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.

Diffusion kinetic process of heavy metals in lacustrine sediment assessed under different redox conditions by DGT and DIFS model.

Different fractions and variations of Mn, Co, Ni, Cu, Cd, Pb, Zn, and Fe in sediment via oxic and anaerobic treatments were investigated using BCR sequential extraction methods, DGT technique, and DIFS model. The results indicated that reducible fraction was the considerable pool apart from residual fraction, suggesting the high desorption potential of heavy metals. The high-resolution DGT measurement indicated that CDGT significantly rose after anaerobic condition and characterized by the relative high R value. Significantly increasing positive fluxes varying from 0.64 to 339.4 µg cm-2 s-1 except Ni suggested that apparent diffusion upward occurred over time from the sediment to the overlying water on anaerobic episode. High proportion of reducible Fe fraction and concurrent reduction of Fe(III) to Fe(II) during anaerobic condition were responsible for the increase of labile metals. The diffusion kinetic parameters including the equilibrium distribution coefficient (Kd), response time (Tc), and rate constant (k1 and k-1) were obtained using DIFS model. These parameters confirmed the partially sustained resupply capacity of heavy metals from solid sediment particle to pore water because of the considerable reducible fractions. Additionally, planar optode (PO) imaging approach demonstrated that low pH accompanied with decreasing dissolved oxygen (DO) concentration on anaerobic condition enhanced the release of labile metal fraction. Generally, anoxia facilitated the reduction of reducible fraction of heavy metals and further strengthened the desorption, resupply and diffusion in the aquatic ecosystems.

In-situ, high-resolution evidence from water-sediment interface for significant role of iron bound phosphorus in eutrophic lake.

Potential release of phosphorus (P) bound to iron (Fe) is critical because of the aggravating effects on P load in aquatic ecosystems. However, the process is largely unknown due to the absence of in-situ high-resolution evidence. Dissolved oxygen (DO), ferrous ion (Fe2+), and dissolved reactive phosphate (DRP) in interstitial water of sediment columns from a eutrophic shallow lake were measured using the novel colorimetric planar optode imaging method and ZrO-Chelex DGT technology during controlled experimental episodes. The solid Fe and P fractions in sediments were also simultaneously evaluated by employing sequential extraction procedure and spectra scanning analysis including SEM-EDS and 57Fe-Mössbauer spectroscopy. The results demonstrated that the DO penetration depths were accordingly regulated with time, the depths depended on the oxygen supply patterns, and oxygen depletion occurred at anaerobic intervals. Considerable increases of concentrations and diffusion of Fe2+ and DRP in interstitial water upward from the deep layer into the overlying water were mirrored by decreased concentrations of solid Fe bound P and mineral phase Fe(II) during an anaerobic episode. This confirmed that the re-dissolution of solid Fe bound P pools is the most important source of labile P, and aggravates the P budget in lake water via anaerobic intervals. The reduction-precipitation mechanism of Fe bound P during different oxidation scenarios indicated that the Fe bound P in sediments can act as intermediates between Po and Ca bound P, and result in the permanent burying of authigenic Ca bound P. Significantly positive correlations (R2 ≥ 0.7783, n = 74) between labile Fe2+ and DRP on both redox conditions also provided explicit evidence for the critical role of redox controlling Fe in labile P cycling at the lacustrine sediment-water interface. These findings provide improved insight for potential controlling effort of Fe coupled P to labile P depending on the oxygen supply in shallow-water hypereutrophic lakes.

Characterization and source identification of organic phosphorus in sediments of a hypereutrophic lake.

High phosphorus (P) load and consequent algal bloom are critical issues because of their harmful effects to aquatic ecosystems. The organic phosphorus (Po) cycling and hydrolyzation pathway in the sediments of a hypereutrophic lake area with high algae biomass were investigated using stable isotopes (δ13C and δ15N) along with C/N ratios, a sequential extraction procedure, 31P NMR spectrum, and alkaline phosphatase activity (APA) was measured simultaneously. C/N ratios lower than 10 combined with lighter δ13C (-23.5 to -25.2‰) and δ15N values (3.7-9.5‰) indicated that endogenous algal debris contributed to the predominant proportions of P-containing organic matter in the sediments. Sequential extraction results showed that Po fractions decreased as nonlabile Po > moderately labile Po > biomass-Po. Decreasing humic-associated Po (HA-Po) in sediments downward suggested the degradation of high-molecular-weight Po compounds on the geological time scale to low-molecular-weight Po including fulvic-associated Po (FA-Po), which is an important source of labile Po in the sediment. An analysis of the solution 31P NMR spectrum analysis showed that important Po compound groups decreased in the order of orthophosphate monoesters > DNA-Po > phospholipids. The significant correlation indicated that orthophosphate monoesters were the predominant components of HA-Po. Rapid hydrolysis of labile orthophosphate diesters further facilitated the accumulation of orthophosphate monoesters in the sediments. Additionally, the simultaneously upward increasing trend demonstrated that APA accelerated the mineralization of Po into dissolved reactive phosphorus (DRP), which might feed back to eutrophication in algae-dominant lakes. The significantly low half-life time (T1/2) for important Po compound groups indicated faster metabolism processes, including hydrolysis and mineralization, in hypereutrophic lakes with high algae biomass. These findings provided improved insights for better understanding of the origin and cycling processes as well as management of Po in hypereutrophic lakes.