低氧、高铵和低光对沉水植物苦草(Vallisneria natans)生长与C-N代谢生理指标的影响特征

湖泊富营养化导致沉水植被大面积衰退和群落逆向演替,诱发一系列次生环境问题,并严重影响到水域生态环境质量.为了从对植物表型生长与C-N代谢生理指标影响的角度深度揭示富营养化水体中沉水植被的致衰退机制,本文以我国长江中下游淡水湖泊常见沉水植物优势种群——苦草(Vallisneria natans)为研究对象,利用L_(16)(4~5)正交试验设计方法,实验模拟研究富营养化水体中低氧、高铵和低光3种重要因素对苦草生长与C-N代谢生理指标的胁迫影响特征.本试验设置了3因素4水平,分别为4个低光照强度(50%、40%、30%和20%自然光照)和4个高铵浓度水平(0.5、1、2和4 mg/L)以及4个低氧处理浓度(7.5、6.5、5.5和4 mg/L).结果显示:光照强度低于30%、溶解氧浓度低于5.5 mg/L时,植株生长与C代谢受阻严重,碳水化合物储存量降低;铵态氮1.0 mg/L时,苦草N代谢活跃,游离氨基酸(FAA)含量明显升高,可溶性糖(SC)/FAA比降低,淀粉呈降低趋势.研究表明富营养湖泊中苦草的衰退是多种因素综合作用的结果,低氧、高铵与低光均会对苦草的生长与C-N代谢产生不利影响;受损沉水植被在藻-草稳态转换的富营养化湖泊中应通过控制水体高铵浓度,严控低氧出现,及时提高水下照度或透明度(如控磷)来予以保护和科学管理;而在次生裸地且藻类占优势的富营养化水体中沉水植被的恢复与重建过程不仅要降低水体营养盐水平尤其是氨氮的水平,还应着重考虑如何有效提高水下光强与溶解氧浓度,并将如上环境因子控制在一定变幅范围内,且控制条件应原则上严于保护受损沉水植被所需的条件.

Integrated effects of hypoxia, high ammonia and low light on the growth and physiological C-N metabolism indices of Vallisneria natans

Lake eutrophication has resulted in the sharp decline of submersed macrophytes and the reversal succession of aquatic communities around the world, subsequently inducing a serial of secondary environmental degradations which has adverse effects on the water quality of lacustrine habitats. The present study was conducted to uncover the potential coupling mechanism of the decline of submersed plants from the perspective of phenotypic growth and C-N physiological stress. We chose Hydrocharitaceae species (Vallisneria natans), which is a cosmopolitan submerged and dominated in the freshwater along the middle-lower reaches of the Yangtze River. The integrated effects of hypoxia, high ammonia and low light, 4 (light:50%, 40%, 30% and 20% underwater sunlight) ×4 (ammonia:0.5, 1, 2 and 4 mg/L) ×4 (oxygen:7.5, 6.5, 5.5 and 4 mg/L), on the growth and C-N metabolism of V. natans were explored by the semi-outdoor simulation experiments with L₁₆(4⁵) orthogonal design methods. Results showed that the growth and C-metabolism would be hindered and these indicators were also in the reduction on the carbohydrate storage capacity when the light intensity was below 30% of sunlight and the oxygen content below 5.5 mg/L. When ammonia >1.0 mg/L, N-metabolism would be active and the content of free amino acids (FAA) was increasing, and the decline of the soluble carbohydrate (SC)/FAA ratio and starch content were also observed. Results suggested that the degradation of V. natans in eutrophic lakes was the coefficient of several environmental factors. The environmental factors e.g. hypoxia, high ammonia and low light could all have negative effect on the growth and C-N metabolism of V. natans. It indicated that the declining submersed macrophyte in eutrophic lakes under the macrophyte-to-algae regime shift need to be protected and effectively managed by inhibiting the high ammonia, controlling the occurrence of hypoxia in the overlying water column tightly and improving the underwater light intensities or the secchi depth (e.g. control of phosphorus increase in overlying water) timely. To restore and re-establish the submersed macrophyte meadow in the eutrophic degraded lakes with the phytoplankton dominance, it''s necessary to decrease the level of all nutrients (especially ammonia) in the water column. To increase both underwater light intensity and dissolved oxygen, it''s needed to control the three environmental factors above within a certain range and the control limits of these factors should be stricter than the protection thresholds of the declined plant population in principle.