Phosphorus Cycling In Sediments Research Articles

Phosphorus cycling in freshwater lake sediments: Influence of seasonal water level fluctuations

Freshwater lakes experience drastic water level fluctuations because of climate change and human activities. However, the influence of such fluctuations on phosphorus cycling in sediments has rarely been investigated. We conducted a geochemical investigation on the phosphorus cycle in a shallow freshwater lake, Dongting Lake; under the influence of human activities and climate change, its water regime undergoes drastic changes. Irrespective of the permanent inundation zone (PIZ) or seasonal inundation zone (SIZ), the phosphorus cycle in sediments was found to be dominated by the reductive dissolution of iron (Fe) (oxyhydr)oxides, degradation of organic matters, and conversion between authigenic phosphorus (Ca-P) and detrital phosphorus in individual seasons. From winter to summer, with increasing water level, the content of Fe-bound phosphorus and organic phosphorus increase due to the deposition of suspended matter, thus increasing total phosphorus in PIZ. Moreover, the rising water level also reduces the dissolved oxygen content and promotes the reductive dissolution of Fe (oxyhydr)oxides. The mineralization of increased organic matter can release CO2 and reduce pH in the vicinity, which can further result in the acidic dissolution of detrital apatite. In turn, most of the released phosphorus can be adsorbed or co-precipitated with calcium minerals, resulting in the significant increase of Ca-P. The mechanisms of phosphorus transformation in SIZ are similar to those in PIZ, but most of the increased organic matter and total P in a core from SIZ are attributable to the decomposition of plant matter. Therefore, the water level rise not only changes the conservative speciation of phosphorus in sediments to active speciation, but also triggers the release of phosphorus adsorbed to oxides and further increases the risk of phosphorus release from sediments to overlying water. Thus, our findings have major implications for freshwater shallow lakes and their P-driven productivity.

Sediment phosphorus cycling in a nutrient-rich embayment in relation to sediment phosphorus pool and release

A semi-enclosed river-mouth bay (Kojima Bay) and the artificial lake within it (Kojima Lake) were studied to examine their nutrient storage and export processes and characteristics. The aim of this study was to perform mass balance analysis using long-term monitoring and historical accumulation data of phosphorus (P) via 210Pb measurements of accumulated sediment. The results are then compared with data based on analysis of P concentrations in core samples to delineate input and output nutrient fluxes in a semi-enclosed bay and an artificial lake, in order to evaluate if the lake sediment is a source or sink of P during the transportation process from the river to the bay. From 1980 to 2008, total P flux into the lake is 3520 tons. The lake sediment phosphorus retention estimated by the model (383 tons) was lower than that obtained by multiplying the average total P concentration in sediment samples by the sediment accumulation rate (1288 tons), possibly indicating that recycling from sediments deposited before 1980 contributed to the recent P cycle. The implication for these data is that the severe eutrophication of the lake in the 1970s led to greatly elevated P levels. The inner artificial lake had a significant amount of trapped P, and the production of diatoms increased in response to P enrichment in lake water, an initial sign of eutrophication. The modeled annual P flux into the lake sediments decreased from ~ 15 g m−2 yr−1 (a flux into the sediment) in 1980 to negative values (a flux out of sediment) by 2008. The decreasing trend of nutrients accumulating in lake sediments suggests the lake is slowly recovering from eutrophication, especially since the 1990s.

Speciation and Transformation of Phosphorus in Sediments During the Redox Cycle

To study the mechanism of phosphorus cycling in sediment during the redox cycle, changes in physicochemical properties of overlying water and various forms of phosphorus in sediments were investigated as a way to quantify the redistribution of phosphorus. Additionally, the effect of the release flux of phosphate from sediments under controlled redox conditions was analyzed. The results showed that the redox potential Eh and the pH system, sulfur system, carbon system, and iron-related changes exhibited periodicity and played an important role in explaining the migration and transformation mechanism in the interface phosphorus of the sediment-water phase. During the redox cycle, the phosphorus content of each species varied with the redox conditions and time. Because of this, quantitative analysis based on changes in water-sediment phosphorus could be obtained. Reducible phosphorus (BD-P) and iron-aluminum-bound phosphorus (NaOH-rP) were reversibly redistributed into weakly adsorbed phosphorus (NH4Cl-P), polyphosphorus/organophosphorous (NaOH-nrP), residual phosphorus (Rest-P), and interstitial water-soluble active phosphorus (SRP). Additionally, 93.7% of phosphorus in the sediment was not released into the water phase during the reduction reaction. The 92% of change in the overlying water total phosphorus (TP) was the SRP of overlying water, which showed that the exchange of the sediment-water phase were mainly soluble active phosphorus in this cycle. According to Fick's First Law, the maximum phosphorus flux was 0.58 mg·(m2·d)-1 during reduction and 0.16-0.22 mg·(m2·d)-1 on day seven of the oxidation phase. In the oxidation stage, the diffusion flux decreased with time, while the opposite trend occurred in the reduction reaction. This indicated that the anaerobic state accelerated the diffusion of phosphorus in sediments, and that oxygen exposure decreased the phosphorus flux in sediments.

Roles of sorption and tube-dwelling benthos in the cycling of phosphorus in Bering Sea sediments

Adsorption of dissolved phosphate onto iron-hydroxides has been shown to be one of the primary regulators of phosphorus cycling in sediments. Bioturbation and bioirrigation by benthic infauna modify this cycling by accelerating the transport of dissolved and particulate phosphorus and by changing rates of reactions that occur in the sediment, such as the adsorption of phosphate by amorphous iron hydroxides. Hydrographic processes vary regionally in the Bering Sea and nutrient exchange between the sediments of the broad shallow shelf and overlying water may influence water column productivity. These characteristics make the Bering Sea a good study site for examining the processes that influence sedimentary cycling of phosphorus. To examine these processes, we collected samples in four domains (southern middle shelf, southern outer shelf, southern off shelf (consisting of the continental slope and Bering Sea basin) and northern Bering shelf) based on hydrographic regime. At each station we directly measured phosphate flux and sediment oxygen consumption using whole–core incubations. We also measured infaunal burrow abundances, amorphous iron-hydroxide concentrations and phosphate sorption. We found that three out of the four domains had a high affinity for trapping phosphate in the sediment, as indicated by their adsorption coefficients (6.59–81.81). However, the measured phosphate fluxes could not be explained by the adsorption capacity of the sediment alone. The results indicated that on the middle shelf, the phosphate flux positively co-varied with infaunal burrow abundances. The high number of organisms in this domain (10–32 burrows per 50cm2 core) enhances the flux of phosphate to the overlying water. Controls on the phosphate flux on the middle shelf cannot be properly understood unless benthic infaunal abundance is taken into account.

Biotic and Abiotic Pathways of Phosphorus Cycling in Minerals and Sediments: Insights from Oxygen Isotope Ratios in Phosphate

A key question to address in the development of oxygen isotope ratios in phosphate (δ(18)O(p)) as a tracer of biogeochemical cycling of phosphorus in ancient and modern environments is the nature of isotopic signatures associated with uptake and cycling of mineral-bound phosphate by microorganisms. Here, we present experimental results aimed at understanding the biotic and abiotic pathways of P cycling during biological uptake of phosphate sorbed to ferrihydrite and the selective uptake of sedimentary phosphate phases by Escherichia coli and Marinobacter aquaeolei. Results indicate that a significant fraction of ferrihydrite-bound phosphate is biologically available. The fraction of phosphate taken up by E. coli attained an equilibrium isotopic composition in a short time (<50 h) due to efficient O-isotope exchange (between O in PO(4) and O in water; that is, actual breaking and reforming of P-O bonds) (biotic pathway). The difference in isotopic composition between newly equilibrated aqueous and residual sorbed phosphate groups promoted the ion exchange (analogous to isotopic mixing) of intact phosphate ions (abiotic pathway) so that this difference gradually became negligible. In sediment containing different P phases, E. coli extracted loosely sorbed phosphate first, whereas M. aquaeolei preferred Fe-oxide-bound phosphate. The presence of bacteria always imprinted a biotic isotopic signature on the P phase that was taken up and cycled. For example, the δ(18)O(p) value of loosely sorbed phosphate shifted gradually toward equilibrium isotopic composition. The δ(18)O(p) value of Fe-oxide-bound phosphate, however, showed only slight changes initially but, when new Fe-oxides were formed, coprecipitated/occluded phosphate retained δ(18)O values of the aqueous phosphate at the time of formation of new Fe oxides. Concentrations and isotopic compositions of authigenic and detrital phosphates did not change, suggesting that these phosphate phases were not utilized by bacteria. These findings support burgeoning applications of δ(18)O(p) as a tracer of phosphorus cycling in sediments, soils, and aquatic environments and as an indicator of paleo- environmental conditions.

Sediment phosphorus cycling in a large shallow lake: spatio-temporal variation in phosphorus pools and release

Sediment and water column phosphorus fractions were recorded monthly for one year (April 2004–April 2005) in a shallow lake recovering from nutrient pollution (Loch Leven, Scotland). Equilibrium phosphate concentration (EPC0) and gross sediment phosphorus (P) release rates were estimated from laboratory experiments. Pore water and organic P pools were lowest during warm water periods whereas bottom water P was lowest during cold water periods. Reductant-soluble, organic, metal oxide-adsorbed, residual and sediment total phosphorus pools all varied significantly with overlying water depth. Short-term, high magnitude, redox initiated P release events occurred in late summer and winter as a result of anoxic sediment conditions. Lower magnitude long-term release conditions were maintained for most of the year, most likely as a result of organic P cycling and maintenance of high concentration gradients between the pore and bottom water P pools. Estimates of summer P uptake/release rates, across an intact sediment-water interface, suggested that maximum gross internal release was ~12 mg SRP m−2 lake surface area d−1 with EPC0 values ranging between 180 and 270 μg P L−1. This study highlights the biological mediation of internal loading in shallow eutrophic lakes, and in particular, the role of sediment algae in decreasing, and sediment bacteria in enhancing, sediment P release.

Phosphorus cycling in sediments of the Bohai and Yellow Seas

Using a box-sampler, surface and core sediment samples were collected from the Bohai and Yellow Seas in 1998–1999. The sediments were sequentially extracted to determine six phosphorus fractions, and non-sequentially extracted for total P (TP) and inorganic P (IP), where the difference between TP and IP was assumed to be organic P (OP). It was found that more than 70% of the total P is inorganic P. The concentrations of phosphorus were high near the river mouth, reflecting the influence from the Huanghe and Changjiang sediment loads. The phosphorus levels in the Bohai Sea are higher than those in the Yellow Sea, which is related to the sediment sources and distribution of chlorophyll a in these areas. Concentrations of TP in the Bohai and Yellow Seas are comparable to the other worldwide coastal areas. Among the six forms of phosphorus, phosphorus distribution ranges were 0.8–2% for loosely sorbed P, 3–10% for iron-bound inorganic P, 4–19% for leachable organic P, 3–15% for authigenic apatite, 35–66% for detrital apatite and 14–36% for refractory organic P. Potentially bioavailable phosphorus accounts for a small amount of TP, while phosphorus load associated with eroded soil may be larger than the anthropogenic load. The regeneration of dissolved P from sediments is a slow process, which could play a role in maintaining low phosphate levels in the seas. Most of the phosphorus was regenerated in a water column, and almost all of the remaining phosphorus was buried in the sediments after its accumulation. The high values of P burial efficiencies in the study areas were related to low benthic phosphate flux, which is due to the high percent of detrital apatite that is not bioavailable and the high sediment accumulation rate.

Temporal and spatial variations in kinetics of alkaline phosphatase in sediments of a shallow Chinese eutrophic lake (Lake Donghu)

Monthly sediment and interstitial water samples were collected in a shallow Chinese freshwater lake (Lake Donghu) from three areas to determine if alkaline phosphatase activity (APA) plays an important role in phosphorus cycling in sediment. The seasonal variability in the kinetics of APA and other relevant parameters were investigated from 1995–1996. The phosphatase hydrolyzable phosphorus (PHP) fluctuated seasonally in interstitial water, peaking in the spring. A synchronous pattern was observed in chlorophyll a contents in surface water in general. The orthophosphate ( o-P) concentrations in the interstitial water increased during the spring. An expected negative relationship between PHP and V max of APA is not evident in interstitial water. The most striking feature of the two variables is their co-occurring, which can be explained in terms of an induction mechanism. It is argued that phosphatase activity mainly contributes to the driving force of o-P regeneration from PHP in interstitial water, supporting the development of phytoplankton biomass in spring. The V max values in sediment increased during the summer, in conjunction with lower K m values in interstitial water that suggest a higher affinity for the substrate. The accumulation of organic matter in the sediment could be traced back to the breakdown of the algal spring bloom, which may stimulate APA with higher kinetic efficiency, by a combination of the higher V max in sediments plus lower K m values in interstitial water, in summer. In summary, a focus on phosphatase and its substrate in annual scale may provide a useful framework for the development of novel P cycling, possible explanations for the absence of a clear relationship between PHP and APA were PHP released from the sediment which induced APA, and the presence of kinetically higher APA both in sediment and interstitial water which permitted summer mineralization of organic matter derived from the spring bloom to occur. The study highlighted the need for distinguishing functionally distinct extracellular enzymes between the sediment and interstitial water of lakes.