Seasonal variations in phosphorus species in the surface sediments of the Gulf of Riga, Baltic Sea

Variation in Sediment Available-Phosphorus in Dianchi Lake and Its Impacts on Algal Growth

Sediment phosphorus (P) is an active component of the P cycle in lakes and its availability and movement could play an important role in eutrophication. Sediments from different depths in five representative sites of Dianchi Lake China, i.e., Haigeng, Dounan, Luojiacun, Xinjie, and Kunyang, were studied from January to December to evaluate the spatial and temporal dynamics in available-P and soluble-P and their impacts on algal growth. The results showed that the average sediment available-P and soluble-P were 41.2 mg kg^-1 and 0.27 mg kg^-1, respectively. Sediment available-P and soluble-P concentrations varied significantly among seasons, sites, and layers, with the following order: surface (0-5 cm) > middle (5-10 cm) > bottom (10-20 cm). The release and transformation characteristics of available-P and soluble-P at the sediment-water interface varied among sites. The Haigeng site, with the most severe algae blooms, had significantly higher average available-P and soluble-P in sediment than other sites. This indicated that phosphorus release and availability were associated with algal growth, and that the sediment surface (0-5 cm) is an important internal source that supports algal growth in Dianchi Lake.

Coagulation removal of phosphorus from a southern China reservoir in different stages of algal blooms: Performance evaluation and AlP matching principle analysis

Due to excessive nutrient discharge, many reservoirs in southern China suffer from eutrophication and harmful algal blooms. Several methods for phosphorus (P) removal have been proposed, including coagulation, adsorption, and biological methods. Among these methods, coagulation is preferable because of its quick effect, simple operation, and low cost. To investigate the effect and mechanism of coagulation on dephosphorization in reservoir water, the performances of Al-based (AlCl₃ and polyaluminum chloride (PACl)) and Fe-based coagulants (FeCl₃ and FeSO₄) were evaluated in this work. For reservoir water with a total phosphorus (TP) concentration of approximately 0.080 mg/L, aluminum salts showed stable advantages in dephosphorization. AlCl3 reduced the TP level by over 90% when treating the water sample collected at the initial stage of algal blooms, and PACl reduced by over 80% during the blooming stage. To reveal the dephosphorization mechanism and AlP matching principle, synthesized water samples were prepared and treated with AlCl₃ and [AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺ (Al₁₃). While simulating the water quality characteristics of reservoir water, important influencing factors were considered. The factors include P content (dissolved phosphorus (DP) and particulate phosphorus (PP)), pH, and extracellular organic matter (EOM). The pH was set to 7.66 and 8.29, with PP proportion set to 20%, 50%, and 80%. Simulated water treatment results indicated that, except for the coagulants species, pH significantly affected the dephosphorization efficiency. Moreover, the effects of P speciation and EOM were confirmed. Based on the coagulation performance and coagulation product characterization, chemical precipitation and inner-sphere complexation were estimated to be the most predominant way that DP and PP match with Al and were efficiently removed by Al-based coagulants.

Chemical behavior of different species of phosphorus in coagulation

Phosphorus is one of the elements that have a significant impact on such environmental problems as eutrophication or algal bloom. Phosphorus compounds in water can be hydrolyzed to orthophosphate that is the only form of phosphorus that algae can assimilate. In this study, phosphorus removal in terms of orthophosphate and total phosphorus from wastewater was studied using alum or ferric ions as coagulants. It was observed that alum shows higher phosphorus removal efficiency than ferric ions in the same mole ratio concentrations. The proportion of orthophosphate among total phosphorus did not change significantly during coagulation process when the coagulant concentration is low. However, the proportion becomes gradually decreased as the coagulant concentration increases. Not only the electrolyte concentration difference in solution, but the characteristics of orthophosphate and polyphosphate such as reactivity and ionic size might also cause the differences in the removal rate. Orthophosphate that has greater reactivity than other phosphorus species would be involved in chemical reactions dominantly when large amounts of coagulants are applied. However, the effect of reactivity was diminished due to the large ionic size of polyphosphate and low concentration of electrolyte in low coagulant concentration during the coagulation process.

Enhancement of dissolved phosphorus release from sediment to lake water by Microcystis blooms--an enclosure experiment in a hyper-eutrophic, subtropical Chinese lake

To clarify the possible influence of Microcystis blooms on the exchange of phosphorus (P) between sediment and lake water, an enclosure experiment was conducted in the hypereutrophic subtropical Lake Donghu during July-September 2000. Eight enclosures were used: six received sediment while two were sediment-free. In mid-August, Microcystis blooms developed in all the enclosures. There was a persistent coincidence between the occurrence of Microcystis blooms and the increase of both total P (TP) and soluble reactive P (SRP) concentrations in the water of the enclosures with sediments. In sediment-free enclosures, TP and SRP concentrations remained rather stable throughout the experiment, in spite of the appearance of Microcystis blooms. The results indicate that Microcystis blooms induced massive release of P from the sediment, perhaps mediated by high pH caused by intense algal photosynthesis, and/or depressed concentrations of nitrate nitrogen (NO3-N).