Chemical and Microbiological Processes as Regulators of the Exchange of Substances between Sediments and Water in Shallow Eutrophic Lakes

It is generally believed that excessive P release from lake sediments, i. e. internal P load, is only a problem in deep lakes with stagnant anaerobic bottom waters. However, substantial amounts of P can also be released from sediments in shallow, well-mixed lakes. The dynamics and magnitude of P release from sediments in these types of lakes are affected primarily by physical factors, such as seasonal variations in water temperature and year-to-year differences in water renewal. These factors, in turn, induce chemical and microbiological processes which regulate the exchange of substances between sediments and water. The fractional distribution of sedimentary P and the chelating capacity of the water are also important factors that can provide insights to the processes involved and their quantitative impact on the P status in shallow, eutrophic lakes.     

Spatial variation of phosphorus fractions in bottom sediments and the potential contributions to eutrophication in shallow lakes

Spatial variation of phosphorus fractions in bottom sediment, pore water and overlying water in three shallow eutrophic lakes, Nishiura, Kitaura and Sotonasakaura, Japan, and the contributions of the fractional P to mobilization of phosphorus from sediment were examined in this study. The vertical distributions of dissolved inorganic phosphorus (DIP) concentrations in overlying and pore water differed with lake and sampling site. In particular, DIP was high in pore water in the surface layer of the sediment for the middle to downlake areas of Lake Kitaura. DIP release flux calculated from a gradient of the concentrations at the sediment–water interface was high compared with other sites. The distribution of fractional P content in sediments was highly variable. The citrate–dithionite–bicarbonate–non-reactive phosphorus (CDB–NRP) fraction, in particular, differed greatly among the three lakes. According to correlation in the ratios between CDB–NRP and loss on ignition, sediments of these lakes were classified in three clusters. The CDB–NRP fraction was suggested to play a role in DIP release from sediment. The possibility of nitrate concentration playing a role in the control of DIP release was considered.     

Mobility of phosphorus fractions in the sediments of Lake Balaton

Sediment phosporus was studied by a combination of the fractionation procedure of Hieltjes & Lijklema and the isotopic dilution technique in a mesotrophic (Tihany) and a hypertrophic (Keszthely) basin of Lake Balaton.In the calcareous sediments the largest part of phosphorus was bound to calcium. Iron-bound and residual P showed higher concentrations at Keszthely than at Tihany. There was little loosely adsorbed P at both locations. Vertical differences in P fractions of the sediments were more pronounced at Keszthely than at Tihany. Exchangeability of the fractions decreased in the following sequence: iron-bound > loosely adsorbed > calcium-bound > residual. Phosphorus, particularly in the calcium-bound fraction, was more mobile at Keszthely, and its exchangeability decreased rapidly downwards at both stations. Four times more potentially mobile phosphorus has been accumulated by the upper sediment layer at Keszthely than at Tihany.     

Different patterns of phosphorus release from lake sediments in laboratory experiments

The phosphorus release from surface sediments of eight lakes, mainly shallow lakes in agricultural areas, was studied in laboratory batch experiments with additions of acetate and/ or nitrate. The lake sediments could be separated into three categories. Some sediments did not release phosphorus under any conditions. The second category showed a high phosphorus release rate when acetate was added, in order to stimulate bacterial activity and oxygen consumption. The addition of nitrate, only, stabilized the redox conditions and prevented phosphorus release. This pattern followed the classical theories of Einsele and Mortimer. The third sediment category released phosphorus up to some level which remained constant throughout the experiment, and was independent of acetate and/or nitrate additions.Several extraction procedures and adsorption-desorption experiments were performed in order to characterize the sediment phosphorus and thus explain the different behaviours of the three sediment categories.     

Chemical composition of modern and pre-acidification sediments in the Tatra Mountain lakes

Concentrations of major nutrients (C, N, P) and acid soluble metals (Ca, Mg, K, Al, Fe, Mn, Pb, and Zn) were determined in modern (0–1 cm) and pre-acidification (5–10 cm) sediment layers collected from 37 alpine and 3 forest lakes in the Tatra Mountains (Slovakia, Poland) in 1996–1998. Sediment composition reflected catchment characteristics and productivity of lakes. In the sediments of alpine lakes, C and N concentrations decreased and Mg increased with a decreasing proportion of vegetation and soil in the catchment. Decreasing Ca:Mg ratios in sediments along the vegetation gradient was inverse to that in water, and could be associated with different ratios of cations in water leachate from catchments and in solids which enter the lake due to soil erosion. Phosphorus concentrations increased with the proportion of moraine areas, with till soils rich in P. Concentrations of C, N, P, and Ca in sediments positively correlated to their concentrations in water. Sediment concentrations of Al and Al:Ca ratios increased with decreasing sediment and water pH. A negative correlation between water pH and concentrations of organic C in water and sediments indicated the important impact of organic acids on the acid status of the lakes exposed to higher terrestrial export of organic matter. Compared to the pre-acidification period, the modern sediments had significantly higher Fe, Mn, Zn, Pb, and K, but lower Mg concentrations. The Zn and Pb enrichment was more evident in oligotrophic alpine lakes than in more productive forest lakes and was independent of lake water or sediment pH. Fe and Mn concentrations in the modern sediments were higher than in ambient soils and bedrock, while those in pre-acidification sediments were similar to contemporary soils and to the rock layer. The enrichment of the modern sediments with Fe and Mn thus probably resulted from both their redox recycling and ecosystem acidification.     

Phosphorus release from sediments in hardwater eutrophic lakes: the effects of redox-sensitive and -insensitive chemical treatments

1. Phosphorus (P) release from bottom sediments is an important source of nutrient enrichment in many lakes in sedimentary basins, such as those in western Canada. On the Boreal Plain, sediment P release is particularly strong during periods of seasonal anoxia.
2. In this study, the effects of reduction–oxidation (redox)-sensitive and -insensitive chemicals on P release were examined in sediment cores collected from three eutrophic lakes.
3. Contrary to expectations, redox-sensitive treatments were no more effective at lowering total phosphorus (TP) in sediment cores than some redox-insensitive treatments. Redox-sensitive treatments with FeCl₃ and FeCl₃ + O₂ reduced TP to 8 and 6%, respectively, of reference concentrations, whereas redox-insensitive treatments with alum or lime + alum reduced TP to 14% of reference concentrations. Lime and O₂ treatments reduced TP concentrations to 35 and 52% of reference concentrations, respectively.
4. The fraction of P that adsorbed and co-precipitated with iron and aluminium in the sediment cores was low (non-apatite phosphorus fractions < 5%), suggesting that P release was controlled by apatite solubility and bacterial metabolism.     

The chemical character and behaviour of phosphorus in poorly oxygenated sediments from open sea to organic-rich inner bay in the Baltic Sea

The chemical composition and vertical distribution of phosphorus (P) in poorly oxygenated sediments in a continuum extending from the open Baltic Sea towards an organic-rich inner bay were characterized by sequential extraction to examine the potential for release of sediment P. The chemical composition of P was related to chemical and physical characteristics of the sediments and the chemistry of pore water and near-bottom water to better understand the behaviour of P. Sediment P increased towards the inner bay, and the concentration of organic matter appeared to dictate its composition: the dominance of apatite-P turned to dominance of organic P (OP). Sediment P burial and, thus, release from sediment P reserves varied depending on the chemical composition of P. Dissolved species at the sediment–water interface suggested fluctuating redox conditions that affect P binding at short time scale. Redox-sensitive, iron (Fe)-bound P was usually relatively low because of poor oxygen (O₂) conditions, which emphasized the role of OP in P release. The results indicate that, over the long term, the abundant organic P reserve may support a significant continuing P release from hypoxic sediments in the severely eutrophied Gulf of Finland (GoF) because capture of P into Fe oxyhydroxides at the sediment surface is restricted. The average long-term minimum annual rate of P release from poorly oxygenated sediments below about 60 m depth in the GoF was approximated on the basis of the vertical distribution of sedimentary P forms and estimates of sedimentation rate.     

Influence of redox potential on phosphate-uptake by sediments in two sub-tropical eutrophic lakes

Biogeochemical reactions in shallow eutrophic lakes areaffected bythe changes in redox potential (Eh) as bottom sedimentsundergotemporal resuspension and settling. The stability of varioussediment P fractions and kinetics of P-uptake were evaluatedfortwo sub-tropical lakes (Lake Apopka and Lake Okeechobee,Florida)using sediment suspensions in closed systems maintained atvariousEh levels ranging from –235 to 555 mV. Redox potential hadminimal effect on the stability of NaOH-P (Fe-/Al-bound P plusmoderately resistant organic P) and HCl-P (Ca-/Mg-P) fractionsinLake Apopka sediments. Increases in ortho-P and NH₄Cl-P(loosely-bound P plus labile organic P) concentrations wereobserved in highly reduced (Eh=–225 mV) Apopkasediments.Phosphate solubility diagrams and mineral equilibriacalculationssuggest that P-uptake by Apopka bottom sediments at elevated Pconcentrations (ortho-P110 M) was due toformationof Ca-P compounds and/or co-precipitation of P withCaCO₃. Incontrast, the ortho-P concentrations for Lake Okeechobeebottomsediments increased exponentially with decreasing Eh. Thequantityof NaOH-P fractions for these sediments decreased withdecreasingEh, suggesting the release of Fe- and Mn-bound P intosolution.Phosphate-uptake by Okeechobee bottom sediments (pH 7.5,ambient)followed first order kinetics, yielding a rate constant (k)of 0.51±0.05 h^-1. Unlike that of Apopka, the mudsediments in Lake Okeechobee have strong affinity for P ineitheraerobic or anaerobic conditions. Results suggest that even incalcareous systems, Fe and Al, when present in highconcentrations(as in the case of Lake Okeechobee), are actively involved inregulating P-uptake and geochemistry.     

富营养化水体沉积物中磷的释放及其影响因素

综述了富营养化水体沉积物中磷的化学形态、释放规律及其影响因素。化学形态分为水溶性磷、铝磷、铁磷、钙磷、还原态可溶性磷、闭蓄磷、有机磷等 7种 ,其分布取决于各形态磷的性质。磷释放受 7种因素影响 ,厌氧、高 pH或低 pH值、高温、扰动、生物活动、底泥与水体含磷量的浓度差值以及钙质沉积物组分等因素均能促进沉积物中磷的释放。     

Sulfate control of phosphorus availability in lakes

During summer stratification large amounts of phosphorus (P) accumulate in anoxic bottom waters of many lakes due to release of P from underlying sediments. The availability to phytoplankton of this P is inversely related to the Fe:P ratio in bottom waters. Using data from 51 lakes, we tested the hypothesis that sulfate concentration in lake water may be critical in controlling the Fe:P ratio in anoxic bottom waters. Results showed that Fe:P ratios in bottom waters of lakes were significantly (p<0.001) related to surface water sulfate concentrations. The higher Fe:P ratios in low sulfate systems is due not only to higher iron concentrations in anoxic bottom waters but also to lower P concentrations in anoxic waters. Thus, our results suggest that anthropogenically induced increases in sulfate concentrations of waters (e.g. from fossil fuel burning) may have a double effect on P cycling in lakes. Higher sulfate concentrations can both increase the magnitude of P release from sediments as well as increase the availability of P released from sediments into anoxic bottom waters.     

Ecological Significance of Nitrogen Cycling by Tubificid Communities in Shallow Eutrophic Lakes of the Danube Delta

Importance of tubificid populations on nitrogen cycle in two categories of shallow eutrophic lakes in the Danube Delta was quantitatively assessed for the 1992-1993 period. The structure of the primary producers in the studied lakes was used to discriminate between the two categories:(i) lakes dominated by macrophytes (A₁) and (ii) lakes dominated by phytoplankton (A₂). In both categories tubificid worms represented important fraction of the entire benthic community (35 and 32%, respectively, as number of individuals). They influence the sediment-water exchange of nutrients. The main processes involved are excretion of nutrients and their continuous release from sediments by molecular diffusion or through channels created by bioturbation. Inorganic nitrogen released from bottom sediments may regulate nitrogen load in the water body and thus, phytoplankton production. In 1992-1993, nitrogen stocks in tubificid biomass accounted for 5.3% in A₁ lakes and 15.6% in A₂ lakes of the amount stocked in phytoplankton, and only for 1.2 and 2.9% respectively, of the nitrogen load in water body. Nitrogen excretion rates ranged between 60.52 and 153.74 mg N m^–2 year^–1, and release rates from sediments between 378.26 and 960.87 mg N m^–2 year^–1, the lowest values being recorded for A₂ category. Differences are related to tubificid biomass, structure and abundance of primary producers and to nutrient load in different ecosystems. Ratios between release rate of inorganic nitrogen by tubificid worms and sedimentation rate of organic nitrogen in the two categories of lakes were 8.3 and 6.4% respectively. Contribution of nitrogen released daily from sediments to the dissolved inorganic nitrogen load in the water column was less than 0.5%. However, in A₁ and A₂ lakes, the released nitrogen had a potential to sustain 24.74 and 8.01%, respectively, of the annual phytoplankton production. These values suggest the significance of tubificids in keeping the eutrophication process at a high level, especially during the periods when nitrogen is the main limiting factor for phytoplankton production.     

Phosphorus dynamics in shallow eutrophic lakes: an example from Zeekoevlei,South Africa

Zeekoevlei is the largest freshwater lake in South Africa and has been suffering from hyper-eutrophic conditions since last few decades. We have used total P (TP), dissolved phosphate (PO₄ ³⁻), organic P (OP), calcium (Ca) and iron (Fe) bound P fractions to investigate the relevant physical, chemical and biological processes responsible for sedimentation and retention of P and to study phosphorus (P) dynamics in this shallow lake. In addition, redox proxies (V/Cr and Th/U ratios) are used to study the prevailing redox conditions in sediments. Adsorption by CaCO₃ and planktonic assimilation of P are found to control P sedimentation in Zeekoevlei. Low concentration of the labile OP fraction in surface sediments restricts the release of P by bacterial remineralisation. Low molar Ca/P and Fe/P ratios indicate low P retention capacity of sediments, and P is most likely released by desorption from wind-induced resuspended sediments and mixing of pore water with the overlying water column. Handling editor: J. Saros     

Nutrient retention and release in a floodplain's aquifer and in the hyporheic zone of a lowland river

Fertilizer applications and other non-point sources result in an increasing diffuse N and P pollution of receiving waters degrading water quality by eutrophication with several adverse impacts. Floodplains are regarded as reactive interfaces between uplands and receiving waters. In the present study groundwater quality on its subsurface flow from an upland area through a lowland floodplain towards the receiving water body of the Spree River was monitored biweekly over 2 years with two transects of 18 groundwater observation wells. Within the floodplain reaction rates of the nutrients are unevenly distributed. On a scale smaller than the floodplain, the hyporheic zone is regarded as reactive interface with unproportional high reaction rates. Therefore, phosphate and dissolved iron were measured with high spatial resolution in the pore water of the riverbed and the oxbow bed to investigate turnover processes and their small-scale spatial variability at the immediate surface–subsurface interface. The biogeochemical composition of subsurface water is characterized by little temporal variability while spatial heterogeneity is high on the hectametre scale of the study site as well as on the centimetre scale of the bed sediments. Nitrate is eliminated very efficiently by denitrification in the anoxic aquifer of the floodplain while ammonium and phosphate concentrations increase under anoxic conditions. Phosphate and ammonium originate from the mineralization of organic matter and phosphate is additionally released by reductive dissolution of iron-bound phosphorus and weathering of bedrock. Sorption–desorption processes equalize temporal fluctuations of phosphate concentrations. Phosphate uptake by plants is assumed as an important process at only one of the groundwater observation wells. Redox conditions required for a phosphate sink are opposite to those involved in nitrate removal by denitrification. Thus, redox patchiness of floodplain aquifers favours nitrate and phosphate removal, i.e. a temporal and spatial sequence of anoxic and oxic conditions eliminates nitrogen and causes phosphate storage. On the groundwater's path from the upland to the river further phosphate is released in the bed sediments. It originates from previously settled particulate compounds containing phosphorus. While the release of iron-bound phosphorus clearly predominates in the riverbed sediments the mineralization of organic matter is an important additional phosphorus release process in the oxbow bed sediments.     

An evaluation of the importance of sulfate reduction and temperature to P fluxes from aerobic-surfaced, lacustrine sediments

We examined the influence of temperature and sulfate reduction rates onP-release from aerobic-surfaced, littoral sediments in a coolingreservoir. Annually, significant differences in P release from sediments at twosites (thermal effluent and non-effluent) were related to differenttemperature regimes, with higher rates of P release and decomposition at highertemperatures. Site-specific differences in solute fluxes were mostpronounced in late summer, when water temperatures reached40°Cat the effluent site and solute fluxes increased dramatically. The thermaleffluent site retained 65% of its annual P load, while thenon-effluent site retained 92%. Relative P release(P-release normalized to inorganic carbon fluxes; RPR) data indicatedthat lake sediments retained P selectively (relative to C) throughout the year,except at the effluent site during late summer when stored P was released inexcess of supply rates.Sulfate reduction rates were often typical of those measured in otherfreshwater lakes, but unusually high rates were measured at thethermal-effluent site especially in early fall and suggested higher DICfluxes than we measured. These high rates suggest that sulfate reduction rateswere overestimated and/or that most sulfide was recycled within the sediments.In any case, the highest sulfate reduction rates did not coincide with thehighest P release rates. Furthermore, the total reduced inorganic sulfurcontentof surficial sediments did not significantly correlate to RPR, althoughconcentrations varied widely throughout the year. Temperature was the onlyvariable examined that significantly correlated to RPR (R² =0.53, P-value = 0.017). Coupling between temperature and sedimentP release was likely mediated through temperature effects on bioturbation andmicrobial metabolic rates.     

Internal P loading in lakes: A different approach to its evaluation

P release rates in five lakes were estimated from a mass balance approach and from laboratory tests. The influence of various environmental parameters on P release has been considered. The internal P loading can contribute phosphorus to overlying waters at levels comparable to the external sources (from 30 to 100%). When considering the recovery from eutrophication of the investigated lakes, the nutrient release from sediments is of critical importance.     

The chemical character and burial of phosphorus in shallow coastal sediments in the northeastern Baltic Sea

The chemical composition and vertical distribution of sediment phosphorus (P) in shallow coastal sediments of the northeastern Baltic Sea (BS) were characterized by sequential extraction. Different P forms were related to chemical and physical properties of the sediments and the chemistry of pore water and near-bottom water. Sediment P composition varied among the sampling sites located in the Archipelago Sea (AS) and along the northern coast of the Gulf of Finland (GoF): the organic rich sites were high in organic P (OP), while apatite-P dominated in the area affected by sediment transportation. Although the near-bottom water was oxic, the sediments released P. Release of P was most pronounced at the site with high sediment OP and reduced conditions in the sediment-water interface, indicating that P had its origins in organic sources as well as in reducible iron (Fe) oxyhydroxides. The results suggest that even though these coastal areas are shallow enough to lack salinity stratification typical for the brackish BS, they are vulnerable to seasonal oxygen (O₂) depletion and P release because of their patchy bottom topography, which restricts mixing of water. Furthermore, coastal basins accumulate organic matter (OM) and OP, degradation of which further diminishes O₂ and creates the potential for P release from the sediment. In these conditions, an abundance of labile OP may cause marked efflux of P from sediment reserves in the long-term.     

Exchange of phosphorus across the sediment-water interface

In this article, principles of phosphorus retention and phosphorus release at the sediment-water interface in lakes are reviewed. New results and hypotheses are discussed in relation to older models of phosphorus exchange between sediments and water. The fractional composition of sedimentary phosphorus is discussed as a tool for interpretation of different retention mechanisms. Special emphasis is given to the impact of biological, particularly microbial, processes on phosphorus exchange across the sediment-water interface and to the significance of biologically induced CaCO₃ precipitation to phosphorus retention in calcareous lakes.     

Characteristics of oligotrophic hardwater lakes in a postglacial land-rise area in mid-Sweden

1. We describe some remarkable ephemeral, oligotrophic hardwater lakes formed because of land rise in the coastal areas of the Baltic Sea, that are unique in Sweden and probably also worldwide. Two younger, coastal lakes were studied by regular sampling for 1 year and compared with an older (i.e. greater altitude) lake, that passed through the oligotrophic hardwater stage some 3–4000 years ago. 2. Despite some differences in composition of the catchment, the two younger lakes were similar with regard to water chemistry and plankton community composition. The concentration of phosphorus was low while nitrogen was high, resulting in very high N/P quotients (101 and 131). Although water colour was moderate, the concentration of organic carbon was extremely high (average values of ≥ 20 μg TOC L^–1), consisting mainly of dissolved compounds (DOC). 3. While the plankton was poorly developed, sediments in both lakes were covered by a layer of photosynthesising micro‐organisms. This substantial `microbial mat', which has not been described in detail before, was up to 15 cm thick and dominated by cyanobacteria and purple sulphur bacteria. The concentration of sediment phosphorus was extremely low (352 μg g^–1 dw) in one of the lakes and dominated by organic‐bound (residual) phosphorus. 4. Deep sediments in the older lake, representing its oligotrophic hardwater period, differed in phosphorus composition from the currently oligotrophic hardwater lakes by having a strong dominance of HCl‐extractable (Ca‐bound) phosphorus. This indicates that phosphorus, initially organic‐bound within the microbial mat, is subsequently bound to calcium. We hypothesise that this is promoted by the environmental conditions created by the benthic photosynthetic activity, in combination with the prevailing hardwater conditions. 5. The rich and flourishing microbial community on the sediments may also explain the high concentration of DOC in the lake.     

环境因子对杭州西湖沉积物各形态磷释放的影响

对西湖沉积物的磷形态、粒径组成、化学组成进行了分析, 模拟研究了上覆水磷含量、光照、pH、温度、水动力条件等不同环境因子对西湖沉积物各形态磷释放的影响。结果表明, 上覆水为蒸馏水时的最大释磷量约为底泥-湖水系统的1.15倍, 且释放形态均以IP中的Fe/Al-P为主。在蔽光条件下的最大TP释放量约为光照条件下最大TP释放量的1.35倍。pH 是影响磷释放的重要因素, 在碱性条件下, 促进Fe/Al-P的释放; 在酸性条件下, 促进Ca-P 的释放。在高温条件下沉积物的释磷量会高于低温条件下的释磷量。沉积物各形态磷的释放量在15h后逐渐趋于平衡扰动状态达到平衡时TP释放量是静态释放平衡状态的1.61倍。研究结果旨在探讨不同环境因子对湖泊沉积物磷迁移转化的生态环境效应, 预测西湖内源磷释放的发展趋势, 为控制沉积物内源污染提供理论基础。     

Effects of a modified zeolite on P and N processes and fluxes across the lake sediment–water interface using core incubations

A new locally produced P-inactivation agent, Z2G1, was tested on sediment cores from Lake Okaro, New Zealand, for phosphorus (P) removal efficacy and any non-target side effects prior to a whole lake trial to manage internal P loading. Z2G1 is a granular product which settles rapidly, and was designed as a sediment capping material. It is a modified zeolite which acts as a carrier for the aluminium (Al)-based P-binding agent. It was found to have a high affinity for P and did not release Al into the water column. Continuous-flow incubation study results showed that a thin layer of Z2G1 (~2 mm) could completely block the release of P from the sediment under aerobic and anoxic conditions, and remove P from the overlying water in contact with the capping layer. The Z2G1 capping layer neither released metals itself nor did it induce the release of metals from the sediments, and the zeolite substrate absorbed arsenic and mercury from the geothermally influenced Lake Okaro sediments. In general, zeolites are strong cation absorbers and the zeolite substrate of Z2G1 absorbed ammoniacal nitrogen, making it the only sediment capping material to actively remove both P and N. There were, however, indications of a suppression effect on microbial denitrification by the Z2G1 capping layer under aerobic conditions. Overall, the Z2G1 sediment capping material is a highly effective P-inactivation agent which might be a useful material for managing internal P loads in eutrophic lakes.