Processes and mechanisms of phosphorus mobility among sediment, water, and cyanobacteria under hydrodynamic conditions.

Phosphorus (P) has an important role in eutrophication and it is essential to explore the processes and mechanisms of P mobility in natural waters. In this study, laboratory experiments were conducted to simulate the SW system (sediment and water) and SAW system (sediment, algae, and water) under four hydrodynamic intensity conditions (static control, 50 rpm, 125 rpm, and 200 rpm treatments), to investigate P mobility. Results in SW system showed that sediment was an important source of P for overlying water, and the released total P (TP) increased with stronger hydrodynamic intensity, when P associated with metal pools (redox-sensitive P [BD-P] and meta-oxides bound P [NaOH-P]) were the most unstable and easier to migrate into the overlying water. Stronger hydrodynamic disturbances could enhance the processes including sediment resuspension, dissolution of particles, and release of P, when P mobility had a close relationship with redox conditions near sediment-water interface (SWI). Therefore, the release of TP, BD-P, and NaOH-P from sediment increased and decreased in the control and 50-200 rpm treatments over time. In SAW system, the release of TP significantly increased from sediment comparing to SW system, and the growth of Microcystis aeruginosa could selectively enhance the release of BD-P, NaOH-P, and organic P (OP). Meanwhile, the released P from sediment was quickly accumulated by algal cells. The maximum accumulation ability of P by cells, the highest photosynthetic efficiency, and the best growth of M. aeruginosa were observed in 125 rpm treatment. But with excessively strong hydrodynamic intensity (200 rpm treatment), the accumulation ability of P and alkaline phosphatase activity (APA) of M. aeruginosa was suppressed, which might hinder algal utilization of P and inhibit algal growth. Overall, our findings demonstrated the patterns of P mobility in natural ecosystems and could contribute to the understanding of P cycling.

Effects of hydrodynamic on the mobility of phosphorous induced by sediment resuspension in a seepage affected alluvial channel.

The phosphorus (P) mobility caused by sediment resuspension was investigated in a tilting flume, considering the important effect of upward seepage. The water level and velocity were observed during the experimental run, and water samples were collected for the measurement of sediment and P concentrations. A lower value of P and sediment concentrations occurred at the upstream end of the test section, and then a stable trend was gradually observed towards the downstream end due to the sediment resuspension and deposition within the test section. A lower P release was identified for the seepage run, accompanied with a lower sediment concentration in the overlying water. These phenomena were directly linked with the turbulent flow characteristics such as time-averaged velocity, Reynolds shear stresses, and turbulence intensity. Results show that the level of turbulence decreased with the upward seepage, indicating lower P and sediment concentrations. The turbulent length scale also decreased with the upward seepage, which led to a decrease in the energy and momentum transfer induced by the larger eddy size in the near-bed region, and eventually, a lower rate of sediment resuspension and P release with upward seepage. The results of the present study are useful for civil or hydraulic engineers since the knowledge of P transport with sediment will improve the management of contaminated sediment in seepage affected alluvial channel.

Cyclical patterns and (im)mobilization mechanisms of phosphorus in sediments from a small creek estuary: Evidence from in situ monthly sampling and indoor experiments.

Internal phosphorus (P) mobility is crucially important to overlying water ecosystems, while its spatiotemporal variations and mechanisms remain to be studied, especially in dynamic estuarine sediments. In this study, in situ monthly field sampling and indoor experiments were combined to measure the soluble reactive P (SRP), soluble Fe and diffusive gradients in thin films (DGT)-labile P/S in the overlying water, sediment and porewater in the Jiuxi River Estuary by employing high-resolution dialysis (HR-Peeper), the DGT technique and a MicroRhizon sampler. The consistent tendency between DGT-labile S and P in most seasons indicates that P mobilization was dominated by intense dissimilatory sulfate reduction (DSR), causing high SRP concentrations and active exchange with the overlying water. The circannual cyclical pattern of P is summarized, where in addition to temperature, monthly changes in runoff and tidal range are crucial external factors to control long-term P cycling via changed redox environments and terrigenous materials inputs. The mobile P, Fe and S present higher values during flood tides and lower values during ebb tides in tidal simulation experiments, demonstrating that the short-term cycling of P, Fe and S in intertidal surface sediments is highly redox-sensitive and controlled by tidal processes. The results also reveal that DSR greatly facilitates P mobility and release, while sediment oxidation and the induced enhancement in DIR and Fe cycling can effectively control P immobilization.

Sulfur cycling in freshwater sediments: A cryptic driving force of iron deposition and phosphorus mobilization.

Sulfur cycling in freshwater ecosystems has been previously considered minor, and the direct evidence of its impacts on iron and phosphorus cycles in freshwater sediments remains unclear. In this study, mesocosms with amended acetate and various sulfate concentrations (1.5-3.0 mmol L-1) were set up to investigate sulfur cycling and its influences on iron-rich freshwater sediments. Acetate addition induced hypoxia and provided substrates, which stimulated the sulfur cycling with evidence of SO42- decline, ΣS2-, S0 increase and corresponding variations of sulfate-reducing bacteria (SRB) and sulfur-oxidizing bacteria. Meanwhile, the growth of iron-reducing bacteria (IRB) was suppressed, and lower Fe(II) release was correspondingly related to larger SRB abundance at higher sulfate level, indicating that microbial iron reduction might be blocked by SRB activities. However, continuous dissolution of Fe(III) oxides and generation of iron sulfides were observed, suggesting that sulfide-mediated chemical iron reduction (SCIR) became the dominant iron-reducing pathway, and Fe(II) was buried as iron sulfides instead of released to water column, which resulted in a transition of iron cycling into unidirectional SCIR. Consequently, continuous dissolution of Fe(III) oxides led to significant increase of PO43- concentration in the water column and sediment pore-water, revealing the phosphorus mobility in sediments derived from the SCIR process. To note, sustained accumulation of iron sulfides was observed even without ΣS2- presence, suggesting that ΣS2- precipitation occurred prior to diffusion. Thus, ΣS2--missing sulfur cycling seemed "cryptic" in this study. To highlight, the transition of the iron-reducing pathway and resulting PO43- release can be induced even under current sulfate level of Lake Taihu, and elevated sulfate levels could significantly intensify SCIR and phosphorus mineralization. Thus, the stimulated iron deposition and the resulting phosphorus release derived from the sulfur cycling should be paid more attention to in the treatment of eutrophic freshwater ecosystems.

High sulfide production induced by algae decomposition and its potential stimulation to phosphorus mobility in sediment.

This study is devoted to addressing the effects of algae blooms on sulfur cycle and the consequent phosphorus mobility in the sediments of freshwater lake ecosystems. A mesocosm experiment was conducted to investigate these effects through monitoring the dynamics of sulfur (S), iron (Fe) and phosphorus (P) in water and sediments, and their diffusion fluxes at the sediment-water interface (SWI). In addition, the abundance of sulfate-reducing bacteria (SRB) in the water column was also detected. The addition of the algae lead to an increase of SRB, a drastic decline of sulfate and a significant increase of total dissolved sulfide (ΣS2-, the peak value of near 3.0 mmol/L on day 6) in the water column. These results suggest the sulfate reduction was dramatically promoted during algae decomposition. Indeed the ΣS2- was 2 to 3 times of SO42- initial concentration, and higher ΣS2- was produced with higher algal biomass. Moreover, the diffusive flux of ΣS2- at the SWI was negative, indicating that diffusion of ΣS2- from water column toward sediment was occurring. These results indicated that algae decomposition might also be another important source of ΣS2- (termed "algae-derived ΣS2-") in addition to sulfate reduction. The increase of Fe(II) in surface sediment pore-water was slightly delayed compared to the ΣS2- generation in the water column, which illustrated that Fe oxyhydroxides in sediments were transformed into Fe(II) through chemical reduction of ΣS2-. Concomitantly, the vertical distribution of PO43- in high amounts algae group suggested that desorption and release of iron oxides-bound PO43- occurred in sediments. Collectively, algae bloom can boost the lake eutrophication not only through direct release of nutrients but also through the high production of ΣS2-and indirect promotion of phosphorus mobility in sediment.

Dissolved reactive phosphorus played a limited role in phosphorus transport via runoff, throughflow and leaching on contrasting cropping soils from southwest Australia.

PURPOSE: Phosphorus (P) lost from agricultural land by erosion, runoff, throughflow and leaching is of major concern for water resource managers worldwide. Previous study on soils from cropping land of southwest Western Australia suggested P loss as dissolved unreactive P (DURP) via leaching, but the implications for processes and rates of P transport in soils are not known. MATERIAL AND METHODS: Two contrasting soil profiles (sand and loam) from cropping land of southwest Western Australia were exposed to artificial rain in packed boxes and field runoff plots to examine P forms and fluxes in runoff, throughflow, leachate and soil solution after three P rates of application (equivalent to 0, 20 and 40kg P/ha). Solutions were analyzed for total P (TP), dissolved reactive P (DRP) and total dissolved P (TDP). Particulate P (PP) and DURP were calculated by subtracting DRP from TP and TDP respectively. RESULT AND DISCUSSION: In the sand profile, about 90% or more of P losses via runoff and leachate were in DURP and PP forms, whereas DRP was a minor contributor. Phosphorus load in soil solution, throughflow, leachate and run-off increased with increasing P rate. The relatively higher affinity of soil for DRP compared to DURP might cause the latter to be more mobile through profile in association with colloidal compounds <0.2µm. Higher PP concentration for loam soil via throughflow is exacerbated by dispersed clay, which could be an additional process influencing P mobility in loam and duplex soils. CONCLUSION: The DRP played a limited role in P transport compared to PP and DURP that both appeared to be associated with soil particles or soil colloids in runoff, throughflow, leachate and soil solution. Further characterization of the latter forms of P is needed so that management practices can be developed to minimize P losses.

Phosphorus mobility among sediments, water and cyanobacteria enhanced by cyanobacteria blooms in eutrophic Lake Dianchi.

This study was focused on the phosphorus mobility among sediments, water and cyanobacteria in eutrophic Lake Dianchi. Four conditions lake water, water and algae, water and sediments, and three objects together were conducted to investigate the effects of cyanobacteria growth on the migration and transformation of phosphorus. Results showed a persistent correlation between the development of cyanobacterial blooms and the increase of soluble reactive phosphorus (SRP) in the lake water under the condition of three objects together. Time-course assays measuring different forms of phosphorus in sediments indicated that inorganic phosphorus (IP) and NaOH-P were relatively more easier to migrate out of sediment to the water and cyanobacteria. Further studies on phosphorus mobility showed that up to 70.2% of the released phosphorus could be absorbed by cyanobacteria, indicating that sediment is a major source of phosphorus when external loading is reduced. Time-course assays also showed that the development of cyanobacterial blooms promoted an increase in pH and a decrease in the redox potential of the lake water. The structure of the microbial communities in sediments was also significantly changed, revealed a great impaction of cyanobacterial blooms on the microbial communities in sediments, which may contribute to phosphorus release. Our study simulated the cyanobacterial blooms of Lake Dianchi and revealed that the cyanobacterial blooms is a driving force for phosphorus mobility among sediments, water and cyanobacteria. The outbreak of algal blooms caused deterioration in water quality. The P in the sediments represented a significant supply for the growth of cyanobacteria.

Mobility of phosphorus induced by sediment resuspension in the Three Gorges Reservoir by flume experiment.

The mobility of phosphorus (P) induced by sediment resuspension have been examined in a circulated flume. During the flume run, the water level and velocity were monitored, and water samples were taken for measurement of sediment and P concentrations. Peak values of both the P and sediment concentrations existed at x=4m, and then decreased slightly along the flume due to deposition. A faster P release was observed for coarser sediment, while a more sustained P release for finer sediment. Combining with the measured data from Yangtze River and sorption experiment, the relation between the load of total P (LTP) and sediment load (Qs) was estimated, and the expressions of distribution coefficient Kd and the concentration of particulate P (PP) were obtained. This study established a bridge between the small-scale sorption experiment and the field observation of natural scale, providing references for the management of contaminated sediment in natural rivers.