岛礁海藻场沉积有机物来源辨析

Identifying sources of sedimentary organic matter in the rocky reef seaweed bed

海藻场的沉积有机物(sedimentary organic matter,SOM)是实现海藻场生态系统服务功能的重要物质基础,本实验以枸杞岛北部海藻场为研究区域,围绕大型海藻周年生活史的幼苗&生长、成熟&茂盛和衰退&凋亡3个阶段,于2014年7月凋亡期、2014年10月生长期和2015年5月茂盛期对海藻场的SOM进行样品采集,并运用碳、氮稳定同位素技术,以C/N、δ^(13)C和δ^(15)N为指标分析了SOM的来源及变化。结果显示,1)7月、10月和翌年5月的SOM C/N的变化范围分别为5.9~6.6、6.0~6.9和5.4~6.2,可判定海藻场SOM是典型的海源性来源;2)7月不同水深的SOMδ^(13)C值空间变化明显,介于–20.3‰~–17.6‰,而10月和翌年5月都不显著,分别介于–22.3‰~–21.7‰和–21.4‰~–21.0‰;3)SOM的δ^(13)C值存在时间变化,而7月δ^(13)C值存在不同水深的空间变化;4)δ^(13)C、δ^(15)N和C/N之间的关系表明,7月SOM主要来源于浮游植物和大型海藻的混合贡献,而10月和翌年5月SOM则主要来源于浮游植物贡献;5)根据碳稳定同位素质量平衡混合模型计算得到,7月大型海藻对该海藻场SOM的平均贡献率最高可达53.71%;6)大型海藻产生的碎屑在SOM占比受波浪等海域动力环境影响显著。

Sedimentary organic matter（SOM） in seaweed beds provides an important material foundation for implementing seaweed bed ecosystem services. In this study, SOM samples were collected from the seaweed bed in northern Gouqi Island during the death period in July 2014, growth period in October 2014, and bloom period in May 2015, in accordance with three macroalgal life history stages： germinate and grow, mature and bloom, and decay and die. The sources and variations of the SOM were determined by carbon and nitrogen stable isotope analysis using the molar carbon-to-nitrogen（C/N） ratio and stable isotope composition（δ13C and δ15N） as indicators. The results showed the following. 1） The molar C/N ratios of the SOM varied in the ranges of 5.9–6.6,6.0–6.9, and 5.4–6.2 in July, October, and the following May, respectively, indicating a typical marine source of SOM from the seaweed bed. 2） The δ13C values of the SOM were significantly different at the various water depths in July（–20.3‰ to –17.6‰）; however, no significant changes were found in October（–22.3‰ to –21.7‰） or in May of the following year（–21.4‰ to –21.0‰）. 3） The δ13C values of the SOM showed temporal variations,whereas spatial variations with water depth were found in July. 4） According to the relations among the δ13C, δ15N,and C/N, the SOM was mainly derived from the mixed contribution of phytoplankton and macroalgae in July, and it was attributed to the contribution of phytoplankton in October and the following May. 5） In July, macroalgae, on average, contributed up to 53.71% of the SOM from the seaweed bed, as estimated by the carbon stable isotope mass-balance mixing model. 6） The proportion of macroalgal detritus in the SOM was significantly affected by waves, among the various hydrodynamic factors in the marine environment.