Phosphorus release from resuspended sediment in the shallow and wind-exposed Lake Arresø, Denmark

Wind-induced sediment resuspension occurs frequently in the shallow and eutrophic Lake Arresø, Denmark. The impact of resuspension on internal phosphorus loading was investigated by laboratory experiments studying P-release from the undisturbed sediment surface and by experiments simulating resuspension events.Phosphorus release from undisturbed sediment sampled in May and August was 12 mg and 4 mg m^–2 d^–1, respectively. During experimental simulation of resuspension, soluble reactive phosphate (SRP) increased by 20–80 µg l^–1, which indicates that a typical resuspension event in the lake would be accompanied by the release of 150 mg SRP m^–2. The internal P loading induced by resuspension is estimated to be 60–70 mg m^–2 d^–1, or 20–30 times greater than the release from undisturbed sediment.SRP release during simulation of resuspension was mainly dependent on the equilibrium conditions in the water column and was basically independent of the increase in suspended solids and the duration of resuspension. A second simulation of resuspension conducted 26 hours later, did not result in any further release of SRP from sediment sampled in May. In contrast, there was an additional SRP release from sediment sampled in August, indicating that an exchangable P pool, capable of altering equilibrium conditions, is built up between resuspension events.It is concluded that resuspension, by increasing the P flux between sediment and water, plays a major role in the maintenance of the high nutrient level in Lake Arresø. A relatively high release rate is maintained during resuspension because of the low Fe:P ratio and the high concentration of NH₄Cl-extractable P in the sediment.     

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.     

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.     

Studying the phosphorus release from the Loosdrecht Lakes sediments,using a continuous flow system

Phosphorus release from the Loosdrecht Lakes sediments was studied, using a continuous flow reactor. The summer release maxima were 4 mg P.m^–2.d^–1 in 1984 and 1.4 mg P.m^–2.d^–1 in 1985. Temperature and downward seepage controlled release rates to a great extent, the pH of the overlying water being only of minor importance. From these results it could be concluded that release processes might be driven by mineralization of particulate organic phosphorus in the sediment. Pore water studies in the sediments of the release reactor confirmed this hypothesis. From the profiles phosphorus dissolution rates were calculated.     

Interactions between sediment and water in a shallow and hypertrophic lake: a study on phytoplankton collapses in Lake Søbygård,Denmark

Short-term changes in phytoplankton and zooplankton biomass have occurred 1–3 times every summer for the past 5 years in the shallow and hypertrophic Lake Søbygård, Denmark. These changes markedly affected lake water characteristics as well as the sediment/water interaction. Thus during a collapse of the phytoplankton biomass in 1985, lasting for about 2 weeks, the lake water became almost anoxic, followed by rapid increase in nitrogen and phosphorus at rates of 100–400 mg N M^–2 day^–1 and 100–200 mg P m^–1 day^–1. Average external loading during this period was about 350 mg N m^–2 day^–1 and 5 mg P m^–2 day^–1, respectively.Due to high phytoplankton biomass and subsequently a high sedimentation and recycling of nutrients, gross release rates of phosphorus and nitrogen were several times higher than net release rates. The net summer sediment release of phosphorus was usually about 40 mg P m^–2 day^–1, corresponding to a 2–3 fold increase in the net phosphorus release during the collapse. The nitrogen and phosphorus increase during the collapse is considered to be due primarily to a decreased sedimentation because of low algal biomass. The nutrient interactions between sediment and lake water during phytoplankton collapse, therefore, were changed from being dominated by both a large input and a large sedimentation of nutrients to a dominance of only a large input. Nitrogen was derived from both the inlet and sediment, whereas phosphorus was preferentially derived from the sediment. Different temperature levels may be a main reason for the different release rates from year to year.     

Release of sedimentary nitrogen and phosphorus in polder ditches of a low-moor peat area

The release of N and P from the sediment of two ditches, one (A) dominated by filamentous algae and the other (B) by water-lilies, was estimated by core and enclosure experiments. The release rates for ditch A tended to be higher than those for ditch B. Sediment cores covered by a filamentous algae layer released about 1.5 times more N and P than those from which the layer had been removed. During the incubation of the cores in the dark at 20°C for 2–3 weeks, about 10% of the N in the filamentous algae layer was mineralized. The mineralization could be described as a first-order reaction with a rate constant of about 0.2 d^–1. On average the cores of ditches A and B released about 40 mg mineral N and 3 mg.m^–2.d^–1 soluble reactive phosphorus. Defining the release from the sediment in the enclosures as the net increase of N and P in the water phase and in the vegetation minus the input, a negative net release,i.e. net accumulation of N and P in the sediment, was found over the summer half of the year. The negative values were due to the significant N and P input, resulting from pumping ditch water into the enclosures in order to compensate for downward seepage. From the enclosure experiments a downward seepage rate of 14 mm.d^–1 and an external load of about 6 g.m^–2 total N and 0.6 g.m^–2 total P during the summer half of the year —i.e. 33 mg.m^–2.d^–1 N and 3 mg.m^–2.d^–1 P. respectively — was calculated for the ditches. Tentative gross release rates — based on the sum of the positive net release of N and P into the water phase over 1–2 weeks intervals and the net increase of N and P in the vegetation — converted to 20°C and allowing for underestimation of the primary production by a factor of 5, amounted to 58 mg mineral N and 7 mg.m^–2.d^–1 soluble reactive phosphorus during the summer half of the year. Combining the rates estimated by cores and enclosures and converting them to rates at the mean water temperature during the summer half of the year, the release of mineral N and soluble reactive phosphorus roughly amounted to 40 and 4 mg.m^–2.d^–1, respectively. The release rates as well as the external load indicated a relatively low eutrophication of the ditches.     

Phosphorus in the sediment of the Loosdrecht lakes and its implications for lake restoration perspectives

In 1984 the external phosphorus load of the shallow eutrophic Loosdrecht lakes was reduced from 3.3 to 1.0 mg m^–2 d^–1. The effect of phosphorus release from the sediment on lake restoration was investigated. Diffusive release under aerobic conditions (20 °C) decreased from 1 mg m^–2 d^–1 in 1984 to 0.3 mg m^–2 d^–1 in 1990. The generation of inorganic phosphorus due to mineralization during summer equals 3 mg m^–2 d^–1, which is much higher than the measured rate of diffusive release. Despite that, the phosphorus release is hardly stimulated by anaerobic conditions, which indicates that only a small amount of phosphorus is adsorbed by ferric iron in the top sediment layer. This apparent discrepancy is probably caused by the uptake of inorganic phosphorus uptake during resuspension and the loss of inorganic phosphorus with downward seepage.The estimated removal of phosphorus due to downward seepage of 0.8 mg m^–2 d^–1 agrees well with the average phosphorus retention in the lake. This indicates that sediment burial and diagenesis are unimportant mechanisms for withdrawing phosphorus from the nutrient cycle.Between 1982 and 1991 the total phosphorus content of the upper 2 cm of the sediment decreased from 0.94 to 0.60 g kg^–1 DW. At present, about 20% of total phosphorus in this layer is potentially bioavailable, but largely incorporated in easily degradable organic matter. This pool is much smaller in deeper layers. Based on the estimated and measured rates and pool sizes, the annual average phosphorus cycle in the lakes was modelled to evaluate the effects of various restoration measures. The main predictions of the model are: 1) further reduction of the external load may cause a gradual decrease of the total phosphorus concentration in the lake water; 2) dredging and iron addition, without reduction of the external load, may give a rapid improvement followed by a slow return to the present situation; and 3) reduction of the external load, combined with a cut off of downward seepage will not improve the water quality.     

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.     

In situ phosphorus release experiments in the Warnow River (Mecklenburg,northern Germany)

Results are presented of in situ benthic phosphorus release experiments in an undercut bank of an impounded river. Due to high sedimentation of phytoplankton biomass high oxygen consumption rates between 259.4 and 947.0 mg O₂ m^–2 d^–1 developed, leading to almost anaerobic conditions and phosphorus releases between 175.2 and 236.3 mgP m^–2 d^–1 over a period of 18 days.In a second series of experiments the water column overlying the sediment was aerated, resulting in much lower P release rates (1.1 to 32.9 mgP m^–2 d^–1) over a period of 30 days. The influence of pH and nitrate was studied by adjusting pH and adding NO₃ ^– to the overlying water. Increasing pH positively affected P release rates and enhanced NO₃ ^– levels led to an increase of benthic P release, too.     

Oxygen consumption and ammonia accumulation in the hypolimnion of Walker Lake, Nevada

Walker Lake (area = 140 km², Z _mean = 19.3 m) is a large, terminal lake in western Nevada. As a result of anthropogenic desiccation, the lake has decreased in volume by 75% since the 1880s. The hypolimnion of the lake, now too small to meet the oxygen demand exerted by decaying matter, rapidly goes anoxic after thermal stratification. Field and laboratory studies were conducted to examine the feasibility of using oxygenation to avoid hypolimnetic anoxia and subsequent accumulation of ammonia in the hypolimnion, and to estimate the required DO capacity of an oxygenation system for the lake. The accumulation of inorganic nitrogen in water overlaying sediment was measured in laboratory chambers under various DO levels. Rates of ammonia accumulation ranged from 16.8 to 23.5 mg-N m^–2 d^–1 in chambers with 0, 2.5 and 4.8 mg L^–1 DO, and ammonia release was not significantly different between treatments. Beggiatoa sp. on the sediment surface of the moderately aerated chambers (2.5 and 4.8 mg L^–1 DO) indicated that oxygen penetration into sediment was minimal. In contrast, ammonia accumulation was reversed in chambers with 10 mg L^–1 DO, where oxygen penetration into sediment stimulated nitrification and denitrification. Ammonia accumulation in anoxic chambers (18.1 and 20.6 mg-N m^–2 d^–1) was similar to ammonia accumulation in the hypolimnion from July through September of 1998 (16.5 mg-N m^–2 d^–1). Areal hypolimnetic oxygen demand averaged 1.2 g O₂ m^–2 d^–1 for 1994–1996 and 1998. Sediment oxygen demand (SOD) determined in experimental chambers averaged approximately 0.14 g O₂ m^–2 d^–1. Continuous water currents at the sediment-water interface of 5–6 cm s^–1 resulted in a substantial increase in SOD (0.38 g O₂ m^–2 d^–1). The recommended oxygen delivery capacity of an oxygenation system, taking into account increased SOD due to mixing in the hypolimnion after system start-up, is 215 Mg d^–1. Experimental results suggest that the system should maintain high levels of DO at the sediment-water interface (10 mg L^–1) to insure adequate oxygen penetration into the sediments, and a subsequent inhibition of ammonia accumulation in the hypolimnion of the lake.     

Benthic nutrient fluxes in a eutrophic,polymictic lake

Sediment release rates of soluble reactive phosphorus (SRP) and ammonium (NH₄) were determined seasonally at three sites (water depth 7, 14 and 20 m) in Lake Rotorua using in situ benthic chamber incubations. Rates of release of SRP ranged from 2.2 to 85.6 mg P m⁻² d⁻¹ and were largely independent of dissolved oxygen (DO) concentration. Two phases of NH₄ release were observed in the chamber incubations; high initial rates of up to 2,200 mg N m⁻² d⁻¹ in the first 12 h of deployment followed by lower rates of up to 270 mg N m⁻² d⁻¹ in the remaining 36 h of deployment. Releases of SRP and NH₄ were highest in summer and at the deepest of the three sites. High organic matter supply rates to the sediments may be important for sustaining high rates of sediment nutrient release. A nutrient budget of Lake Rotorua indicates that internal nutrient sources derived from benthic fluxes are more important than external nutrient sources to the lake.     

Pore water dynamics in the sediment of a shallow and hypertrophic lake

Seasonal variations in pore water with main stress on pH and phosphate were investigated in the sediment of the shallow and hypertrophic Lake Søbygaard, Denmark. The purpose was to evaluate factors affecting the internal phosphorus loading. Pore water was obtained by in situ incubation of ceramic cups, sampled anaerobicaly from a fixed position in the sediment. The method is evaluated. During summer, pH and phosphate concentrations increased in the upper 8–10 cm of the sediment. Increased pH was most pronounced in the upper 5 cm, where pH increased to between 9 and 10. This is believed to be caused by the photosynthetically elevated pH in the above lake water. Phosphate concentrations increased with depth, from 0–2 mg P 1^–1 in the upper 5 cm to 3–6 mg P 1^–1 in 6–10 cm depth. Average phosphate gradient in the upper 6–8 cm was 1.0 mg P 1^–1 cm ^–1 in the summer decreasing to 0.2 mg P 1^–1 cm ^t1 in the autumn/winter. In spite of low redox potential, Fe(II) was not present in the upper 20 cm. The seasonal variation in pore water phosphate is believed mainly to be due to the variations in pore water pH inducing a substitution of phosphate ions with hydroxyl ions on ironhydroxides during summer. A considerable sedimentation of organic bound phosphorus and decomposition in the sediment is also considered important. Phosphorus release from the sediment is facilitated by bio- and gas turbation and by the frequent occurrence of resuspension caused by windaction. Net release rate is highly variable during the season. The summer average is 40 mg P m^–2 d^–1.     

Fixation of phosphorus in lake sediments using iron(III)chloride: experiences,expectations

After a reduction of the external phosphorus loading to a lake, an internal loading from the sediments may delay the improvement of the water quality. The accepted method to combat internal loading is careful dredging of the upper sediment layers (Cooke et al., 1986), but this method is costly and time consuming. Addition of phosphorus binding agents to the sediments might offer an alternative. In the Netherlands the use of aluminum compounds, the most common phosphorus binding agent, for water quality improvement purposes is not favoured. Therefore a sediment treatment with a solution of iron(III)chloride was tested. Iron was chosen because it is considered to be a natural binder of phosphate. 100 g m^–2 of Fe³⁺ were added to the sediments of the shallow (1.75 m average depth) and eutrophic Lake Groot Vogelenzang (The Netherlands) in October and November 1989. The iron(III)chloride solution was diluted 100 times with lake water and mixed with the surface sediments with a water jet.Following the addition the concentrations of total phosphorus (Fig. 1), chlorophyll-a and suspended solids decreased. This improvement of the water quality lasted for three months. After this time the total phosphorus concentration increased again, but remained at a lower level than in spring and summer of 1989. The phosphorus release rate from the sediments as measured from intact sediment cores decreased from 4 to 1.2 mg P m^–2 d^–1 (n = 5), and the bioavailability of the sediment phosphorus, as measured with bioassays, decreased from 34 to 23% (n = 5) shortly after the treatment. One year after the treatment the release rate was increased to 3 mg P m^–2 d^–1 (n = 5). Before treatment, the lake was thought to have a residence time of over one year. However, the chloride added to the lake disappeared according to a dilution rate of 0.03 d^–1 or a retention time of about 35 days. A high external loading due to rapid flushing with phosphorus-rich water from surrounding lakes possibly prevented a more durable improvement in water quality. Another possibility is that the iron addition has lost its phosphate binding capacity due to reduction or binding with other anions like carbonate or sulphide. Therefore the suitability of the method to reduce internal loading and especially the long term availability of added iron to bind phosphorus needs additional proof.The treatment of the 18 ha area of Lake Groot Vogelenzang took three weeks. The operational costs were about US$ 125000. This is fast and cheap compared to dredging. Application of the technique is limited to those cases where the sediments are not polluted with micro-pollutants and the water depth need not be increased.     

Benithic-pelagic coupling of nutrient metabolism along an estuarine eutrophication gradient

The shallow, brackish (11–18% salinity) Roskilde Fjord represents a eutrophication gradient with annual averages of chlorophyll, ranging from 3 to 25 mg chl a m^–3. Nutrient loadings in 1985 were 11.3–62.4 g N m^–2 yr^–1 and 0.4–7.3 g P m^–2 yr^–1. A simple one-layer advection-diffusion model was used to calculate mass balances for 7 boxes in the fjord. Net loss rates varied from –32.2 to 17.9 g P m^–2 yr^–1 and from –3.3 to 66.8 g N m^–2, corresponding to 74% of the external P-loading and 88% of the external N-loading to the entire estuary.Gross sedimentation rates measured by sediment traps were between 7 and 52 g p m^–2 yr^–1 and 50 and 426 g N M^–2 yr^–1, respectively. Exchangeable sediment phosphorus varied in annual average between 2.0 and 4.8 g P m^–2 and exchangeable sediment nitrogen varied from 1.9 to 33.1 g N m–1. Amplitudes in the exchangeable pools followed sedimentation peaks with delays corresponding to settling rates of 0.3 m d^–1. Short term nutrient exchange experiments performed in the laboratory with simultaneous measurements of sediment oxygen uptake showed a release pattern following the oxygen uptake, the changes in the exchangeable pools and the sedimentation peaks.The close benthic-pelagic coupling also exists for the denitrification with maxima during spring of 5 to 20 mmol N m^–2 d–1. Denitrification during the nitrogen-limited summer period suggests dependence on nitrification. Comparisons with denitrification from other shallow estuaries indicate a maximum for denitrification in estuaries of about 250 µmol N m^–2 h^–2 achieved at loading rates of about 25–125 g N m^–2 yr^–1.     

The influence of Procambarus clarkii (Cambaridae,Decapoda) on water quality and sediment characteristics in a Spanish floodplain wetland

This study describes the effects of the American red swamp crayfish, Procambarus clarkii Girard, on water quality and sediment characteristics in the Spanish floodplain wetland, Las Tablas de Daimiel National Park. Our short term enclosure study during a summer drawdown revealed that crayfish acted as a nutrient pump that transformed and translocated sediment bound nutrients to the water column. Water quality impoverishment was mainly due to the increase of dissolved inorganic nutrients (soluble reactive phosphorus and ammonia), and a significant increase of total suspended solids occurred likely as a result of crayfish associated bioturbation. At the same time, crayfish reduced the content of organic matter in the sediment and we observed a slight increase of total sediment phosphorus and nitrogen content as a result of crayfish benthic activity. P. clarkii effects, in terms of internal nutrient loading (229.91 mg TP m^–2 d^–1), were shown to be important on a local scale, indicating the significance of internal nutrient supply to water column primary producers particularly under low external supply (summer). Extrapolations to the whole ecosystem, however, revealed a negligible crayfish contribution (0.06%) to total internal nutrient loading (0.035 mg TP m^–2 d^–1). Hence, crayfish spatial heterogeneity patterns are important in global and local matter fluxes and nutrient cycles in wetlands.     

A mathematical model of the phosphorus cycle in Lake Loosdrecht and simulation of additional measures

The phosphorus cycle in the ecosystem of the shallow, hypertrophic Loosdrecht lakes (The Netherlands) was simulated by means of the dynamic eutrophication model PCLOOS. The model comprises three algal groups, zooplankton, fish, detritus, zoobenthos, sediment detritus and some inorganic phosphorus fractions. All organic compartments are modelled in two elements, carbon and phosphorus. Within the model system, the phosphorus cycle is considered as completely closed. Carbon and phosphorus are described independently, so that the dynamics of the P/C ratios can be modelled. The model has been partly calibrated by a method based on Bayesian statistics combined with a Range Check procedure.Simulations were carried out for Lake Loosdrecht for the periods before and after the restoration measures in 1984, which reduced the external phosphorus loading to the lake from ca. 2 mgP m^–2 d^–1 to 1 mgP m^–2 d^–1. The model outcome was largely comparable withthe measured data. Total phosphorus has slowly decreased from an average 130 µgP l^–1 to ca. 80 µgP l^–1, but chlorophyll-a (ca. 150 µg 1^–1, summer-averaged) and seston concentrations (8–15 mgC 1^–1) hardly changed since the restoration measures. About two-thirds of the seston consisted of detritus, while the phytoplankton remained dominated by filamentous cyanobacteria. The P/C ratio of the seston decreased from ca. 1.0% to 0.7%, while the P/C ratios of zooplankton, zoobenthos and fish have remained constant and are much higher. The system showed a delayed response to the decreased phosphorus loading until a new equilibrium was reached in ca. five years. Major reasons for the observed resilience of the lake in responding to the load reduction are the high phosphorus assimilation efficiency of the cyanobacteria and the high internal recycling of phosphorus. A further reduction of nutrient loading, perhaps in combination with additional measures like biomanipulation, will be the most fruitful additional restoration measure.     

Hydrological and nutrient budgets of freshwater and estuarine wetlands of Taylor Slough in Southern Everglades, Florida (U.S.A.)

Hydrological restoration of the Southern Everglades will result in increased freshwater flow to the freshwater and estuarine wetlands bordering Florida Bay. We evaluated the contribution of surface freshwater runoff versus atmospheric deposition and ground water on the water and nutrient budgets of these wetlands. These estimates were used to assess the importance of hydrologic inputs and losses relative to sediment burial, denitrification, and nitrogen fixation. We calculated seasonal inputs and outputs of water, total phosphorus (TP) and total nitrogen (TN) from surface water, precipitation, and evapotranspiration in the Taylor Slough/C-111 basin wetlands for 1.5 years. Atmospheric deposition was the dominant source of water and TP for these oligotrophic, phosphorus-limited wetlands. Surface water was the major TN source of during the wet season, but on an annual basis was equal to the atmospheric TN deposition. We calculated a net annual import of 31.4 mg m^–2 yr^–1 P and 694 mg m^–2 yr^–1N into the wetland from hydrologic sources. Hydrologic import of P was within range of estimates of sediment P burial (33–70 mg m^–2 yr^–1 P), while sediment burial of N (1890–4027 mg m^–2 yr^–1 N) greatly exceeded estimated hydrologic N import. High nitrogen fixation rates or an underestimation of groundwater N flux may explain the discrepancy between estimates of hydrologic N import and sediment N burial rates.     

杭州西湖北里湖沉积物氮磷内源静态释放的季节变化及通量估算

通过采集北里湖不同季节的柱状芯样,在实验室静态模拟沉积物氨氮(NH+4-N)和可溶解性磷酸盐(PO3-4-P)的释放,同时研究了沉积物间隙水中NH+4-N及PO3-4-P的垂直分布特征.结果表明,沉积物间隙水NH+4-N随深度的增加有上升的趋势,PO3-4-P随深度的增加呈先升后降的趋势.氮、磷营养盐在沉积物—水界面均存在浓度梯度,表明存在自间隙水向上覆水扩散的趋势.沉积物NH+4-N在春季、夏季、秋季、冬季的释放速率分别为0.074 mg·m-2· d-1、0.340mg· m-2· d-1、0.087 mg· m-2· d-1、0.0004 mg·m-2·d-1,pO3-4-P的释放速率则分别为0.340 mg·m-2·d-1、0.518 mg·m-2·d-1、0.094 mg·m-2·d-1、-0.037 mg· m-2·d-1.不同采样点表现出明显的季节和空间差异性,释放速率表现为夏季＞春季、秋季＞冬季.根据静态模拟出的不同季节下内源氮、磷释放速率计算,全湖内源氮、磷营养盐的贡献分别为0.0037、0.0057t/a.该研究可为北里湖富营养化及内源污染的治理提供基础数据.     

Seasonal variation of methane emissions from a temperate swamp

Methane flux measurements were made at four sites in a freshwater temperate swamp over the 13 month period of April 1985 through May 1986. Emissions were highly variable both between sites and over time at any one site. Ebullition from sediments was an important component of methane release. Although release of methane through bubbling occurred in only 19% of the measurements made between April and June 1985, when instrumentation allowed us to separate diffusive and bubble fluxes, ebullition accounted for 34% of the total flux during this period. Methane release rates showed a strong seasonal variation, with highest emission rates observed in early spring and again in late summer, which was associated with changes in plant growth and physiology. Emission rates were partially correlated with sediment temperature, but the relationship was not straightforward, and resembled a step function. Emissions responded strongly to temperature change through the range of 10–16°C. At winter sediment temperatures between 4–9°C, CH₄ flux continued at low rates (0–28 mg CH₄ m^–2d^–1; average = 7.9 mg CH₄m^–2d^–1) and appeared insensitive to changes in sediment temperature. Annual methane emission from three constantly flooded sites (mean water depth = 35 cm) was 43.7 +/- 7.8 gm^–2 (standard error); annual flux from a bank site was 41.4 +/- 20.5 gm^–2. A comparison of flux measurements from fresh and saline wetlands in the immediate area of Newport News Swamp emphasizes the importance of edaphic factors in controlling flux.     

Sulfate reduction and S-oxidation in a moorland pool sediment

In an oligotrophic moorland pool in The Netherlands, S cycling near the sediment/water boundary was investigated by measuring (1) SO₄ ^2– reduction rates in the sediment, (2) depletion of SO₄ ^2– in the overlying water column and (3) release of³⁵S from the sediment into the water column. Two locations differing in sediment type (highly organic and sandy) were compared, with respect to reduction rates and depletion of SO₄ ^2– in the overlying water.Sulfate reduction rates in sediments of an oligotrophic moorland pool were estimated by diagenetic modelling and whole core³⁵SO₄ ^2– injection. Rates of SO₄ ^2– consumption in the overlying water were estimated by changes in SO₄ ^2– concentration over time in in situ enclosures. Reduction rates ranged from 0.27–11.2 mmol m^–2 d^–1. Rates of SO₄ ^2– uptake from the enclosed water column varied from –0.5, –0.3 mmol m^–2 d^–1 (November) to 0.43–1.81 mmol m^–2 d^–1 (July, August and April). Maximum rates of oxidation to SO₄ ^2– in July 1990 estimated by combination of SO₄ ^2– reduction rates and rates of in situ SO₄ ^2– uptake in the enclosed water column were 10.3 and 10.5 mmol m^–2 d^–1 at an organic rich and at a sandy site respectively.Experiments with³⁵S^2– and³⁵SO₄ ^2– tracer suggested (1) a rapid formation of organically bound S from dissimilatory reduced SO₄ ^2– and (2) the presence of mainly non SO₄ ^2–-S derived from reduced S transported from the sediment into the overlying water. A³⁵S^2– tracer experiment showed that about 7% of³⁵S^2– injected at 1 cm depth in a sediment core was recovered in the overlying water column.Sulfate reduction rates in sediments with higher volumetric mass fraction of organic matter did not significantly differ from those in sediments with a lower mass fraction of organic matter.Corresponding author