The sources characteristics of stable isotope organic carbon and nitrogen in suspended particles and surface sediments in Lake Erhai and their water quality implications

Abstract

为探究湖泊水体悬浮颗粒物和沉积物有机碳、氮来源及水质指示意义，分析了2013-2014年洱海悬浮颗粒物和表层沉积物有机碳同位素（δ¹³C）、氮同位素（δ¹⁵N）和C/N比值时空变化特征及与水质的关系.结果表明：①洱海悬浮颗粒物δ¹³C、C/N、δ¹⁵N在旱、雨季差异显著（P<0.05），旱季变化范围分别为-31.75‰~-18.21‰（均值-25.34‰±4.14‰）、9.1~16.9（均值13.3±2.7）、4.9‰~7.4‰（均值6.4‰±1.3‰），雨季变化范围分别为-14.7‰~-23.8‰（均值-20.2‰±3.3‰）、4.6~8.9（均值7.1±1.6）、7.4‰~10.8‰（均值9.3‰±1.8‰）.悬浮颗粒物有机碳来源在旱季以陆源C3植物为主（46.0%±6.9%），转变为雨季以浮游植物为主（43.3%±6.1%）；氮来源在旱季以陆源植物为主（40.7‰±6.5%），转变为雨季以湖内水生植物和浮游植物为主（39.9%±6.6%）.表层沉积物δ¹³C（-24.0‰~-14.6‰（均值为-18.7‰±4.7‰））和C/N（9.1~15.5（均值为12.1±3.3））均无显著季节差异（P>0.05），δ¹⁵N在旱、雨季差异显著（P<0.05），变化范围分别为1.9‰~4.9‰（均值为3.6‰±1.5‰）和0.7‰~7.8‰（均值为4.2‰±1.8‰）.表层沉积物有机碳来源在旱、雨季均以陆源C4植物为主（48.2%±19.1%），氮来源旱季以陆源植物为主（44.3%±10.1%），转变为雨季以化肥为主（30.3%±6.8%）.两者有机碳与氮来源差异揭示水生和浮游植物来源的有机碳与氮易降解，对水质影响较大，而陆源C4植物和土壤来源的有机碳和化肥来源氮易沉积，对水质影响相对较小.②随机森林回归分析表明，悬浮颗粒物有机质（POM）来源差异（δ¹³C、C/N）、氧化还原电位和水温（WT）是影响水体多营养循环指标的重要因子，重要度为4.0%~6.9%；POM、C/N和WT是影响叶绿素a的重要因子，重要度为9.3%~10.7%，说明POM来源特征结合水环境因子显著影响水质，而表层沉积物有机质来源相对稳定，对水质无显著影响.为防控洱海水质持续下降，除了加强外源控制，雨季还应重点采取控藻措施.;In order to explore the sources of organic carbon and nitrogen in suspended particles (SP) and sediments, and their implication for water quality, the spatiotemporal variation characteristics of stable isotopic carbon (δ¹³C), nitrogen (δ¹⁵N) and C/N in SP and surface sediments of Lake Erhai were determined in 2013-2014, and their effects on water quality were analyzed. The results showed that, ① The δ¹³C, C/N and δ¹⁵N of SP were significantly different between dry and wet seasons (P<0.05), and varied in the range of -31.75‰ to -18.21‰(the mean was -25.34‰±4.14‰), 9.1 to 16.9(13.3±2.7) and 4.9‰ to 7.4‰(6.4‰±1.3‰) in the dry season, and -23.8‰ to -14.7‰(-20.2‰±3.3‰),4.6 to 8.9(7.1±1.6) and 7.4‰ to 10.8‰(9.3‰±1.8‰) in the wet season, respectively. The end-member mixing model showed, the sources of organic carbon in SP transformed from the terrestrial C3 plants dominated (46.0%±6.9%) in the dry season to phytoplankton dominated (43.3%±6.1%) in the wet season; the nitrogen sources in SP transformed from the terrestrial plants dominated (40.7%±6.5%) in the dry season to aquatic plants and phytoplankton dominated (39.9%±6.6%) in the wet season. There were no significant differences in δ¹³C and C/N in surface sediments between dry and wet seasons (P>0.05), and the δ¹³C and C/N varied in the range of -24.0‰ to -14.6‰(-18.7‰±4.7‰)) and 9.1 to 15.5(12.1±3.3), respectively. The δ¹⁵N of surface sediments was significantly different between dry and wet seasons(P<0.05), and varied in the range of 1.9‰ to 4.9‰(3.6‰±1.5‰) and 0.7‰ to 7.8‰(4.2‰±1.8‰), respectively. The organic carbon in surface sediments in dry and wet seasons mainly originated from the terrestrial C4 plants, which contributed 48.2%±19.1% of the total organic carbon, and the nitrogen sources in surface sediments transformed from the terrestrial plants dominated (44.3%±10.1%) in the dry season to fertilizers dominated (30.3%±6.8%) in the wet season. The different sources of organic carbon and nitrogen between SP and surface sediments revealed that the organic carbon and nitrogen from phytoplankton were easily degraded, leading to the deterioration of water quality; while the organic carbon from terrestrial C4 plants and soil erosion and nitrogen from fertilizer are easy to deposit, which had little effect on water quality. ② Correlation analysis and random forest regression analysis showed that the source difference of particulate organic matter(POM) (δ¹³C and δ¹⁵N), oxidation-reduction potential and water temperature (WT) in SP were key indicators in the multi-nutrient cycling index for lake water with the importance for 4.0% to 6.9%, while POM, C/N and WT in SP were key indicators in chlorophyll-a for lake water with the importance for 9.3% to 10.7%, which indicated that the source characteristics of POM combining with environmental factors had a significant effect on water quality. However, the source characteristics of organic carbon and nitrogen in surface sediments have no significant effect on water quality. For the protection of Lake Erhai, in addition to further strengthening the control of external loads, the phytoplankton biomass should be mainly controlled in the wet season to avoid further deterioration of water quality.