Laboratory-determined Phosphorus Flux from Lake Sediments as a Measure of Internal Phosphorus Loading

Eutrophication is a water quality issue in lakes worldwide, and there is a critical need to identify and control nutrient sources. Internal phosphorus (P) loading from lake sediments can account for a substantial portion of the total P load in eutrophic, and some mesotrophic, lakes. Laboratory determination of P release rates from sediment cores is one approach for determining the role of internal P loading and guiding management decisions. Two principal alternatives to experimental determination of sediment P release exist for estimating internal load: in situ measurements of changes in hypolimnetic P over time and P mass balance. The experimental approach using laboratory-based sediment incubations to quantify internal P load is a direct method, making it a valuable tool for lake management and restoration.Laboratory incubations of sediment cores can help determine the relative importance of internal vs. external P loads, as well as be used to answer a variety of lake management and research questions. We illustrate the use of sediment core incubations to assess the effectiveness of an aluminum sulfate (alum) treatment for reducing sediment P release. Other research questions that can be investigated using this approach include the effects of sediment resuspension and bioturbation on P release.The approach also has limitations. Assumptions must be made with respect to: extrapolating results from sediment cores to the entire lake; deciding over what time periods to measure nutrient release; and addressing possible core tube artifacts. A comprehensive dissolved oxygen monitoring strategy to assess temporal and spatial redox status in the lake provides greater confidence in annual P loads estimated from sediment core incubations.     

减轻二滩水库泥沙淤积的生态系统服务价值评估

基于ArcGIS 9.2,构建了生态系统减轻水库泥沙淤积的评估模型,并结合泥沙输移比（SDR）和通用水土流失方程（USLE）,模拟了二滩水库集水区的产沙量和保沙量及其空间分布特征以及水库使用年限内其生态系统服务价值.结果表明：2000年,二滩水库集水区土壤保持量12.1×10⁸ t·a^-1,土壤保持量高值区主要分布于雅砻江干流和支流水网附近;泥沙输移比的高值主要分布在河道附近和水库周边,水库周边是最主要的产沙区和保沙区;研究区实际产沙量为629.3×10⁴ t·a^-1,占实际土壤侵蚀量的12.7%;农田是研究区最主要的产沙源,其产沙量占集水区总产沙量的62.9%;林地平均单位面积的保沙价值最高.在二滩水库使用年限内（100 a）,集水区对于减轻二滩水库泥沙淤积的总价值为27.53亿元.     

Phosphorus dynamics and loading in the turbid Minnesota River (USA): controls and recycling potential

Phosphorus (P) dynamics in the agriculturally-dominated Minnesota River (USA) were examined in the lower 40 mile reach in relation to hydrology, loading sources, suspended sediment, and chlorophyll to identify potential biotic and abiotic controls over concentrations of soluble P and the recycling potential of particulate P during transport to the Upper Mississippi River. Within this reach, wastewater treatment plant (WWTP) contributions as soluble reactive P (SRP) were greatest during very low discharge and declined with increasing discharge and nonpoint source P loading. Concentrations of SRP declined during low discharge in conjunction with increases in chlorophyll, suggesting biotic transformation to particulate P via phytoplankton uptake. During higher discharge periods, SRP was constant at ~0.115 mg l⁻¹ and coincided with an independently measured equilibrium P concentration (EPC) for suspended sediment in the river, suggesting abiotic control over SRP via phosphate buffering. Particulate P (PP) accounted for 66% of the annual total P load. Redox-sensitive PP, estimated using extraction procedures, represented 43% of the PP. Recycling potential of this load via diffusive sediment P flux under anoxic conditions was conservatively estimated as ~17 mg m⁻² d⁻¹ using published regression equations. The reactive nature and high P recycling potential of suspended sediment loads in the Minnesota River has important consequences for eutrophication of the Upper Mississippi River.     

Longitudinal Gradients in the Chemical and Microbial Composition of the Bottom Sediment in a Channel Reservoir (Saidenbach R., Saxony)

Saidenbach Reservoir is situated in the Erzgebirge Hills, Saxony, in a catchment with agriculture as the principal use and a high sensitivity to soil erosion. The high elimination of total phosphorus (ca. 70%) mainly results from a) the sedimentation flux of planktonic diatoms and b) the adsorption to and sedimentation with mineral particles rich in iron. At stations with a high sulfide concentration in the 0–5 cm sediment layer (about 0.2%), the P content was likewise high (about 0.3%), probably due to the high iron concentration (more than 2.5% of dry weight). A fractionation procedure showed that the major part of the sediment phosphorus was not sensitive to reductants (dithionite) but has been found only in the following extraction step (NaOH extract).     

Sediment resuspension: rescue or downfall of a thermally stratified eutrophic lake?

The aim of this study was to estimate the effect of sediment resuspension, a common phenomenon in many lakes, on the phosphorus budget of a eutrophicated lake. We used two different approaches, mass balance calculations and spatially comprehensive resuspension measurements, to determine the level of phosphorus loading from which rehabilitation action is started in a dimictic north temperate lake. The effect of resuspension was assumed to be significant, since it often is a governing process for cycling of material in lakes. Internal loading was multifold to that of external loading as determined by the budget calculation. Spatially comprehensive sedimentation and resuspension measurements were necessary, since deep site versus spatially comprehensive measurements had a marked difference in their results. Resuspension of P slightly exceeded the internal loading assessed by budget calculations and thereby proved its significance as a governing in-lake process that influences P cycling strongly. The shallow areas were of importance, since most of the total P load originated from there. The fate of P after resuspension depends on the retention capacity of resuspended particles in addition to prevailing biological and physico-chemical conditions. Therefore, sediment resuspension can either strengthen or diminish internal nutrient load and the processes of the shallow zones are of importance.     

Changes in the cycling of phosphorus in the Upper Kis-Balaton Reservoir following external load reduction

1. The Upper Kis-Balaton Reservoir was opened in 1985 to control eutrophication in Lake Balaton. High external TP load of the reservoir halved in 1991 due to improved sewage treatment. Annual TP retention efficiency of the reservoir dropped immediately from 50 to 20%, while the hypertrophic algal biomass started to decrease after 1995.
2. We examine mass balances of the main forms of P in three consecutive areas (1, 2 and 3) of the reservoir. Biologically available P (BAP) includes soluble reactive P and algal P, whereas the rest of TP is considered to be non-available (nBAP).
3. Important pathways for P cycling varied in both space and time. A reduction in external load induced an enhanced net internal P load in the reservoir, and a variety of changes in the three areas. Thus, in the smallest Area 1, BAP retention dropped almost to zero, and the net sedimentation of nBAP decreased significantly. In Area 2 net BAP retention was characteristic before management, and net internal P load was observed afterwards. In Area 3 net internal P load has been substantial in some years ever since the reservoir was filled. The OECD model, corrected for internal P load, reasonably predicted the measured TP concentration in each area both before and after reduction in load.
4. The external load-algal biomass curve differed in the three areas. An immediate response occurred in the upstream Areas 1 and 2, while a delay of 4–5 years was observed in Area 3. Different long-term sediment dynamics may underlay these load-response relations in the three areas. Intense sediment redistribution, however, sets a serious constraint to the simple calculation of a mass balance. An evaluation of the long-term sediment dynamics is needed in order to predict future performance of the reservoir.     

A Simple Model of Phosphorus Retention Evoked by Submerged Macrophytes in Lowland Rivers

This study was designed to quantify and model the effects of macrophytes on phosphorus retention in a lowland river. The seasonal course of phosphorus retention was calculated from the measured difference in TP between beginning and end of a 30-km river course and the estimated lateral P input. The coverage of submersed macrophytes was mapped and coincided with the difference between theoretical water level (without vegetation) and the observed one. Therefore, the increase in water level was used as measure of the macrophytes’ abundance. In years with rare vegetation (1991–1994), P was retained in winter and remobilized in summer. In years with dense stands of macrophytes (1995–2002), net P retention was highest in summer and amounted up to 20% of TP load, and was negative during winter. The annual P budget was close to zero in both periods. The found sinusoidal annual pattern of total phosphorus retention was used to create a retention model for vegetated lowland rivers.     

Internal and external loading as regulators of nutrient concentrations in the agriculturally loaded Lake Pyhäjärvi (southwest Finland)

This study assesses the effects of external and internalloading on the nutrient concentrations in an agriculturallyloaded shallow lake. Using 13 years of observations of thelake's input and outflow, we calculated the long-term balancesof Tot-P and Tot-N. A more detailed balance, which includeddissolved nutrients and suspended solids, was estimated for anice-free period of one year. The contribution of the externalload was assessed using a mass-balance model. The internalload was estimated from the nutrient balances and on the basisof sedimentation measurements and bioassays. The drainagebasin of the lake provided most of the external nutrientinput; the remaining load was derived from atmosphericdeposition to the lake. The proportions of river-transported Pand N in dissolved form were 25% and 77%, respectively. Thelake retained >80% of the external load. Particulatenutrients settled to the bottom and were probably resuspendedseveral times before permanent sedimentation. Dissolvednutrients were bound by primary producers and a highproportion of dissolved P was removed with the fish catch.Dissolved N was also lost via denitrification. Themass-balance model showed that external loading only partlyregulated the mean annual nutrient concentrations in the lake.The regulation was probably due to internal loading, which washigh despite the efficient net retention of nutrients. Duringthe ice-free period, the temporal variations in nutrientconcentrations were controlled almost solely by internalprocesses, such as resuspension of inorganic and organicbottom matter. Although the internal load of bioavailable Pmay, under favourable conditions, exceed the external load,the mechanism by which bioavailable P is translocated from thebottom sediments to the water could not be fully identified.Abbreviations used in this paper follow the editor'srules.     

Spatial and temporal variability of particulate phosphorus fractions in seston and sediments of Lake Kinneret under changing loading scenario

Over a period of three years, the flux of particulate phosphorus to the sediment–water interface of Lake Kinneret was monitored by using seston traps deployed near the bottom of both accumulation and resuspension zones. The trap material was subjected to sequential phosphorus extraction. The obtained data set was compared to the phosphorus distribution in the surface layer of bottom sediments. Due to the sequence of drought years less allochtoneous phosphorous is reaching the lake resulting in a continuous decline of total particulate phosphorus (TPP) in the upper sediment layer. The observed decline in sedimentary TPP in spite of increased TPP sedimentation can be seen as a dilution effect due to the sedimentation of material with a relatively lower P content. The change in sedimentation can be seen as the result of increased resuspension at low lake levels. With sedimentary P in the littoral zone being unaffected by the drop in the external P load, the changes observed in the profundal zone appear to be driven by internal wave activity.     

Distribution and release of sedimentary phosphorus in Lake Ladoga

Sedimentary phosphorus fractions and phosphorus release from the sediments were studied in Lake Ladoga at altogether 46 sampling sites, representing the full range of sediment types encountered in the lake. Determination of P fractions and physico-chemical analyses were made of surface sediment cores (10–20 cm long, each sampled at 3–4 levels) and in the overlying water. The range of total phosphorus per dry weight of sediment was 0.2–3.3 mg g^–1, and that of inorganic P 0.1–2.5 mg g^–1. The levels of interstitial soluble phosphorus, range 2–613 µg 1^–1 for total P and 1–315 µg 1^–1 for inorganic P, were higher than those of dissolved P concentrations in the overlying water. Diffusive fluxes of phosphate from sediment to the overlying water were estimated using three independent methods. The estimated range was 4–914 µg P m^–2 d^–1; the mean value for the whole bottom area, 0.1 mg P m^–2 d^–1, is lower than previously published estimates. The estimated annual contribution of sedimentary inorganic P flux to Lake Ladoga water is equal to 620 tons of P per year, which amounts to more than 10% of the estimated external P load into the lake. 68% of the total diffusive flux emanates from deep water sediments, which are not exposed to seasonal variation of conditions. In deep lakes, such as Lake Ladoga, phosphorus release from the sediments is controlled primarily by diffusive mechanisms. Wave action and currents as well as bioturbation are probably of importance mainly in shallow near-shore areas. Phosphorus release by gas ebullition and macrophytes is considered negligible.     

The influence of catchment characteristics on suspended sediment properties

In order to investigate the influence of catchment characteristics on suspended sediment properties, suspended sediment samples were collected from the outlets of 60 small catchments located throughout Southwest England over a 14-month period. The catchments were selected to provide a representative range of topographic, land use, soil and geological characteristics. The suspended sediment samples were collected using time-integrating trap samplers, emptied at monthly intervals. Laboratory analysis focused on a selection of suspended sediment properties, including particle size composition, cation exchange capacity, and organic carbon, nitrogen, metal, radionuclide and phosphorus content. The results presented show that despite the substantial variability of catchment characteristics associated with the 60 sampling sites, the suspended sediment properties generally fell within a relatively limited range. However, statistical analysis identified significant spatial variability of suspended sediment within Southwest England, which in turn can be linked to catchment characteristics.     

Excessive Phosphorus Loading to Dal Lake,India: Implications for Managing Shallow Eutrophic Lakes in Urbanized Watersheds

Extensive watershed development has resulted in excessive total phosphorus (TP) loads to Dal Lake, a high altitude Himalayan lake known for its tourism and economic potential. External and internal TP loads of 5 and ∼1 g m^–2 yr^–1, respectively, were estimated for the lake. These loading rates are high in relation to the lake's critical tolerance range of 0.1–0.2 g m^–2 yr^–1, and, over time, have resulted in severe eutrophication in view of extremely high macrophyte biomass (average = 3.2 kg m^–2‐fresh weight) and bottom sediment enrichment (79 tons of TP reserves which contribute 88% of the annual TP budget). This study emphasizes the importance of external TP load reduction as a primary management objective to counteract internal TP loading and P storage within bottom sediments resulting from historic anthropogenic loads. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The role of Gloeotrichia echinulata in the transfer of phosphorus from sediments to water in Lake Erken

The abundance of Gloeotrichia echinulata colonies in the sediments of Lake Erken and their phosphorus content were investigated to determine the contribution of Gloeotrichia colonies to total sediment phosphorus. Moreover, the potential size of the algal inoculum and the migration to the water during summer were estimated.The surplus phosphorus content of the resting colonies in the sediment was about 45% of total phosphorus, which maximized at 8.5 µg P (mg dw)^–1 or 81 ng P colony^–1. The C:P ratio (by weight) in the early colonies appearing in the lake water was 50:1, while the ratio stabilized at 150 during the major migration period. The internal supply of surplus phosphorus was used during the pelagic growth of the colonies.The internal phosphorus loading to the epilimnion of Lake Erken due to Gloeotrichia migration could, from the measurements of the increase in particulate epilimnetic phosphorus, be estimated at 40 mg P m ^–2 or 2.5 mg P m^–2 d^–1 in late July and early August. Determination of the number of colonies in the sediment before and during the migration verified this value to be a conservative estimate of the internal phosphorus loading due to Gloeotrichia migration to the epilimnion in Lake Erken.The sediment P content calculated from the P concentration in early epilimnion colonies resulted in a value of 35 µg P (g dw)^–1 as a maximum. This corresponds to only 3% of the total phosphorus content in Lake Erken sediment.     

Temporal changes in the metal and phosphorus content of suspended sediment transported by Yorkshire rivers,U.K. over the last 100 years,as recorded by overbank floodplain deposits

Cores of overbank deposits were collected from locations along the Rivers Swale and Aire in Yorkshire, U.K. The middle and lower reaches of the River Aire drain heavily industrialized and urbanized areas, whereas the River Swale drains a predominantly rural catchment, although Pb and Zn mining in the headwater areas in the 19th century introduced contaminated sediment. Downcore changes in the heavy and trace metal (Al, Cr, Cu, K, Pb, Sr and Zn) and phosphorus (P) content of the floodplain sediment were used to provide evidence of temporal changes in the contaminant content of the fine-grained sediment transported by the study rivers over the last ca. 100 years. The core collected from the downstream site on the River Aire shows upcore increases in P content, which reflects the expansion of the urban area during this period. Variations in the metal content of the deposited sediment over the period represented by the core reflect changes in the extent and type of industrial activity in the catchment. For the cores from the River Swale, there are no major upcore changes in the P content of the sediment, but there was a period characterized by increased levels of Pb and Zn which can be linked to metal mining activities.     

Phosphorus Fractions and Fluxes in the Water Column and Sediments of a Tropical Reservoir (Lobo‐Broa – SP)

Sedimentation rates of organic and inorganic matter, chlorophyll a, P fractions, Ca, Mn, Fe and Al, were determined by sediment traps in a tropical oligo‐mesotrophic reservoir of São Paulo (Brazil). Vertical profiles of the sediments were analyzed for organic content, metals, P and surface P fraction composition. Estimated mean sedimentation rates, corrected for resuspension were: total solids, 1068 g m^—2 y^—1 (OM = 44.7%); chlorophyll a, 2.1 g m^—2 y^—1 and total phosphorus, 2.9 g m^—2 y^—1. The predominant P fraction in the settling flux was associated with aluminum minerals while surface sediments were dominated by organic P. The reservoir exhibited low sediment retention of P (13.0%), Al (9.9%), Fe (9.9%), Mn (1.4%) and Ca (traces), compared to trap sedimentation. This feature was related with a high vertical dynamics (resuspension and bottom release) and with the low retention time of the system.     

Eight years of internal phosphorus loading and changes in the sediment phosphorus profile of Lake Søbygaard,Denmark

During each of the first 8 years following an 80–90% reduction in external phosphorus loading of shallow, hypertrophic Lake Søbygaard, Denmark in 1982, phosphorus retention was found to be negative. Phosphorus release mainly occurred from April to October, net retention being close to zero during winter. Net internal phosphorus loading was 8 g P m^–2 y^–1 in 1983 and slowly decreased to 2 g P m^–2 y^–1 in 1990, mainly because of decreasing sediment phosphorus release during late summer and autumn. The high net release of phosphorus from Lake Søbygaard sediment is attributable to a very high phosphorus concentration and to a high transport rate in the sediment caused by bioturbation and gas ebullition. Sediment phosphorus concentration mainly decreased at a depth of 5 to 20 cm, involving sediment layers down to 23 cm. Maximum sediment phosphorus concentration, which was 11.3 mg P g^–1 dw at a depth of 14–16 cm in 1985, decreased to 8.6 mg P g^–1 dw at a depth of 16–18 cm in 1991. Phosphorus fractionation revealed that phosphorus release was accompanied by a decrease in NH₄Cl-P + NaOH-P and organic phosphorus fractions. HCl-P increased at all sediment depths. The Fe:P ratio in the superficial layer stabilized at approximately 10. Net phosphorus release can be expected to continue for another decade at the present release rate, before an Fe:P ratio of 10 will be reached in the sediment layers from which phosphorus is now being released.     

Retention of Sediments and Nutrients in the Iron Gate I Reservoir on the Danube River

This work addresses an intensively debated question in biogeochemical research: “Are large dams affecting global nutrient cycles?” It has been postulated that the largest impoundments on the Lower Danube River, the Iron Gates Reservoirs, act as a major sink for silica (Si) in the form of settling diatoms, for phosphorus (P) and to a lesser extent for nitrogen (N). This retention of P and N in the reservoir would represent a positive contribution to the nutrient reduction in the Danube River. Based on a 9-month monitoring scheme in 2001, we quantified the nutrient and the sediment retention capacity of the Iron Gate I Reservoir. The sediment accumulation corresponded to 5% TN (total nitrogen), 12% TP (total phosphorus) and 55% TSS (total suspended solids) of the incoming loading. A mass balance revealed that more N and P are leaving the reservoir than entering via the inflow. Based on these current results, the reservoir was temporarily acting as a small nutrient source. The nutrient accumulation in the sediments of the Iron Gate I Reservoir represents only 1% of the “missing” load of 10⁶ t N and 1.3 × 10⁵ t P defined as the difference between the estimated nutrient export from the Danube Basin and the measured flux entering the Black Sea. This result disproves the hypothesis that the largest impoundment on the Danube River, the Iron Gates Reservoir, plays a major role in N and P elimination.     

Estimation of the in-situ settling velocity of particles in lakes using a time series sediment trap

SUMMARY 1. We outline a simple method for estimating the in-situ settling velocity of particles in wind disturbed lakes. The total sedimentation rate (primary and secondary sedimentation) was measured during storm events in two lakes using a time series sediment trap.
2. The sediment trap had 12 sample bottles which were programmed to open for periods of 3–4 h, giving total deployment times ranging from 36 to 48 h.
3. Wave mixed layer theory was used to infer if and when sediment resuspension occurred and the first order rate equation was used to estimate the settling velocity of resuspended particles.
4. The settling velocity during three resuspension events in Lough Neagh and one in Lough Macnean were 34.3, 29.5, 24.1 and 77.3 m day⁻¹, respectively. These are similar to values obtained using recent in-situ video imaging in marine environments, but much larger than previous lake sediment trap studies.     

Lead-210 dating of sediments compared with accumulation rates estimated by natural markers and measured with sediment traps

Methods to provide accurate accumulation rates for lake models are discussed. Cores were taken in 1979 in two basins of Lake Lucerne, Switzerland, and accumulation rates were calculated by using Pb-210 dating and by a natural landslide marker of 1795 in one basin (Weggis). In the other basin (Horw Bay) the sediment accumulation rates based on the lead method were compared with yearly sedimentation rates measured by sediment traps in 1969/70. At the Weggis station, the core dating yielded sediment accumulation rates of about 400 g dry wt. m^–2 y^–1 with the lead method, averaged over a sediment depth of 4–20 cm; accumulation was about 700 g dry wt. m^–2 y^–1 with the marker method, averaged over 0–33 cm. In Horw Bay, the trap method yielded about 1300 g dry wt. M^–2 y^–1 compared with 400–1000 g dry wt. m^–2 y^–1 obtained with the lead method and related to various depth intervals. The characteristic sources of error of the three methods as well as several hypotheses for these discrepancies are discussed.