Spatial and seasonal variation in greenhouse gas and nutrient dynamics and their interactions in the sediments of a boreal eutrophic lake

Dynamics of greenhouse gases, CH₄, CO₂ and N₂O, and nutrients, NO ₂ ^– + NO ₃ ^– , NH ₄ ⁺ and P, were studied in the sediments of the eutrophic, boreal Lake Kevätön in Finland. Undisturbed sediment cores taken in the summer, autumn and winter from the deep and shallow profundal and from the littoral were incubated in laboratory microcosms under aerobic and anaerobic water flow conditions. An increase in the availability of oxygen in water overlying the sediments reduced the release of CH₄, NH ₄ ⁺ and P, increased the flux of N₂O and NO ₂ ^– + NO ₃ ^– , but did not affect CO₂ production. The littoral sediments produced CO₂ and CH₄ at high rates, but released only negligible amounts of nutrients. The deep profundal sediments, with highest carbon content, possessed the greatest release rates of CO₂, CH₄, NH ₄ ⁺ and P. The higher fluxes of these gases in summer and autumn than in winter were probably due to the supply of fresh organic matter from primary production. From the shallow profundal sediments fluxes of CH₄, NH₄ ⁺ and P were low, but, in contrast, production of N₂O was the highest among the different sampling sites. Due to the large areal extension, the littoral and shallow profundal zones had the greatest importance in the overall gas and nutrient budgets in the lake. Methane emissions, especially the ebullition of CH₄ (up to 84% of the total flux), were closely related to the sediment P and NH ₄ ⁺ release. The high production and ebullition of CH₄, enhances the internal loading of nutrients, lake eutrophication status and the impact of boreal lakes to trophospheric gas budgets.     

Seasonal and interannual variation in nutrient fluxes from tributary inputs, consumer recycling and algal growth in a eutrophic river impoundment

We measured tributary inputs, algal nutrient demand and excretion rates of consumers (gizzard shad and zooplankton) at a eutrophic river impoundment. During two summers with contrasting flow regimes, tributary inputs accounted for 38% (1998) and 3% (1999) of algal N demand and 95% (1998) and 17% (1999) of algal P demand. Gizzard shad contributions averaged 14% and 20% of algal demand for N whereas P contributions were 31% and 58% (1998, 1999; respectively). Zooplankton recycling accounted for a comparable fraction of algal P demand (47%) but a larger fraction of N demand (43%) because their excretia were N rich (N:P = 13:1) compared to fish (7:1). Nutrient release by one of the consumers (gizzard shad) was compared with tributary loading over a nine-year period to assess inter-annual variation in their relative importance. Historical records of inflow chemistry, discharge and gizzard shad biomass showed that variation in tributary inputs was the primary determinant of seasonal and inter-annual variation in nutrient loading. Consumer-derived nutrients were important in late-summer and during years when tributary inputs were low. We propose a conceptual model in which primary production is regulated by external nutrient loading and consumer recycling acts to stabilize and sustain production during periods of diminished external inputs.     

Benthic nutrient fluxes in Cadiz Bay (SW Spain)

During summer and autumn 1988, benthic fluxes of nutrients and oxygen were measured in the Bay of Cadiz. The study was carried out using benthic chambers and in addition by determining gradients of nutrient concentration in interstitial water. Fluxes ranged between 13.5–24.3, 3.4–7.8, 6.1–28.4 and (− 99.4)−(− 188.5) mmol m^{− 2} d⁻¹ for NH₄ ⁺ , o-P, SiO₂ and O₂ respectively. These values are far higher than those reported by other authors for locations at similar latitudes. The stoichiometry of O, N and P transference suggest that benthic degradation of principally allochthonous organic matter takes place mainly through anaerobic pathways.     

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     

Sediment nutrient accumulation and nutrient availability in two tidal freshwater marshes along the Mattaponi River, Virginia, USA

Sediment deposition is the main mechanism of nutrient delivery to tidal freshwater marshes (TFMs). We quantified sediment nutrient accumulation in TFMs upstream and downstream of a proposed water withdrawal project on the Mattaponi River, Virginia. Our goal was to assess nutrient availability by comparing relative rates of carbon (C), nitrogen (N), and phosphorus (P) accumulated in sediments with the C, N, and P stoichiometries of surface soils and above ground plant tissues. Surface soil nutrient contents (0.60–0.92% N and 0.09–0.13% P) were low but within reported ranges for TFMs in the eastern US. In both marshes, soil nutrient pools and C, N, and P stoichiometries were closely associated with sedimentation patterns. Differences between marshes were more striking than spatial variations within marshes: both C, N, and P accumulation during summer, and annual P accumulation rates (0.16 and 0.04 g P m^–2 year^–1, respectively) in sediments were significantly higher at the downstream than at the upstream marsh. Nitrogen:P ratios <14 in above ground biomass, surface soils, and sediments suggest that N limits primary production in these marshes, but experimental additions of N and/or P did not significantly increase above ground productivity in either marsh. Lower soil N:P ratios are consistent with higher rates of sediment P accumulation at the downstream site, perhaps due to its greater proximity to the estuarine turbidity maximum.     

The effect of wave-reduction engineering on sediment resuspension in a large, shallow, eutrophic lake (Lake Taihu)

Frequent resuspension of sediments is recognized as an important process in large shallow lakes, impeding the recovery of eutrophic lakes. A large-scale project, including a wave barrier (3.3 km long) and a soft enclosure, was implemented to reduce wave energy and sediment resuspension in Lake Taihu, eastern China. The effects of the wave-reduction engineering on sediment resuspension and internal nutrient loading were investigated. Results showed that sediment resuspension rates as well as suspended solids (SS) in the areas protected by the wave barrier and the soft enclosure were significantly lower than in the unprotected areas. There was a positive relationship between total phosphorus (TP) and SS; thus internal loading of phosphorus was significantly reduced by the wave-reduction structure. High nutrient levels and phytoplankton biomass persisted during the experiment period, suggesting that additional measures, such as re-establishment of the macrophyte community, must be included to help restore the water quality in such a large, shallow and eutrophic lake.     

Response of a eutrophic, shallow subtropical lake to reduced nutrient loading

1. Lake Apopka (FL, U.S.A.) was subjected to decades of high nutrient loading from farms developed in the 1940s on converted riparian wetlands. Consequences included perennially high densities of cyanobacteria, low water transparency, elimination of submerged vegetation, modified fish community, and deposition of nutrient‐rich, flocculent sediments. 2. Initial steps were taken to reduce phosphorus (P) loading. Through strengthened regulation and purchase of farms for restoration, external P loading was reduced on average from 0.56 to 0.25 g P m^?2 year^?1 (55%) starting in 1993. The P loading target for the lake is 0.13 g P m^?2 year^?1. 3. For the first 6 years of P loading reduction the annual sedimentation coefficient (σ) averaged 13% less than the prior long‐term value (0.97 versus 1.11 year^?1). The sedimentation coefficient, σ, was lower in the last 3 years of the study, but this period included extreme low‐water conditions and may not be representative. Annual σ was negative (net P flux to the water column) only 1 year. 4. Wind velocity explained 43% of the variation in σ during the period before reductions in total phosphorus (TP) concentration of lake water, but this proportion dropped to 6% after TP reductions. 5. Annual mean TP concentrations differed considerably from values predicted from external loading and hydraulic retention time using the Vollenweider–Organization for Economic Co‐operation and Development relationship. Reductions in lake water TP concentration fit model predictions better when multiyear (3‐year) mean values were used. 6. Evidence available to date indicates that this shallow, eutrophic lake responded to the decrease in external P loading. Neither recycling of sediment P nor wind‐driven resuspension of sediments prevented improvements in water quality. Reductions in TP concentration were evident about two TP‐resident times (2 × 0.9 year) after programmes began to reduce P loading. Improvements in concentrations of chlorophyll a and total suspended solids as well as in Secchi transparency lagged changes in lake‐water TP concentration but reached similar magnitudes during the study.     

Importance of biological and abiotic factors for geochemical cycling in a freshwater eutrophic lake

Here we report the results of a comprehensive biogeochemical monitoring of Rostherne Mere in 1998, including changes in dissolved oxygen, organic carbon and nitrogen, nitrate/nitrite, ammonia, Al, Na, S, K, Mg, Ca, Si, Fe, Mn, orthophosphate, particulate N & P, suspended solids, temperature, pH, chlorophyll-a and zooplankton. The results demonstrated the major influence of primary producers on the overall geochemical cycling of N, P and Si, and suggested that the significance of zooplankton might have been previously underestimated. For major anions and cations, however, the influence of biota on lake water concentrations appeared to be negligible, reflecting the fact that these chemicals were present far in excess of plankton requirements. Thus changes in concentrations of Ca, K, Na, Mg and S were rather limited and must have reflected changes in hydrological and meteorological parameters. K, however, demonstrated a transitional pattern, reflecting some influence of biological uptake. During the stratification period, the slow processes of bacterial decomposition in the hypolimnion gradually released chemicals contained in the materials accumulated in the bottom layer, remarkably increasing the concentrations of dissolved compounds of those elements present in amounts comparable with the pool stored in the sedimenting detritus (e.g. orthophosphate P, ammonia N, Si and DOC). The decomposition also resulted in a drop in the redox potential, followed by partial denitrification and chemical release from the sediments. The hypolimnion of the Mere was confirmed to remain at the stage of Mn release, characterised by accumulation of DOC, orthophosphates, ammonia and initial stages of denitrification. High levels of P released from the sediments during the stratification period suggest that the lake’s recovery after sewage diversion might be further delayed.     

Nutrient limitation in a tropical saline lake: a microcosm experiment

There is increasing evidence that nitrogen limitation is of widespread occurrence in tropical lakes. Nonetheless, data on the deep tropical Lake Alchichica (Mexico) show that dissolved inorganic nitrogen (DIN) to soluble reactive phosphorus (SRP) ratio fluctuates widely. To elucidate further the role of nitrogen and phosphorus limitation on the phytoplankton growth in tropical saline lakes, we present the results of a series of nutrient enrichment experiments with natural assemblages of Lake Alchichica phytoplankton conducted monthly for a year. Our assays indicate that phosphorus and nitrogen alternate in limiting Lake Alchichica phytoplankton biomass. Phosphorous limited phytoplankton growth most (41.7%) of the time, followed by nitrogen (33.3% of the time), and both nutrients for the rest of the time (25.0%). This alternation in nitrogen and phosphorus responsible for phytoplankton growth limitation in Lake Alchichica is attributed to the combination of natural conditions (e.g., young volcanic terrain rich in phosphorus) that would favor nitrogen limitation and anthropogenic impacts (e.g., agricultural nitrogen fertilization) which would cause phosphorus limitation. Guest Editors: J. John & B. Timms Salt Lake Research: Biodiversity and Conservation—Selected papers from the 9th Conference of the International Society for Salt Lake Research     

Benthic shear stress gradient defines three mutually exclusive modes of non-biological internal nutrient loading in shallow lakes

Assessment of the importance of internal nutrient loading is essential for managing and restoring eutrophic shallow lakes. To date, studies of internal loads have tended to focus on one of two abiotic processes, either molecular diffusion or sediment/nutrient entrainment (resuspension). This study presents a new approach to determining the non-biological fluxes of nitrogen (N) and phosphorus (P) from the sediment to the water column of shallow lakes. Three mutually exclusive flux processes: (i) molecular diffusion, (ii) turbulent diffusion (eddy diffusivity) and (iii) wind-induced resuspension of N and P, were related to a gradient of benthic shear stress. A model presented here allowed the durations and magnitudes of different non-biological fluxes to be calculated over time, based on benthic shear stress. Two site-specific critical shear stress thresholds determined which of the three flux processes dominated for any benthic shear stress value. The model was calibrated for a shallow lake and the continuous flux of nutrient from the sediment to the overlying water generated by each process during that period was calculated, enabling the estimation of the relative importance of each of the three flux processes over a one-year period. Wind-induced resuspension dominated the internal nutrient flux, operating for 38% of the time and contributing 0.9 T P year⁻¹ and 10.2 T N year⁻¹ to the internal nutrient load. In contrast, molecular diffusion only contributed 0.01–0.02 T P year⁻¹ and 0.12–0.20 T N year⁻¹ to the water column, while turbulent diffusion provided up to 0.6 T P year⁻¹ and 6.2 T N year⁻¹. Our model suggests that turbulent diffusion is a neglected and potentially important process contributing to internal nutrient loading in shallow lakes, whereas molecular diffusion appears to be relatively unimportant in lakes that experience turbulence at the sediment–water interface. Handling editor: L. Naselli-Flores     

Effects of hydrodynamics on phosphorus concentrations in water of Lake Taihu,a large,shallow, eutrophic lake of China

To understand the effect of hydrodynamical process on water phosphorus concentration, wind, wave, and several water quality indices were observed in Meiliang Bay, a shallow and eutrophic bay locates in north of Lake Taihu. During the 7 day observation period, wind speed and significant wave height were recorded more than 3 h per day, and water samples were collected in five water-depth layers once a day. Hydrodynamical disturbance had no significant correlationship with the water quality at the top layer when the significant wave height was smaller than 30 cm, but it significantly increased suspended solids (SS) concentration of the bottom water layer. Concentrations of nutrients showed no positive correlationship with SS concentration in the water body. Intensive sediment resuspension may not have occurred when the hydrodynamic stress on sediment was only a little higher than the critical stress for sediment resuspension. A new method for confirming the critical stress for intensive sediment resuspension and nutrient release still needs to be developed. The range of the water quality indices was quite high during the seven days of observation. High variation seems to be a common character of large shallow lakes like Taihu.     

Effects of low nitrogen-phosphorus ratios in the phytoplankton community in Laguna de Bay, a shallow eutrophic lake in the Philippines

The effects of low nitrogen-phosphorus ratios on microalgae from a large eutrophic freshwater lake in the Philippines were investigated. Natural microalgal populations from Laguna de Bay, the largest lake in the Philippines, were cultured using three different nitrogen-phosphorus weight ratios (2N:1P; 6N:1P and 12N:1P) at two phosphorus concentrations (0.25 and 0.5 mg l^–1) in each case. The growth and genera composition of the cultures under the different treatments were followed for a 12-week period. Community level responses were assessed based on species richness (s), Shannon-Wiener Index (H), Simpson Index () and Evenness (J). Among the different microalgal groups, only the chlorophytes showed a significantly higher density in response to the 12N:1P treatment at the higher P concentration, indicating that the nutrient ratio had a significant interaction with the nutrient levels used in the experiments. The genera found in the different treatments were generally similar; however, the degree of dominance of some varied with treatment during the experiment. The succession of dominant genera also differed among the N:P treatments. The diatoms like Fragilaria, Aulacoseira (= Melosira) and Nitzschia dominated the lowest N:P. On the other hand, chlorophytes (Kirchneriella and Scenedesmus) dominated the highest N:P treatment, particularly from the second to the seventh week of the experiments with the diatoms becoming co-dominant only towards the eighth week until the end of the experimental. The 6N:1P treatment showed a mixed dominance between the diatoms and the chlorophyte genera. The various indices of diversity indicate significantly lower diversity only in the 12N:1P at 0.5 mg l^–1 P and not in 12N:1P at 0.25 mg l^–1 P.     

Biogeochemical response to physical forcing in the water column of a warm monomictic lake

Based on microstructure measurements of temperature and horizontalcurrent velocity the physical structure in the water column of Lake Kinneretwascharacterized as a five layer system consisting of a surface mixed layer, lowerepilimnion, metalimnion, upper hypolimnion and benthic boundary layer. Usingoxygen and hydrogen sulfide as natural chemical tracers, the time scale ofchemical change was identified in relation to advection, mixing and biologicalprocesses. Rapid changes due to advection that took place on an hourly timescale were removed by referring the data back to the temperature of the water.Biological activity dominated the hydrochemical changes observed in the meta-and upper hypolimnion. These were expressed by DO depletion rates of 2.0 and0.4g m^–2 d^–1, respectively.Verticaland horizontal mixing were shown to occur on a seasonal time scale. Once thechemical stratification process was completed the slow mixing through thebenthic boundary layer became the limiting factor for subsequent reactions inthe water column.     

Nitrogen dynamics and microbial food web structure during a summer cyanobacterial bloom in a subtropical,shallow, well-mixed,eutrophic lake (Lake Taihu,China)

Nitrogen dynamics and microbial food web structure were characterized in subtropical, eutrophic, large (2,338 km²), shallow (1.9 m mean depth), and polymictic Lake Taihu (China) in Sept–Oct 2002 during a cyanobacterial bloom. Population growth and industrialization are factors in trophic status deterioration in Lake Taihu. Sites for investigation were selected along a transect from the Liangxihe River discharge into Meiliang Bay to the main lake. Water column nitrogen and microbial food web measurements were combined with sediment–water interface incubations to characterize and identify important processes related to system nitrogen dynamics. Results indicate a gradient from strong phosphorus limitation at the river discharge to nitrogen limitation or co-limitation in the main lake. Denitrification in Meiliang Bay may drive main lake nitrogen limitation by removing excess nitrogen before physical transport to the main lake. Five times higher nutrient mineralization rates in the water column versus sediments indicate that sediment nutrient transformations were not as important as water column processes for fueling primary production. However, sediments provide a site for denitrification, which, along with nitrogen fixation and other processes, can determine available nutrient ratios. Dissimilatory nitrate reduction to ammonium (DNRA) was important, relative to denitrification, only at the river discharge site, and nitrogen fixation was observed only in the main lake. Reflecting nitrogen cycling patterns, microbial food web structure shifted from autotrophic (phytoplankton dominated) at the river discharge to heterotrophic (bacteria dominated) in and near the main lake.     

The threshold responses of phytoplankton community to nutrient gradient in a shallow eutrophic Chinese lake

Excessive nutrient loads resulted in cascading trophic effects and ecosystem responses. Aims of this study were to determine if the thresholds in nutrient gradient related to phytoplankton community composition could be identified in eutrophic lake, and further to analyze the change of phytoplankton assemblage along the nutrient concentration based on Threshold Indicator Taxa ANalysis (TITAN). The results presented the significant community thresholds estimate for negative taxa declining at 1.650 mg/L TN and 131.5 μg/L TP, as well as simultaneously increasing for positive taxa at 1.665 mg/L TN and 151.5 μg/L TP along nutrient enrichment gradient. However, there was unremarkable change point determined for TN:TP ratios in Lake Dianchi. Elevated TN and TP altered the phytoplankton assemblage, even may induce the fade of algal blooms across the threshold in the hypertrophic lake. The findings could provide implications for deeply deciphering abrupt transitions for phytoplankton assemblage and developing nutrient tactics to protect the lake ecosystems.     

Effect of ecological engineering on the nutrient content of surface sediments in Lake Taihu, China

Ecological engineering was carried out in Meiliang Bay of Lake Taihu beginning in 2003 in order to improve water quality. There were two main objectives: to improve the growth environment for macrophytes, and to restore macrophyte assemblages. We examined surface sediments once per month beginning in April 2005 to study the response of sediment nutrient content to the ecological engineering. Average total nitrogen (TN) and total phosphorus (TP) concentrations in the surface sediments were 7043 and 1370 mg kg⁻¹, respectively, in May 2005, while after 1 year, TN concentration was reduced to 2929 mg kg⁻¹ and TP concentration was reduced to 352 mg kg⁻¹. We conclude that ecological engineering can lower the nutrient content in surface sediments when it is used to improve water quality.     

Effects of temperature and oxygenavailability on greenhouse gas and nutrient dynamics in sediment of a eutrophic mid-boreal lake

The effects of oxygen conditions and temperature on dynamics of greenhousegases (CH₄, CO₂, N₂O) and nutrients(NH₄ ⁺, NO₂ ^–+NO₃ ^–, tot-P) were studied in sediment of hyper-eutrophic LakeKevätön, Finland. Undisturbed sediment cores were incubated at 6, 11,16, and 23 °C in a laboratory microcosm using a continuouswater flowtechnique with an oxic or anoxic water flow. The production of CO₂increased with increasing temperature in both oxic (Q₁₀ 3.2 ±0.6) and anoxic (Q₁₀ 2.3 ± 0.4) flows. The release ofCH₄ increased with temperature in anoxic conditions (Q₁₀2.3 ± 0.2), but was negligible with the oxic flow at all temperatures.The release of NH₄ ⁺ increased with temperature with the oxic and anoxic flows(Q₁₀ 2.4 ± 0.1). There was a net production of NO₂ ^–, NO₃ ^– and N₂O with the oxic flow at temperatures below16 °C. The release of phosphorus was greater from the anoxicsediments and increased with temperature with both the anoxic (Q₁₀2.9 ± 0.5) and oxic (Q₁₀ 1.9 ± 0.1) flows. It isprobable that the temperature of boreal lakes and the associated oxygendeficiency will increase as the climate becomes warmer. Our experiments showedthat this change would increase the global warming potential of greenhousegasesreleased from sediments of eutrophic lakes predominately attributable to theincrease in the CH₄ production. Furthermore, warming would alsoaccelerate the eutrophication of lakes by increasing release of phosphorus andmineral nitrogen from sediments, which further enhance CH₄productionin sediments.     

Internal loading of nutrients and certain metals in the shallow eutrophic Lake Myvatn, Iceland

Sub-arctic Lake Myvatn is one of the most productive lakes in the Northern Hemisphere, despite an ice cover of 190 days per year. In situ, transparent and dark flux chambers were used for direct measurements of benthic fluxes of dissolved oxygen, nutrients, silica and certain metals, taking into account primary production and mineral precipitation. The range of benthic flux observed for dissolved oxygen (DO), dissolved inorganic carbon (DIC), ammonium, ortho-P, silica, calcium, and magnesium was –45.89 to 187.03, –99.32 to 50.96, –1.30 to 1.27, –0.51 to 0.39, –62.3 to 9.3, –33.82 to 16.83, and –23.93 to 7.52 mmol m^–2 d^–1, respectively (negative value indicating flux towards the lake bottom). Low benthic NH₄ ⁺ and ortho-P fluxes were likely related to benthic algal production, and aerobic bottom water. Ortho-P fluxes could also be controlled by the dissolution/precipitation of ferrihydrite, calcite, and perhaps hydroxyapatite. The negative silica fluxes were caused by diatom frustule synthesis. Benthic calcium and magnesium fluxes could be related to algal production and dissolution/precipitation of calcium and/or Ca,Mg-carbonates. Fluxes of DO, DIC, pH and alkalinity were related to benthic biological processes. It is likely that some of the carbon precipitates as calcite at the high pH in the summer and dissolves at neutral pH in the winter. Mean of the ratio of gross benthic DIC consumption and gross benthic DO production was 0.94 ± 0.18, consistent with algal production using NH₄ ⁺ as N source. During the summer weeks the water column pH remains above 10. This high pH is caused by direct and indirect utilisation of CO₂, HCO₃ ^–, CO₃ ^–2, H₄SiO₄ ^° and H₃SiO₄ by primary producers. This study shows that in shallow lakes at high latitudes, where summer days are long and the primary production is mostly by diatoms, the pH is forced to very high values. The high pH could lead to a positive feedback for the Si flux, but negative feedback for the NH₄ ⁺ flux.     

Benthic fluxes of nutrients and some trace metals in the Tamar estuary,SW England

Pore water concentration gradients and fluxes of chemical components have been studied in sediments from six intertidal sites in the Tamar Estuary, SW England over the course of a year. Fluxes of nutrients (ammonia, nitrate, nitrite, phosphate and silicate) and trace metals (iron, manganese, zinc, copper and cadmium) were determined using a laboratory microcosm incorporatingin situ pore water samplers. Nutrients (except nitrate) were transported out of the sediment throughout the year, but nitrate fluxes were directed into the sediment in the summer (denitrification) and out of the sediment in the winter (nitrification). The activities of benthic macrofauna resulted in enhanced fluxes but these differed between sites depending on population structure and density and whether irrigation or sediment reworking predominated. Fluxes of trace metals were seasonally and spatially variable and specific differences were observed that could be attributed to both chemical and biological activity.     

Phosphorus speciation in Myall Lake sediment, NSW, Australia

The amount of phosphorus and its fractions in the sediment of Lake Myall, NSW, Australia, was assessed using a sequential extraction technique. Five sedimentary phosphorus reservoirs were measured, namely loosely sorbed phosphorus (NH₄Cl–P), iron associated phosphorus (BD–P), calcium bound phosphorus (HCl–P), metal oxide bound phosphorus (NaOH–P) and residual phosphorus (organic and refractory P, Res-P). Samples were taken from the deep and shallow sites of the lake. During the analysis, the average concentrations of each fraction of phosphorus was calculated. The results depicted that the total phosphorus (TP) content and chemically extractable phosphorus in both fine and coarse sediment fractions from the deep sites of the lake were significantly higher than those of the shallow sites, except for HCl–P extracted from the fine sediment fraction. Sediment TP was also strongly and positively correlated to sediment Fe. The phosphorus in the sediment mainly consisted of BD–P and Res-P, while NH₄Cl–P and HCl–P only contributed a minor part. The rank order of the different phosphorus extracts was similar for the two sites, namely Residual-P > BD–P > NaOH–P > HCl–P > NH₄Cl–P.