Discussion on Phosphorous Release from Lake Sediment

Abstract

对东太湖三种类型9个样品的沉积物-上覆水柱状原样在滤除藻类、蔽光、室温、 微量充气条件下连续培养50d,上覆水中的PO₄^3--P浓度在起初15d内增高了2个数量级,在随后 的33d中又下降了1个数量级并趋于稳定．这一过程有别于一般意义上的沉积物磷释放,本文提 出了另外一种解释．认为前期上覆水中的PO₄^3-积累源于沉积物表面活性有机碎屑层的分解释放 磷,后期上覆水中的PO₄^3-被沉积物吸附沉淀．并提出了以下假设：在自然条件下,沉积物表面活性 有机碎屑层释磷反应强烈,形成很高的PO₄^3-P浓度并向沉积物和上覆水中扩散,在湖水中由于水 生植物吸收维持了较低的PO₄^3--P浓度,在间隙水中由于扩散阻力较高而形成较陡的PO₄^3--P浓度 梯度。与内源磷污染负荷等同的沉积物磷释放是指沉积物磷输出（到湖水中）通量与（自湖水）输入 通量之差值,在东太湖那样湖底条件比较稳定的湖泊中,磷的生物循环有可能严重干扰柱状原样 模拟实验,测定内源磷负荷采用全湖磷平衡法或原位围隔模拟实验法比较可靠．;Nine sediment-water columns were taken from three stations of East Taihu Lake (about lm in depth) in June, 1997. The overlying water of the cores was filtered (0.45μm), and then the cores were placed in a dark room for 50 days. Air-bubbling was used to keep oxidized conditions in the overlying water and upper sediment, without resuspending the sediment. The room temperature was between 23.5 and 31.2℃ , and the pH value of the overlying water and upper sediment was 7.2±0.2. Concentration of PO₄^3--P in the overlying water was measured every 2 or 3 days to follow the P-release process. In the first 15 days,PO₄^3--P level in the overlying water increased 2 orders of magnitude. But in the following 33 days, the PO₄^3--P level decreased 1 order of magnitude and then stabilized. Such a process was also found in the West Lake, Hangzhou and Gehu Lake, Changzhou. We think it not a normal sediment-P-release process, but more complex as follows. In natural conditions of East Taihu Lake, the sediment is covered by a layer of detritus. Biological degradation is so intensive in this layer that PO₄^3- accumulates to a high level and diffuses to both the overlying water and the sediment beneath it. In the sediment a steep PO₄^3--P concentration gradient forms because of the high diffusive resistance. But in the overlying water, PO₄^3--P is used by aquatic plants and is kept at a low level. We are not able to measure PO₄^3--P concentration in the detritus layer, but an indirect evidence was found in the same lake: a PO₄^3--P concentration gradient from 200mg m^-3 in the upper sediment layer to 80mg m^-3 in the sediment of 18cm depth existed when PO₄^3--P concentration in the overlying water was only 22 mg m^-3. In the core experiment, phosphorous biological cycling was broken when phytoplankton was removed from the overlying water by filtration, and thus, PO₄^3- accumulated in the water quickly. After 15 days, the P-release reaction in the detritus layer was about to end, and P-diffusion from overlying water to the sediment triggered. In East Taihu Lake and the other similar lakes with stable bottom conditions, detritus layer may exist and affect P-release core experiment seriously. In such a situation, it is better to measure internal-P-loading by other means, such as whole lake P-budget and in-situ enclosure experiment.