Fisheries management as an additional lake restoration measure: biomanipulation scaling-up problems

Restoration of the highly eutrophic Reeuwijk lakes (ca. 700 ha) started in 1986 by reducing the external phosphorus loading. As an additional measure to improve the quality of the lake water, the structure of the fish population in Lake Klein Vogelenzang (18 ha) was altered in 1989 by the removal of ca. 100 kg ha^–1 bream from the lake in April and December. This constituted about 50% of the total bream biomass in the lake.The fish-stock reduction in April, 1989, was initially followed by high phosphorus concentrations, probably the result of considerable phosphorus release from the sediments. The resulting heavy algal blooms that occurred reduced the transparency to very low values. During the summer the zooplankton population increased markedly in numbers coinciding with reductions in total suspended matter including (blue-green) algae. A great improvement in Secchi-disc transparency was observed and by the end of December, 1989, the bottom of the lake (1.5–2.0 m) was visible. After heavy storms in January and February 1990, transparency dropped to < 1 m as a result of resuspension of high concentrations of suspended matter from the bottom sediments. Although transparency over the rest of 1990 was higher than in 1988, i.e. the year preceding the removal of fish (biomanipulation), it was lower than expected, based on the results of 1989. The study shows that technical and biological factors can cause serious management problems for the implementation of biomanipulation in larger water bodies.     

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 influence of chironomus plumosus larvae on the exchange of dissolved substances between sediment and water

4th instar Chironomus plumosus larvae (about 1000·m^–2) were added to tubes containing sediment and overlying water. At a temperature of 20°C the larvae greatly increased the trasnport of silica, phosphorus and iron from the sediment to the water. Oxygen concentrations did not influence the exchange of silica. For two non-calcareous sediments the exchange of phosphorus and iron was much higher under anaerobic than under aerobic conditions while the difference was small for sediment from a hardwater lake. Exchange of inorganic nitrogen was little influenced by added chironomid larvae.     

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 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.     

Pore water phosphorus and iron concentrations in a shallow,eutrophic lake — indications of bacterial regulation

Peak pore water SRP and iron(II) concentrations were found during summer in surface sediments in the shallow and eutrophic L. Finjasjön, Sweden, and the concentrations generally increased with water depth. The SRP variation in surface sediments (0–2 cm) was correlated with temperature (R² = 0.82–0.95) and iron(II) showed a correlation with sedimentary carbon on all sites (R² = 0.42–0.96). In addition, sedimentary Chla, bacterial abundances and production rates in surface sediments (0–2 cm) varied seasonally, with peaks during spring and fall sedimentation. Bacterial production rates were correlated with phosphorus and carbon in the sediment (R² = 0.90–0.95 and R² = 0.31–0.95, respectively), indicating a coupling with algal sedimentation. A general increase in sediment Chla and bacterial abundances towards sediments at greater water depth was found. Further, data from 1988–90 reveal that TP and TFe concentrations in the lake were significantly correlated during summer (R² = 0.81 and 0.76, in the hypolimnion and epilimnion, respectively). The results indicate that the increase in pore water SRP and Fe(II) in surface sediments during summer is regulated by bacterial activity and the input of organic matter. In addition, spatial and temporal variations in pore water composition are mainly influenced by temperature and water depth and the significant correlation between TP and TFe in the water suggests a coupled release from the sediment. These findings support the theory of anoxic microlayer formation at the sediment-water interface.     

Arsenic transport between water and sediments

Arsenic discharged into the Moira River has accumulated in the sediments of Moira Lake during the past century. The chronology of arsenic concentrations in the sediments, established using Pb-210 dating, has a subsurface concentration maximum (> 1000 g g^–1) that reflects higher inputs to the lake 15 to 45 years ago. The distribution coefficient (K_d) of arsenic in the surficial sediments was low (4000–6000 L kg^–1) and decreased below the sediment water interface. Higher concentrations of exchangeable As also were extracted deeper in the sediments. As a result, arsenic is mobile in the sediment column and the flux of arsenic via diffusion and particle resuspension from the sediments into the water is greater than current external loading from the Moira River. Less than 20% of the external input of arsenic is buried in the lake sediments. Using these flux measurements and a one dimensional model of arsenic transport in the sediment column, we constructed the history of arsenic exchange between water and sediments throughout the past century. The simulations predict that arsenic input into the water from the sediments has been > 20 % of external loading for the past 25 years and will continue to be important in the future as diffusion and resuspension regenerate arsenic from the mixed layer of the sediments into the overlying water.     

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.     

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.     

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.     

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.     

Wind-induced increases in trophic state characteristics of a large (27 km 2), shallow (1.5 m mean depth) Florida lake

Nutrient and chlorophyll concentrations in Lake Newnan (27 km², mean depth 1.5 m), Florida showed dramatic increases from 1991 to 1998. Historical data showed Lake Newnan never had sufficient aquatic macrophyte abundance for a shift in alternate stable states to account for increases in trophic state characteristics. External phosphorus and nitrogen loads from incoming streams were monitored from August 1997 to July 1998 to determine if external supplies of nutrients were responsible for increases in lake nutrient and chlorophyll concentrations. During the study period, external nutrient loading rates were not correlated to lake nutrient concentrations. Phosphorus and nitrogen models based on the external loading estimates predicted the lake total phosphorus and total nitrogen concentrations to be 370% and 680% less, respectively, than the observed lake total phosphorus and total nitrogen mean concentrations. Consequently, phosphorus and nitrogen exports were 280% and 540% greater, respectively, than stream input loading. Data during the study period revealed strong inverse relations between lake stage and total phosphorus (r=–0.78), total nitrogen (r=–0.71), and chlorophyll (r=–0.90) concentrations. Long-term data (1965–1998) also revealed inverse correlations (r=–0.48 to –0.52) between lake stage and total phosphorus, total nitrogen, and chlorophyll concentrations. Applying fundamental wave theory and using a bathymetric map, it is probable that as much as 70% of the lake bottom sediment could be subjected to resuspension 50% of the time when the lake stage falls below 19.9 m mean sea level (msl). Above a lake stage of 19.90 m msl, less than 20% of the lake bottom sediment can potentially be resuspended 50% of the time. A percent frequency distribution from 1991 to 1998 showed that over 30% of the lake stages fell below 19.9 m msl. However, from 1967 to 1990, only 8% of the lake stage values fell below 19.9 m msl. Increases in total phosphorus, total nitrogen and chlorophyll concentrations in Lake Newnan were likely caused by an increased probability of internal loading due to decreased lake levels, and not to external loading of phosphorus and nitrogen.     

Control of phosphorus loading and flushing as restoration methods for Lake Veluwe,The Netherlands

As a result of high nutrient loading Lake Veluwe suffered from an almost permanent bloom of the blue-green algaOscillatoria agardhii Gomont. In 1979, the phosphorus loading of the lake was reduced from approx. 3 to 1 g P.m^–2.a^–1. Moreover, since then the lake has been flushed during winter periods with water low in phosphorus. This measure aimed primarily at interrupting the continuous algal bloom. The results of these measures show a sharp decline of total-phosphorus values from 0.40–0.60 mg P.l^–1 (before 1980) to 0.10–0.20 mg P.l^–1 (after 1980). Summer values for chlorophylla dropped from 200–400 mg.m^–3 to 50–150 mg.m^–3.The increase in transparency of the lake water was relatively small, from summer values of 15–25 cm before the implementation of the measures to 25–45 cm afterwards. The disappointing transparency values may be explained by the decreasing chlorophylla and phosphorus content of the algae per unit biovolume. Blue-green algae are gradually loosing ground. In the summer of 1985 green algae and diatoms dominated the phytoplankton for the first time since almost 20 years. To achieve the ultimate water quality objectives (transparency values of more than 100 cm in summer), the phosphorus loading has to be reduced further.     

Impacts of bleak (Alburnus alburnus) and roach (Rutilus rutilus) on water quality,sedimentation and internal nutrient loading

Lake Vesijärvi, southern Finland, suffered sewere eutrophication by sewage effluent from the city of Lahti during the 1960's and the early 1970's. The municipal sewage loading was diverted from the lake in 1976 and the lake started to recover. However, in the 1980's blue-green algal blooms increased again and the recovery of the lake faded. Enclosure experiments demonstrated that high roach (Rutilus rutilus) biomass is one of the key factors in the fading recovery of the lake. In this study, the influence of roach and another cyprinid fish species (bleak, Alburnus alburnus) to planktonic algal productivity and biomass in Lake Vesijärvi was examined. Enclosure experiments in the field showed the impacts of planktivorous bleak on water quality; in an enclosure with a density of 1 fish m^–2 average daily algal production (1370 mg C m^–2) and chlorophyll-a concentration (50–90 µg 1^–1) were more than twice that in an enclosure without fish. Laboratory experiments showed that the availability of planktonic food affects the foraging behaviour of roach and consequently the internal nutrient loading from the sediment into the water. Roach caused the highest phosphorus loading and turbidity when there was no zooplanktonic food available in the water. The possible interactions between planktivorous and omnivorous fish species are discussed.     

Phosphorus Sequestration in Lake Sediment with Iron Mine Tailings

Internal phosphorus loading can lead to eutrophication in lakes when anoxic sediments release bioavailable phosphorus into the water column. In laboratory experiments, iron mine tailings helped to sequester phosphorus in sediment from a eutrophic lake. Phosphorus release from the sediments after extraction with distilled water or 0.02 N H ₂ SO ₄ was significantly reduced when mine tailings were added (1:1 w/w), even when the system was anaerobic (～ 1 mg O ₂ /L). The degree of sequestration was enhanced when glucose (1% w/w) was added to stimulate the growth of microorganisms, suggesting that the process was microbially mediated. We suggest that oxidized iron in the mine tailings served as an electron sink for microbial respiration via dissimilatory Fe³⁺ reduction. The reduced iron released into solution sequestered phosphorus, either as it re-oxidized and formed hydrous ferric oxide complexes containing phosphorus (HFO-P), or through precipitation. Since mine tailings are inexpensive, they may prove useful for preventing phosphorus from entering surface waters, as well as reducing internal phosphorus loading.     

Available phosphorus in lake sediments in The Netherlands

The amount of phosphorus available to algae in the sediments of four lakes in the western part of the Netherlands has been assessed by means of chemical extraction and bioassay techniques. In addition to direct chemical sediment analyses, extractions were carried out with an NTA column method and a stepwise NH₄ Cl-NaOH-HCI shaking method, the latter supposedly separating the weakly bound, the Fe- and Al-bound and the Ca-bound phosphates in the sediments. Bioassays, with sediment as the sole source of P, were made withScenedesmus quadricauda in modified Skulberg's 28 medium to determine the amount of phosphates available to algae.The average total P concentration of the sediments varied from 0.8 to 3.6 mg P g^–1 dry wt and correlated well with the net external P loading of the lakes. Uptake of P by algae in the bioassays varied from 0.4 to 36% — while NTA extracted 36–69% of the total P. The ratio NH₄Cl extracted/ NaOH extracted/ HCI extracted phosphates is different from lake to lake, although in all lakes the highest extractions (27–62% of total P) are found in the NaOH fraction. However, in the peaty sediments of these lakes, the NaOH step extracted not only the Fe- and Al-bound phosphates but, also, large amounts of humus compounds. Hence, this fraction also contains non-available organic P.The results are related to soil type and chemical characteristics of the sediments, and compared with data from other authors. A positive correlation was found between phosphate available to algae and NTA- and NaOH-extractable P, but the correlation with total phosphorus was higher. Moreover, algal-extractable P proved to be positively correlated with total iron and clay content and negatively with the amount of organic matter.It is concluded that the sediments in the investigated lakes show great variability and that the chemical extraction techniques cannot replace the bioassays to assess the amount of phosphorus available to algae.     

Changing suspended sediment and particulate phosphorus loads and pathways in underdrained lowland agricultural catchments; Herefordshire and Worcestershire,U.K.

Data obtained from a small (150 ha) experimental catchment, Hereford U.K., have shown that land drains contributed >50% of the total catchment suspended sediment (SS) yield over a two year period. In one of the monitored drains, annual sediment yields were 964 and 978 kg ha^–1. Particulate phosphorus (PP) contributed >60% of total phosphorus lost through the drains and at least 73% of the total drain load came from topsoil. A major question that was not answered by the monitoring programme was whether SS and PP loads had increased since the drains had been installed between the 1960s and 1980s. To address this problem, a sediment yield record was reconstructed from the Kyre Pool catchment in Worcestershire that has a similar drainage history and soil types. Reservoir sediments were dated using the ²¹⁰Pb crs method and results suggest a fourfold increase in SS yield (300–1170 kg ha^–1 yr^–1) and PP loads (0.19–0.79 kg ha^–1 yr^–1) since the 1960s. The most recent SS and PP loads are comparable to those obtained from catchment monitoring. The high lake sediment ¹³⁷Cs inventory suggests a significant influx of eroded topsoil and ¹³⁷Cs activities in the most recent lake sediments are comparable to those of monitored land drains, supporting the hypothesis that land drainage has had a major impact on SS and PP yields and pathways.     

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.     

Measurement and verification of rates of sediment phosphorus release for a hypereutrophic urban lake

The contribution of sediment release to the phosphorus budget of hypereutrophic Onondaga Lake was determined through laboratory measurements made on intact cores. Rates ranged from 9–21 mg P m^–2 d^–1 with a mean of 13 mg P m^–2 d^–1, values similar to those observed in other lakes of comparable trophic state. There was no statistically significant trend in rates in time (July versus September) or in space (location along the major N/S axis of the lake). Rates of sediment phosphorus release measured in the laboratory compared favorably with the observed rate of soluble reactive phosphorus accumulation in the lake's hypolimnion. The sediments are the second largest source of phosphorus for Onondaga Lake, contributing 24% of the overall phosphorus load to the system.     

Iron content of sediment and phosphate adsorption properties

Phosphorus can occur in sediments in different forms and accordingly its availability varies. The distinction between the phosphorus fractions is made with two chemical extraction methods; an ammonium oxalate-oxalic acid extraction and an extraction according to Hieltjes & Lijklema (1980).The iron and aluminum liberated with the ammonium oxalate-oxalic acid extraction method is linearly correlated (r ² = 0.73) with the phosphorus liberated in the first two steps of the Hieltjes and Lijklema extraction by: P = 0.035 (Fe + Al) + 0.001 (P, Fe and Al in mmol g^–1).The iron and aluminum (hydr)oxides are very important fractions in the sediment adsorption capacity for phosphorus. The phosphorus sorption capacity (PSC) is 0.080 mol P (mol (Fe + Al))^–1 and the adsorption constant (k) is 11.9 µmol P l^–1. Here it is assumed that iron and aluminum (hydr)oxide have the same affinity for phosphorus.