Nitrogen and phosphorus control thresholds of cyanobacterial blooms in Lake Taihu

Abstract

藻类生长与营养盐浓度存在藻类几何级数增长的营养盐浓度变化的下限阈值和藻类生长不受氮磷浓度增加影响的上限阈值,但由于蓝藻水华的形成受多种因素的综合影响,不同湖泊、不同区域及不同时段的氮磷浓度对蓝藻水华的影响差别较大,使得蓝藻生长的氮磷控制阈值难以确定.针对控制蓝藻水华暴发的氮磷阈值的研究虽然有所开展,但多集中在实验室研究阶段或对经验值的判断,虽然也有基于野外实测数据的研究,但也限制于某一特定区域,而基于野外长序列实测数据并且覆盖整个湖泊的氮磷阈值研究则是空白.太湖作为具有较高营养背景的富营养化浅水湖泊,蓝藻水华的发生受氮磷影响较大.对太湖总磷(TP)、总氮(TN)和叶绿素a(Chl.a)浓度的时空变化分析发现,太湖西北湖区的TP、TN与Chl.a浓度明显较高,并且TP、TN与Chl.a均呈显著性正相关.为探究太湖蓝藻水华暴发的TP和TN控制阈值,以轻富营养化等级下的Chl.a分级标准(10,26］作为表征水华暴发的条件,采用郑丙辉等的频率分布法,确定了太湖蓝藻水华暴发的TP和TN控制阈值分别为0.05~0.06和1.71~1.72 mg/L;通过空间验证,太湖藻型区TP和TN浓度远高于同级营养水平下全湖区TP和TN控制阈值,表明藻型区高氮磷水平为蓝藻水华发生提供充足营养盐条件,即使氮磷全湖平均浓度控制在蓝藻水华暴发的氮磷阈值水平之下,但在气象水文等因素适宜条件下,藻型区水华发生风险仍然较高;并且在高氮磷背景下,即便在水华发生风险低的季节,水华发生风险仍然较大.近十几年来,虽然太湖经历了大规模的高强度治理,但由于环太湖流域的湖西区入湖负荷占比大,导致太湖藻型区氮磷浓度仍处于高位运行状态,为蓝藻水华的暴发提供了充足的营养盐基础,因此,湖西区的控源减排仍然是太湖富营养化及蓝藻水华防控的重点.;Algae growth and nutrient concentration has a lower threshold for algae affected by nutrient concentration with geometric increase and an upper threshold for algae that is not affected by the increase of nitrogen and phosphorus concentration. However, the formation of cyanobacterial blooms is affected by many factors, the effects of nitrogen and phosphorus on cyanobacterial blooms vary greatly in different lakes, different areas, and different periods, making it difficult to determine the nitrogen and phosphorus control thresholds for cyanobacteria growth. Although studies on nitrogen and phosphorus thresholds for algal growth and control of cyanobacterial blooms have been carried out, they are mostly focused on laboratory studies or empirical judgments. Although there are also studies based on field-measured data, they are also limited to a specific area, while studies on nitrogen and phosphorus thresholds based on long series of field measurements and covering the entire lake are lacking. As a eutrophic shallow lake with a high nutrient background, the occurrence of cyanobacterial blooms in Lake Taihu is strongly influenced by nitrogen and phosphorus. Analyzing the temporal and spatial changes of total phosphorus (TP), total nitrogen (TN) and chlorophyll-a concentration (Chl.a) in Lake Taihu, it is found that TP, TN and Chl.a in the northwestern of the lake were significantly higher, and TP, TN and Chl.a are all positively correlated. To investigate the control thresholds of TP and TN for cyanobacterial bloom outbreaks in Lake Taihu, the Chl.a grading standard (10, 26] under the light eutrophication level was used as the condition to characterize the bloom outbreak, referring to the frequency distribution method put forward by Zheng Binghui, the TP and TN control thresholds were determined to be 0.05-0.06 mg/L and 1.71-1.72 mg/L, respectively. Through spatial verification, the concentration of TP and TN in the phytoplankton-dominated area of Lake Taihu is much higher than the control thresholds of TP and TN in the whole lake area under the same nutritional level. This indicated that the high nitrogen and phosphorus levels in the phytoplankton-dominated area provide sufficient nutrient basis for the occurrence of cyanobacterial blooms. Even if the average concentration of nitrogen and phosphorus in the whole lake is controlled below the threshold level, the risk of algal blooms in the phytoplankton-dominated area is still relatively high under suitable conditions of meteorology and hydrology. In addition, under the background of high nitrogen and phosphorus in the phytoplankton-dominated area, even in the low-risk seasons for cyanobacterial blooms that were once considered, the possibility of blooms is still high. In the past ten years, although Lake Taihu has undergone large-scale and high-intensity treatment, the nitrogen and phosphorus concentrations in the phytoplankton-dominated area of Lake Taihu are still at a high level because of the large proportion of the inflow pollution load from Huxi District around Lake Taihu Basin, which provides sufficient nutrients for the cyanobacterial blooms. In consequence, source control and emission reduction in Huxi District is still the key measurement for the prevention and control of eutrophication and cyanobacterial blooms in Lake Taihu.