that dredging can effectively reduce the risk of the DRP potential release in dredged pore water, but also would induce the DRP fulminating release in the short time under hydrodynamic action. Generally, dredging was usually deployed during the summer and the autumn. Considering Taihu Lake is a large, shallow, eutrophic lake and the contaminant distribution is spatially heterogeneous, it is vital to determine the optimal time, depth and scope of dredging.

[Impacts of Asian clams (Corbicula fluminea) on lake sediment properties and phosphorus movement].

To examine the impact of Corbicula fluminea on sediment properties and phosphorus dynamics across sediment-water interface in lake, the microcosm experiment was carried out with sediment and lake water from the estuary of Dapu River, a eutrophic area in Taihu Lake. Rhizon samplers were used to acquire pore water, and soluble reactive phosphorus (SRP) flux across sediment-water interface and sediment properties were determined. The activity of C. fluminea destroyed the initial sediment structure, mixed sediment in different depths, increased oxygen penetration depth, sediment water content, and total microbial activity in sediment. The downward movement of overlying water was enhanced by the activity of C. fluminea, which decreased Fe2+ in pore water by oxidation. The production of ferric iron oxyhydroxide adsorbed SRP from pore water and decreased SRP concentration in pore water, and this increased iron bound phosphorus in corresponding sediment. The emergence of C. fluminea accelerated SRP release from sediment to overlying water, and enhanced SRP flux increased with the rise of introduced C. fluminea density. Metabolization of C. fluminea might play an important role in accelerating SRP release.

[Environment effects of algae-caused black spots: driving effects on the N, P changes in the water-sediment interface].

The impact and driving effect of deposited algal cells in the water-sediment interface on the N, P changes were studied through continuous extracted pore water with home-made static experiment. Results showed that dissolved oxygen in water-sediment interface was depleted in 50 min after algal cells settled. Soon the dead algal cells formed the anoxia and strong reducing environment and the dead cells had a severe anaerobic mineralization in the water-sediment interface, also the water bodies had a intense black and stink phenomenon. PO4(-3) -P, NH4(+) -N concentration in water-sediment interface increased from the 2nd day after added the algal cells to the sediment interface, and its concentration was 4.00 mg/L and 39.45 mg/L, respectively. Its concentration was the 10 fold and 241 fold higher than that the control experiments at the same time (PO4(-3) -P, NH4(+) -N concentration in control experiments was 0.42 mg/L and 0.16 mg/L, respectively). Anaerobic mineralization of dead cells in sediment surface drove the nutrients diffusing upward the overlying water, added the nutrients concentration in water bodies, and it also supplied the nutrient materials for the algal blooms happened again.

[Estimation of releasing fluxes of sediment nitrogen and phosphorus in Fubao Bay in Dianchi Lake].

NH4(+)-N and PO4(3-)-P release from the sediment of Fubao Bay in Dianchi Lake were simulated using static incubations of intact sediment cores. The results showed that the releasing rates of sediment NH4(+)-N and PO4(3-)-P were 22.941-163.117 mg x (m2 x d)(-1) and 0.90-2.06 mg x (m2 x d)(-1), respectively, with large variation in different sampling areas. The lower releasing rates were associated with declined concentration gradients between sediment pore water and overlying water,which might be related to the high input of sewages in the bay. Using peeper (in situ dialysis membrane samplers) method, concentration gradients of NH4(+)-N and PO4(3-)-P were obtained at the interstitial water-overlying water profiles. Based on calculations using the Fick's First Law,the releasing rates of sediment NH4(+)-N and PO4(3-)-P were estimated at 2.85-81.96 mg x (m2 x d)(-1) and 0.118-0.265 mg x (m2 x d)(-1), respectively. Comparison of the two methods using to estimate the fluxes of sediment NH4(+)-N and PO4(3-)-P, the NH4(+)-N and PO4(3-)-P flux from the calculation using Fick's First Law was lower than that using static incubations. For the whole bay,the area-weighted releasing fluxes of NH4(+)-N and PO4(3-)-P from static incubations were estimated to be (49.9 +/- 8.8) t x a(-1) and (0.79 +/- 0.53) t x a(-1), respectively.

Effects of hydrodynamics processes on phosphorus fluxes from sediment in large, shallow Taihu Lake.

The turnover of phosphorus (P) in lake sediments, a major cause of eutrophication and subsequent deterioration of water quality, is in need of deep understanding. In this study, effects of resuspension on P release were studied in cylindrical microcosms with Y-shape apparatus. The results indicated that there was a positive correlation between flux of suspended substance across sediment-water interface (F(SS)) and the wind speed, and an increasing F(SS) during each wind process followed by a steady state. The maximal F(SS) under light, moderate, and strong wind conditions were 299.9 +/- 41.1, 573.4 +/- 61.7, and 2093.8 +/- 215.7 g/m2, respectively. However, flux of P across sediment-water interface (F(P)) did not follow a similar pattern as F(SS) responding to wind intensity, which increased and reached the maximum in initial 120 min for light wind, then decreased gradually, with maximal flux of 9.4 +/- 1.9 mg/m2. A rapid increase of F(P) at the first 30 min was observed under moderate wind, with maximal flux of 11.2 +/- 0.6 mg/m2. Surprisingly, strong wind caused less F(P) than under light and moderate wind conditions with maximal flux of 3.5 +/- 0.9 mg/m2. F(SS) in water column declined obviously during the sedimentation process after winds, but F(P) varied with wind regime. No obvious difference was detected on F(P) after 8 h sedimentation process, compared with the initial value, which means little redundant P left in the water column after winds.