Field observation of shallow lake covered by Charophytes for purifying water quality

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Covering the lake bottom, Charophytes have a substantial potential in purifying water in lakes. However, lack of research on water quality in charophyte dominant Japanese lakes as well as the disappearance of charophyte bed in Japan make it difficult to evaluate the efficiency of purifying water by charophytes in shallow lakes. Field observation we conducted at Myall lake is NSW, Australia, where the entire lake was covered with charophytes and the underlying gyttja layer, then chemicals of plant, gyttja, and water samples were analyzed in laboratory to clarify ecology of charophytes and effects on water quality based on their life style. Then, following are clarified.1)Charophytes inhibit disturbing the bottom sediment surface, and gyttja originated from charophytes are extremely fine and viscous, stabilizing the bottom surface against wind currents. Thus, the recovery of nutrient in the sediment to the overlying water is hampered.2) Gyttja is extremely soft, then is hard to anchor other submerged plants with high buoyancy. Thus the invasion of other plants is inhibited.3)Charophytes trap phosphate when they produce calcite using calcium in water. This system seems to be available even in Japanese lakes with relatively small calcium concentration.4)Gyttja is composed of extremely fine particles thus its layer is very anoxic. Thus, the interstitial water is rich in ammonium rather than nitrate. As phosphate concentration is low, thus the nitrogen and phosphorus ratio is very high. When charophytes uptake nutrient, they highly fractionate ammonium composed of light nitrogen rather than heavy one. Thus, the nitrogen stable isotope ratio of charophyte tissues becomes extremely low.