Mechanism On Phosphorus Transfer In Overlying Water And Sediment By CaCO₃-P Coprecipitation On The Leaf Surface Of Potamogeton Crispus

Phosphorus is the major limiting factor to cause water eutrophication. The removal of dissolved phosphorus in water column and the control of sediment phosphorus are keys to prevent eutrophication. Calcium is an important nutrition element in water, which can be redistributed by migration and transformation through absorption/release by aquatic plants and precipitation/dissolution by itself. The coprecipitation of phosphate with calcium carbonate is widespread in water, and its removal of phosphorus as a self-cleaning mechanism has important ecological significances to control phosphorus.In order to study the formation of CaCO-P coprecipitates in the presence of submerged plant Potamogeton crispus and the effect on phosphorus in water, a series of pot experiments with different lake sediments and different contents of calcium and phosphorus were designed in "overlying water-submerged plant-sediment" system, to analyze the changes of overlying water properties, calcium and phosphorus contents of P. crispus, characteristics of coprecipitates, calcium release by P. crispus, and changes of phosphorus in sediments, to explore the formation of coprecipitates on the leaf surface of P. crispus and it’s effect on phosphorus in treatments with different sediments and calcium and phosphorus levels. The main conclusions are as follows:(1) Overlying water pH value and concentrations of Ca2+and SRP were closely related to qualities of sediments and exogenous calcium and phosphorus. The acidity of the sediment directly influenced the overlying water pH value. The concentration of Ca2+increased to100～180mg/L in the overlying water with sediment added1000mg/kg calcium, improving the overlying water EC. The increased overlying water Ca2+could promote SRP settlement and reduce high pH value caused by the growth of P. crispus.(2) P. crispus had significant effects on overlying water properties, especially pH value. In the growth stage of P. crispus, the strong photosynthesis and respiration caused difference on pH value over day and night. The overlying water pH value could be more than10in the day, about11%on average higher than that in the night. Suitable light and temperature could induce P. crispus to release Ca2+form leaves. P. crispus caused a significant negative correlation between calcium and phosphorus, and maintained overlying water SRP less than0.02mg/L(3) The calcium and phosphorus levels in sediment or overlying water directly influenced total calcium and total phosphorus in the stems and leaves of P. crispus. High calcium sediment significantly increased total calcium, but reduced total phosphorus of P. crispus. Supplying calcium to the water could increase total calcium of P. crispus grown in low calcium sediment. Supplying phosphorus to the water could increase total phosphorus of P. crispus grown in both high and low phosphorus sediments. But when the sediment phosphorus was insufficient for the growth of P. crispus, the uptake of phosphorus by the stems and leaves was more obvious in higher phosphorus concentration of the water. High level of Ca2+in leaves could cause precipitation of calcium in the cells, which fixed phosphate or formed calcium salt crystals, and caused negative impact on the absorption and transfer of phosphorus by P. crispus. But the plant could release Ca2+to adjust the intracellular calcium concentration.(4) The growth of P. crispus provided appropriate conditions for the formation of CaCO3-P on the leaf surface. P. crispus absorbed CO2or HCO3-for photosynthesis, causing the rise of pH, especially pH of the liquid diffusion layer around leaves, providing pH condition for CaCO3precipitation. And the leaves of P. crispus could release calcium to improve the oversaturation level of CaCO3. The microorganisms, algae and other particles in water provided nuclei for CaCO3, and the secretions regulated and controlled the crystal growth. Some of these secretions were likely to wrap the surface of the crystal particles and inhibit their farther developing. But the high concetrations of Ca2+and SRP could promote the occurrence of CaCO3-P coprecipitation. However, the respiration of P. crispus could reduce the pH and lead to the dissolution of CaCO3in water.(5) The coprecipitates presented varieties of morphology, including granular, plate-like, spherical, needle, etc. These coprecipitates were affected by the conditions of the leaves. During the vigorous growth period of P. crispus, CaCO3-P mainly deposited to the sediment. While during the subsequent weak growth period, parts of the coprecipitates maintained on the leaf surface. Due to the low phosphorus concentration in the overlying water at this time, the coprecipitates contained less phosphorus.(6) Coprecipitation of CaCO3-P could maintain overlying water phosphorus at a low concentration and control the release of sediment phosphorus. In the crystallization process, CaCO3could adsorb phosphorus, depositing in the sediment surface finally to increase the content of Ca-P. Ca-P in surface sediment could transform to Al-P, Fe-P and steady state Ca10-P, with some dissolve and release to the pore water or overlying water, supplying Ca2+and PO43-for submerged plant and CaCO3-P coprecipitation.