Kinetics of phosphorus release from a natural mixed grain-size sediment with associated algal biofilms

Experiments using flumes containing mixed grain-size sediment with an associated algal biofilm, from two sites on the R. Tame, investigated the sediment-water exchanges in heterogeneous sediment deposits. These results were considered in the light of findings of a companion study [Gainswin BE, et al. The effects of sediment size fraction and associated algal biofilms on the kinetics of phosphorus release. Sci Total Environ, this issue.] by considering this natural system in relation to the effects of the different sizes of material comprising the sediment. Sediment samples were collected in trays installed in the river over a period of one growth cycle (March 2001-April 2002) and placed in flume channels with controlled water flow. The temperature, pH, and dissolved oxygen of the solution overlying the sediment were monitored automatically whilst filtered samples were obtained at 2-0h intervals over 48 h. The biomass, expressed as chlorophyll a, of the algal component of the biofilm from the surface of the sediment was estimated using methanol extraction. The composition of the sediment, viz. size fractions, organic matter and porosity, were determined at the end of the experiments. The equilibrium phosphate concentration and a phosphorus transfer index were used to establish that a net uptake of phosphorus by some of the samples that occurred at the time of sampling. The results were modelled using a Diffusion Boundary Layer model and the maximum flux from the sediment (or limiting diffusion flux) compared for each of the samples. The limiting diffusion flux was highest at the most contaminated site--reaching approximately 180 nmol m(-2) s(-1) (normalised with respect to the river bed area). The limiting diffusion flux calculated for the composite samples was in agreement with the flux estimated from the contributions expected from the individual size fractions [Gainswin BE, et al. The effects of sediment size fraction and associated algal biofilms on the kinetics of phosphorus release. Sci Total Environ, this issue.]. The dominance of the flux contribution from the stones size fraction (>20 mm) confirms that sediment having a filamentous biofilm and associated particulate material results in a greater flux than a silt sediment without such a biomass.     

Iron and hydrogen sulfide interference in the analysis of soluble reactive phosphorus in anoxic waters

Anoxic water from eight lakes containing various amounts of ferrous iron, hydrogen sulfide, calcium and total phosphorus was analysed for soluble reactive phosphorus (SRP). Hydrogen sulfide concentrations higher than 1 mg l⁻¹ and ferrous iron concentration above 0.20 mg l⁻¹ produce interferences in the SRP analysis in many occasions (e.g. 80% underestimation of SRP). Interfering concentrations of these materials are shown to be present in anoxic water from a large number of lakes, groundwater springs and ocean basins.The mechanisms of the interferences are discussed and methods described to prevent these analytical errors. Ferrous iron is not problematic if the sample is kept anoxic before and during filtration. On the other hand vigorous aeration is obligatory if hydrogen sulfide is present. Simple methods to test for both the possible interfering compounds are presented. It is shown for the water from the anoxic hypolimnia of eight lakes that a modified analysis of total (unfiltered) reactive phosphorus (TRP) yields on average only 2% lower phosphorus concentrations than SRP analysis. TRP hence can replace the complicated SRP analysis in anoxic waters.     

Suppression of phosphorus release from lake sediments by the addition of nitrate

An assessment was made of the effectiveness of nitrate in reducing phosphorus release from the anoxic sediments of a small dimictic lake, White Lough, N. Ireland. Laboratory experiments on sediment cores showed that nitrate delayed and reduced phosphorus release with an input of 61 gN m⁻² causing complete suppression. The addition of 24 gN m⁻² of nitrate to the sediment of White Lough resulted in a delay and reduction of phosphorus release similar to that observed in the laboratory core experiments. Sediment release of iron in the lake was also delayed and reduced but manganese release and the rate of hypolimnetic deoxygenation were unaffected by the nitrate addition. Ammonium release rates in the sediment cores and in the whole lake experiment did not vary with nitrate input. Comparing the costs of using nitrate or iron/aluminium salts to suppress sediment phosphorus release indicated that the nitrate method was at least 80% more expensive.     

A comparison of phosphorus retention in lakes determined from mass balance and sediment core calculations

Phosphorus retention (TP_acc) was determined in 7 lakes in Ontario, Canada from mass balances and from the direct measurement of phosphorus accumulation in the sediments. Phosphorus retentions determined using the mass balance technique ranged between 20 ± 4.0 and 95 ± 23 mg/m²/yr and were slightly lower than those calculated by multiplying sediment total phosphorus (TP) concentrations by sediment accumulation rates (49 ± 14 to 148 ± 37 mg/m²/yr). The lower TP_acc values measured using the mass balance approach may result from the underestimation of external TP inputs. Alternatively, it is suggested that in the more acidic lakes, a decrease in TP concentrations in the lakewaters and/or a decrease in TP export from the catchment may have resulted in a decrease in TP_acc through time. Thus, relatively higher TP_acc values would be calculated using the sediment approach (since TP_acc values calculated from the sediment data are averaged over a greater number of years than those calculated from the mass balance data).Provided several cores are collected from the lake, the direct measurement of sediment TP and sediment accumulation rates for estimating TP_acc may be a suitable alternative to mass balances, especially in situations where it is not possible to measure all inputs of TP to the lake.     

Lake restoration by hypolimnetic Ca(OH)2 treatment: impact on phosphorus sedimentation and release from sediment

A whole-lake hypolimnetic Ca(OH)₂ addition, that induced calcium carbonate precipitation, combined with deep water aeration has been applied to eutrophic Lake Luzin, Germany during 1996-1998. In this study we investigated the dynamic of phosphorus and its binding forms in seston and sediment before and during the treatment. The sedimentation rates of phosphorus increased within three years of induced calcite precipitation. The phosphorus binding forms shifted to the calcite-bound phosphorus in the settling matter. The increase of calcite-bound P in the settling material did not coincide with the maximum induced CaCO₃-precipitation caused by the hypolimnetic addition of Ca(OH)₂. An impact of chemicals additions and pH on phosphorus binding forms in seston and surface sediments has been studied in laboratory experiments with sediment core incubations and slurry experiments.Laboratory studies showed that the lowest phosphorus flux from sediment was related to the experiment with pH = 7 in overlaying water adjusted with Ca(OH)₂. The adjusting of pH with Ca(OH)₂ leads to a lower P flux of 2.3 mg P m⁻² d⁻¹, while the highest P-flux is attributed to the experiment with the pH which was adjusted with NaOH. Phosphorus fraction which reflects phosphorus binding on carbonates in surface sediments increased within one year of treatment, enhancing the phosphorus retention capacity of sediments.     

Sediment-water exchange in Lough Ennell with particular reference to phosphorus

The significance and evaluation of sediment phosphorus in the continuing eutropic conditions noted in Lough Ennell was assessed. A high positive correlation exists between the concentrations of total Fe, citrate dithionite bicarbonate (CDB) extractable Fe, organic content, total Al and a number of phosphorus fractions (total P, inorganic P, organic P, apatite P and CDB extractable P) for all samples recovered. A significant inverse correlation was noted between CaCO₃ and most of the other parameters measured whereas no correlations were found for Mn. The sediment adsorption capacity for orthophosphate was found to be about 450 μg P g⁻¹ sediment. Phosphate release from hypolimnetic anaerobic sediments was considered unimportant in the biological sense due to the lack of mobility of this nutrient to the photic zones. Bioavailable P in the littoral zones (assumed to be non-apatite inorganic phosphorus, NAIP) comprised 14.5 tonnes in the uppermost 2 cm sediment layer, which is about 2.5 times the average annual external total P load. Littoral zone sediments, which are normally aerobic, accounted for an internal areal loading of 134 mg P m⁻² yr⁻¹ which is equivalent to 17 and 30% of the average external total P and inorganic P loadings respectively. This internal loading is a major contributory factor in the continuing eutrophic status of the lake.     

Seasonal variation of phosphorus release from the sediments of Shallow Lake Balaton (Hungary)

Phosphorus release was low from intact sediment cores of the mesotrophic area of the lake throughout the year, and amounted to 0.3 mg P m⁻² day⁻¹ during autumn in short-term incubations. In the hypertrophic area maximum release (2.8 mg Pm⁻² day⁻¹) was measured during summer.

Phosphorus release showed a rapid increase from long-term incubated intact sediment cores with the increasing pH of the overlying water. At the ecologically real maximum pH the release may amount to 0.8 and 4.0 mg P m⁻² day⁻¹ in the mesotrophic and hypertrophic areas, respectively. A release of 2.0–3.9 mg P m⁻² day⁻¹ was estimated from sediment suspensions of the hypertrophic area within a pH range of 8–9. These values are similar to the external phosphorus loadings of the respective areas.

The most important phosphorus mobilizing factors are pH and the decomposition of the organic matter in the sediments. Redox conditions may play a significant indirect role in the regulation of the internal loading.

A positive feedback is hypothesized between the internal phosphorus loading and primary production both processes being affected by the external loading in different ways.     

Inorganic nitrogen release kinetics and exchangeable inorganic nitrogen of the sediments from shallow lakes in the middle and lower reaches of the Yangtze River region

Inorganic nitrogen (IN) release kinetics and exchangeable inorganic nitrogen (EIN) of the sediments from shallow lakes in the middle and lower reaches of the Yangtze River region were investigated. The results showed that the EIN contents of the studied sediments ranged from 131 to 274 mg/kg. With the increase of the total nitrogen (TN) contents, their EIN also significantly increased. Their ratios of EIN to TN ranged from 5 to 17%, and with the increase of their TN contents, their ratios decreased. The relative contribution of NH₄⁺‐N and NO₃^‐N to EIN ranged from 83 and 11% to 89 and 17%, respectively. The curves of the NO₃^‐N release kinetics of the studied sediments were not completely consistent with those of IN and NH₄⁺‐N. The amounts of the IN released from sediments not only relate to their EIN contents, but also to other N fractions.     

Vertical distribution of sediment enzyme activities involved in the cycling of carbon, nitrogen, phosphorus and sulphur in three boreal rural lakes

Vertical distribution of eleven hydrolytic enzyme activities were investigated with fluorogenic model substrates in boreal rural lake sediments, Lake Uurainen (13 km(2)), Lake J?mij?rvi (9 km(2)) and Lake Pyylampi (0.068 km(2)), in Finland. The eleven hydrolytic enzyme activities were high into deep sediment layers indicating potential for turnover of organic matter in the permanently anoxic zones. The activities of beta-glucosidase, sulphatase and N-acetyl-glucosaminidase were similar in the three lakes independent on the ecological status of the lake. Acetate- and butyrate-esterase were more active than the other enzymes in the three lakes. These unspecific esterases had sediment activities in forest Lake Pyylampi close to those reported for boreal coniferous forest soils in Finland. Similar beta-cellobiosidase activities throughout the sediment depths indicated constant depolymerisation potential for cellulose.